How Paleozoic vines and lianas got off the ground: on scrambling and climbing Carboniferous--early Permian pteridosperms.
Pteridosperms form a heterogeneous group of gymnospermous plants with fernlike foliage and were widely distributed during the late Paleozoic. Paleoecological and sedimentological studies suggest that these plants mostly inhabited the warm and humid coal-swamp forests. Pteridosperms were morphologically diverse and displayed a wide range of growth forms. Some taxa were small to medium-sized, or even large, upright, self-supporting trees (Seward, 1917; Scott, 1923; Bertrand & Corsin, 1950; Stewart & Delevoryas, 1956; Laveine, 1986), some with massive stems (e.g., up to 48 cm in diameter in Medullosa stellata var. gigantea, cf. Sterzel, 1896: 66-69). Based on biomechanical analyses of such stems, Mosbrugger (1990: 90) suggests that the largest trees may have been up to 13 m tall. Other forms appear to have been leaners that gained support from larger trees and/or loosely intermingled with components of the surrounding vegetation, or they may have grown in dense stands (clumps), in which they supported each other by intertwining stems and foliage (Andrews, 1948; Wnuk & Pfefferkorn, 1984; Mosbrugger, 1990; Stewart & Rothwell, 1993). For Medullosa thompsonii and similar species (subgenus Anglorota; cf. Schopf, 1939), different growth habits have been suggested, based on the height of the plant. They may have been small to medium-sized plants that were self-supporting; but tall and lax individuals may have grown in clumps for support (Andrews, 1945). Some pteridosperms were true vines or lianas (Potonie, 1898). Scrambling and/or climbing growth habits have been documented for compression taxa based on gross-morphological features including narrow, wavy or sinuous stems and small fronds (e.g., Potonie, 1898; Danze-Corsin, 1953; Galtier et al., 1985) and/or specialized climbing aids such as climber hooks, tendrils, and tendrils terminating in adhesive pads (Huth, 1912; DiMichele et al., 1984; Krings & Kerp, 1997, 1999). Stem diameter and anatomy also suggest that a number of permineralized forms were vine- or liana-like (e.g., Sterzel, 1896; Baxter, 1949; Stidd & Phillips, 1973; Rothwell, 1975; Hamer & Rothwell, 1988; Dunn et al., 2003).
Although permineralized specimens may provide valuable information on fossil vine- and liana-like plants (e.g., on internal anatomy, biomechanies, and physiology), rarely do they provide a sound basis for interpreting gross morphology and growth habits. Rather, information on the latter aspect is better determined from impression/compression specimens since these often display large(r) plant parts, often in organic connection. Particular features (e.g., narrow, wavy or sinuous stems, long internodes, small compact fronds, and orientation of the fronds to one side) may indicate a vine- or liana-like life-form (Potonie, 1898; Hamer & Rothwell, 1988; Krings et al., 2001a). Unfortunately, even large stem and foliage specimens often cannot provide unequivocal evidence. For example, large specimens of Neuropteris rarinervis from the Westphalian of Germany display rather large fronds, up to 80 cm long, that are produced on narrow stems less than 3 cm in diameter. As for Medullosa thompsonii, different growth habits, depending on the height of the plants, can be visualized for N. rarinervis. If the stems are short, they still may be mechanically stable, and the plants may have been self-supporting. However, if the stems of N. rarinervis were several meters long it would be difficult to envisage a self-supporting, treelike growth habit for a plant exhibiting this complement of features and therefore may have had a leaning posture or was a true vine or a liana. More useful in unequivocally documenting vine- and liana-like growth habits in fossil plants is the demonstration of plant parts modified into specialized scrambling/climbing aids that are similar or even identical to those seen in extant vines and lianas. Cuticular analysis appears to be especially valuable for elucidating pteridosperm scrambling/climbing aids (cf. Krings, 1997; Kerp & Krings, 1998); a considerable amount of the evidence on scrambling/climbing pteridosperms here presented is based on cuticle preparations.
This article reviews the information gathered to date on vine- and liana-like forms among late Palcozoic pteridosperm taxa from the Upper Carboniferous-Lower Permian of Europe and North America based on impression/compression material and cuticle preparations. Three general aspects are considered: 1) the various modes of attachment utilized by vine- and liana-like pteridosperms to both anchor and support the plant; 2) morphological features that may represent adaptations to special physiological requirements necessary in the scrambling/climbing growth habit; and 3) the role vine- and liana-like pteridosperms may have played in late Paleozoic coal-swamp forest ecosystems.
III. Attachment Modes of Scrambling and Climbing Pteridosperms
Vines and lianas are herbaceous and woody plants that begin their life as self-supporting seedlings but, as growth continues, become mechanically unstable and rely on some kind of external support for their continued vertical growth (Holbrook & Putz, 1996). Various modes of attachment to an external support (Klettermodi sensu Schenck, 1892) among extant scrambling and climbing vascular plants can be characterized based on morphological and physiological behavior: scrambling, hook-supported scrambling/climbing, tendril climbing, (adventitious or shoot-borne) root climbing, and twining/winding (Darwin, 1867; Schenck, 1892; Stocker, 1932; Hegarty, 1989, 1991). Various modes of attachment, very similar to those seen in extant angiosperms, have been demonstrated to occur among Late Carboniferous and Early Permian pteridosperms.
A. SCRAMBLING AND HOOK-SUPPORTED SCRAMBLING/CLIMBING
Scrambling plants are not self-supporting but rather grow loosely intermixed with and supported by the surrounding vegetation. They sprawl over other plants or grow in dense stands or thickets, in which individuals support each other by intertwined parts (Schenck, 1892; Hegarty, 1991). The majority of scramblers does not develop specialized structures in order to attach themselves to a support medium (Schenck, 1892: 71-77). Thus, scrambling growth cannot be determined directly from the impression/compression fossil record of pteridosperms. However, certain features of pteridosperm frond architecture (e.g., large fronds with lax appearance, thin, flexuous stipes and rachides, wide-angled pinnae [Fig. 1]) may suggest scrambling life-forms, since these features make it difficult to envisage the plant as self-supporting, but rather imply that it grew either intermixed with and supported by other individuals of its kind and/or the surrounding vegetation.
[FIGURE 1 OMITTED]
Hook-supported scrambling/climbing is traditionally regarded as a specialized form of scrambling (rather than a form of climbing; e.g., Schenck, 1892: 77-78), in which attachment of the plant to the surrounding vegetation is secured by various types of recurved, hook-like structures (Treub, 1886a, 1886b; Huth, 1888; Schenck, 1892; Ewart, 1898; Stocker, 1932). However, although hooks are much more restricted in locating a support than are, for example, twining stems or tendrils (cf., e.g., De Vries, 1874; Penalosa, 1982), many hook-scramblers/ climbers are still able to respond morphologically to the presence of a suitable support (Treub, 1886a, 1886b; Ewart, 1898; Putz & Holbrook, 1991), and thus may be regarded as climbers rather than as scramblers, which lack any form of morphological or physiological response (so-called prehensile mechanisms; cf. Penalosa, 1982) to the presence of a suitable support (Schenck, 1892: 78; Hegarty, 1991). Since such morphological responses are difficult to document based on the fossil record, we here refer to these forms as hook-supported scramblers/climbers. However, hook-like scrambling/climbing aids, as they occur in fossil plants, are generally termed "climber hooks" (Boersma, 1972, 1991; DiMichele et al., 1984; Kerp & Krings, 1998; Li & Taylor, 1999). Hook-supported scrambling/climbing was rather common among mariopterid pteridosperms (e.g., Karinopteris, Mariopteris, and Pseudomariopteris; Huth, 1912; Remy & Remy, 1977).
These plants generally were small to medium-sized vines, with stems that rarely exceeded 2 cm in diameter and fronds up to 45 cm long (Potonie, 1898, 1899; Kidston, 1925; Boersma, 1972; Krings et al., 2001a). Although mariopterid fronds were relatively small, it appears unlikely that stems of this diameter were self-supporting. Rather, these plants scrambled/climbed with specialized climber hooks, usually developed from apical prolongations (sensu Boersma, 1972 [= apical/terminal extensions]) of pinna axes. Such climber hooks display considerable interspecific morphological variability. For example, the climber hooks of numerous Namurian and Westphalian Karinopteris and Mariopteris species from France and Germany (e.g., Fig. 2), and those of Pseudomariopteris busquetii (Fig. 3) from the upper Stephanian and Autunian/ Rotliegend of France and Germany, are simple (usually up to 1.5 cm long) prolongations of the pinna axes with recurved tips (Huth, 1912; Corsin, 1932; Danze-Corsin, 1953; Boersma, 1972, 1991; Josten, 1991; Kerp & Barthel, 1993; Steur, 1995; Kerp & Krings, 1998; Krings & Kerp, 2000; Krings & Schultka, 2002), whereas the hooks of Pseudomariopteris cordato-ovata from the Stephanian of central France (Figs. 4 & 5) terminate in two recurved secondary hooks (Kerp & Krings, 1998; Krings & Kerp, 2000). The climber hooks of Karinopteris acuta from the Namurian of VoBacker (near Frondenberg, Germany) possessed club-shaped tips bearing numerous recurved secondary hooks (Fig. 6; Schultka, 1995), whereas, in Karinopteris sp. from the Indiana paper shale (Middle Pennsylvanian, U.S.A.), recurved secondary hooks are distributed along the entire abaxial side of the (up to 5 cm long) climber hooks (Fig. 7; DiMichele et al., 1984; Kerp & Barthel, 1993).
[FIGURE 2-7 OMITTED]
Mariopteris occidentalis from the Middle Pennsylvanian of Oklahoma (Fig. 8) also utilized hooks to scramble/climb but apparently did not possess climber hooks in the form of prolongations of pinna axes; rather, this species had numerous recurred hooks (Figs. 9-11) regularly distributed on the abaxial sides of the pinna axes (Krings et al., 2001b). Climber hooks similar to those seen in mariopterids have also been documented for a few other pteridosperms (e.g., Eremopteris lincolniana, cf. White, 1943: pl. 24, figs. 1, 7, 8), suggesting that hook-supported scrambling/climbing may have been a more widely spread mode of attachment in this group of ancient gymnosperms. The large number of seed ferns that possess climber hooks indicates that this scrambling/climbing strategy was effective. Scrambling/climbing with large numbers of small hooks may be regarded as particularly suitable for small to medium-sized vines/lianas in areas of dense vegetation (Menninger, 1970). The many small hooks produce frictional resistance by catching and ratcheting between the leaves and branches of the support plants and thus form a loose but effective interaction.
[FIGURE 8-11 OMITTED]
The considerable variation that exists among scrambling/climbing mariopterid pteridosperms relative to size and morphology of hooks raises the question as to whether the different forms of climber hooks may represent adaptations of individual taxa to certain morphological features of their most frequently utilized support media. One may argue that climber hooks that bear recurved secondary hooks (e.g., those of Karinopteris sp. [Fig. 7] from the Indiana paper shale with numerous recurved secondary hooks on the abaxial side; cf. DiMichele et al., 1984; Kerp & Barthel, 1993) would be more effective and better adaptated than simple hook-like prolongations of pinna axes (e.g., in Pseudomariopteris busquetii [Fig. 3] from the European Stephanian and Rotliegend; cf. Krings & Kerp, 2000). However, any adaptative significance of the different climber hook morphologies can only be assumed since no information on the actual scrambling/climbing process is available in the fossil record. Nevertheless, the climber hooks of M. oceidentalis (Figs. 9-11) may be regarded as representing a rather primitive form of hook for scrambling/climbing (Krings et al., 2001b). Fronds of M. occidentalis may be restricted to the surface of a support medium (e.g., large tree fern fronds) in order to effectively anchor the plant body, whereas fronds that bear climber hooks in the form of apical extensions can anchor either by extending out on the support medium or by hanging in a thicket, suspended between components of the one or several supports (e.g., in tree-fern or pteridosperm foliage or in calamite branching systems and foliage). Moreover, climber hooks in the form of apical axis extensions may also provide greater stability, owing to the capacity of the hooks to attach more securely.
Particular stem specimens of Pseudomariopteris busquetii from the upper Rotliegend of Bad Sobernheim (Saar-Nahe Basin, Germany) bear fronds that extend from one side of the stem and lack climber hooks (Krings et al., 2001a), suggesting that these stems had been attached to a support medium. Orientation of leaves to one side (presumably toward the sunlight and free space but away from the host, especially its shade) is also commonly found in extant climbing plants with firmly attached stems (Schenck, 1892; Givnish & Vermeij, 1976; Madison, 1977). To date, however, the mode of attachment of the stems from Sobernheim (either by aerial adventitious roots or by means of twining) remains unknown. Nonetheless, the information gathered from these large specimens from Sobernheim suggests that the mode of attachment of P. busquetii may have changed during the life of a single plant or that individuals of P. busquetii could, under different habitat conditions, develop a variety of strategies, each adaptated to the respective habitat (Krings et al., 2001a). Change of mode(s) of attachment based on stage of development or environmental factors (e.g., availability and quality of supports) and simultaneous use of different modes of attachment are not unusual among extant (tropical) vines and lianas (Hegarty, 1989, 1991). Some Bignoniaceae, for example, climb with their twining shoots and simultaneously utilize leaf tendrils but later produce adventitious roots that are effective in attaching specialized long shoots (Schenck, 1892: 6-7).
Gigantopterids, enigmatic Permian gymnosperms from North America and Cathaysia, are characterized by slender stems and large, often massive leaves. Li and Taylor (1998, 1999) suggested that several gigantopterid taxa were scrambling/climbing plants that also utilized various types of climber hooks to anchor the plant body to a support medium. The scrambling/ climbing growth habit of gigantiopterids, however, has been questioned by Wang (1999), who suggests that they may have been amphibious or aquatic plants.
B. ROOT CLIMBING
Callistophyton sp. from the Middle and Upper Pennsylvanian of North America has been reconstructed as a scrambling shrub with supporting shoot-borne roots (Rothwell, 1975). However, Callistophyton was certainly not a true root climber, like the species of the extant angiosperm genera Hedera (Schenck, 1892: 92-95) or Monstera (Madison, 1977), which are able to tie their stems firmly to a host medium with numerous shoot-borne roots. Fronds of Dicksonites pluckenetii, a taxon that is considered to represent the compression equivalent of Callistophyton (Stidd & Barthel, 1979; Rothwell, 1981; Meyen & Lemoigne, 1986), from the Stephanian and Rotliegend of Europe are often rather large and possess slender, sinuous primary axes (Fig. 1). These features suggest that the plant may have scrambled in thickets of other plants, thereby gaining additional support from roots. Moreover, a specimen of D. pluckenetii from the upper Rotliegend of Bad Sobernheim (Germany) demonstrates that this taxon also could develop axial tendrils (Fig. 12). True root climbing is probably difficult to demonstrate in the fossil record. The outer regions of stems (e.g., of arborescent lycopsids, calamites, cordaites), to which root climbers may have been attached, are often not well preserved, and the climbers may have been separated from the stems during fossilization and/ or diagenesis.
[FIGURE 12 OMITTED]
C. TENDRIL CLIMBING
The morphologically most specialized and presumably most advanced vine- to liana-like pteridosperms were the tendril climbers. Tendril climbing has to date been demonstrated for Dicksonites pluckenetii from the upper Rotliegend of Bad Sobernheim (Germany) (see above) and for two taxa from the Stephanian of central France.
Lescuropteris genuina (Fig. 13) climbed by leaflet tendrils, developed from specialized apical portions of pinnae, in which one or several of the pinnules are variously modified into tendrils (Fig. 14; Krings & Kerp, 1997) similar to the leaflet tendrils developed by many Fabaceae (e.g., Pisum or Vicia species; Stocker, 1932). In addition, one specimen (Fig. 15) indicates that tendrils could also develop from apical extensions of normally shaped pinnules (i.e., prolongations of one vein through the pinnule apex). Tendril development from apices of normally shaped leaves is not rare among extant tendril climbers (Cremers, 1973, 1974). However, a different explanation for this specimen may also exist. Czapek (1909) observed that, in Entada polystachya (Mimosaceae), the tendrils develop from leaf petioles, and older tendrils, which did not find a support, subsequently develop into normal leaves. Thus, the pinnule with extended apex of L. genuina (Fig. 15) may also represent an older tendril that did not function in support and subsequently developed a photosynthetic function. The tendrils of L. genuina were apparently well adaptated to adhere to the support medium. The epidermal cells of these tendrils bear large papillae (Figs. 16 & 17), not found in any other part of the plant. These papillae were no doubt advantageous for securing a young and developing tendril to the surface of a support medium (Lisk, 1924; Jaffe & Galston, 1968). Moreover, papillae, as they occur in tendrils of extant climbing plants, are known to be effective in the transmission of contact stimuli, which are important in directing tendril growth around the support (Sachs, 1888; Lisk, 1924; Tronchet, 1977).
[FIGURE 13-17 OMITTED]
The climbing aids of Blanzyopteris praedentata, on the other hand, which most probably represent highly modified fronds, consist of a main axis (= tendril) that bears lateral branches (Fig. 18) with numerous apically widened branchlets terminating in adhesive pads (Fig. 19). The adhesive pads in this fossil are morphologically comparable to those in, for example, Parthenocissus tricuapidata, a member of the grape family (Vitaceae; Krings & Kerp, 1999; Speck et al., 2000). Interestingly, the climbing aids of B. praedentata also are characterized by a feature that may be interpreted as an adaptation to secure initial attachment to a support medium. In contrast to Lescuropteris genuina, however, the epidermal cells of the climbing organs of B. praedentata do not possess papillae; rather, on the lower surface of the main axis of the climbing aid are large and apically bifurcate trichomes that often form little hooks (Figs. 20 & 21). The morphology and distribution of these trichomes suggest that they may have served for the initial attachment of the developing climbing organ to a support medium until the lateral branches and adhesive pads had become fully developed. Because climbing organs of B. praedentata were quite large (the branched part was about 10 cm wide and up to 15 cm long; cf. Krings & Kerp, 1999), we hypothesize that the main axes may have attached to the support medium prior to the development of lateral branches and adhesive pads. Fully developed lateral branches would presumably even hamper an immature climbing organ's "search" for a suitable support. The effectiveness of the mature climbing aid is no doubt based on the number of adhesive pads that are firmly attached to a support. Delayed development of lateral branches and adhesive pads (i.e., after attachment of the main axis) would enable oriented growth of the adhesive pads toward the support and adjustment of the climbing organ's morphology to the surface features of the support.
[FIGURE 18-21 OMITTED]
Unequivocal evidence for pteridosperms that climb with twining/winding stems has not yet been documented. Of special interest, however, is a specimen from the Westphalian of northern England, reported by Cleal and Thomas (1999). This highly informative fossil displays a slender stem with mariopterid fronds still attached and wound around the stump of an arborescent lycopsid. However, it remains uncertain whether this mariopterid climbed solely with its twining stem or was attached to the tree by means of shoot-borne roots. An upright standing Sigillaria stem from the Westphalian of Poland clearly shows imprints of a larger winding axis (Gradzinski & Doktor, 1995). However, it remains uncertain as to whether these imprints have come from a twining pteridosperm or a fern, such as is well documented to occur among some late Paleozoic coenopteridalean and filicalean ferns (e.g., Ankyropteris brongniartii; el. Robler, 2000).
IV. Special Adaptations of Vine- and Liana-Like Pteridosperms
The vine- or liana-like growth habit represents a physiological challenge for the plant, in that sufficient water and nutrients must be transported through a long, narrow stem to supply a large area of photosynthetically active leaf tissue (Gessner, 1956; Lerch, 1991). This special physiological requirement may necessitate certain adaptations. The occurrence of vessels in gigantopterids (e.g., Li et al., 1996; Li & Taylor, 1998, 1999) is one anatomical feature that may be related to the special physiological requirements of the vine- or liana-like growth form. However, since the present article focuses on information obtainable from impression/compression material or cuticle preparations, we restrict ourselves to those features with possible adaptative significance that can be observed from hand specimens directly or may be obtained through cuticular analysis.
Several features of pteridosperm stomatal complexes may have had some significance as adaptations to special physiological requirements of the vine- or liana-like habit. The reduction of water loss as a result of excessive transpiration is of critical importance to vines and lianas. Low stomatal densities, comparatively small sizes of the stomatal apparatus, sunken guard cells, and partial closure of the outer stomatal chambers by overarching papillae extending from the subsidiary cells are known to function protectively with regard to excessive water loss (e.g., Hill, 1998). Minute stomatal complexes with deeply sunken guard cells have been documented for several vine- and liana-like pteridosperm taxa, including the hook-scramblers/climbers Karinopteris beneckei (Krings & Schultka, 2002), Mariopteris occidentalis (Krings et al., 2001b), Pseudomariopteris busquetii, and P. cordato-ovata (Krings & Kerp, 2000). The tendril-climbing Lescuropteris genuina is characterized by tiny stomatal apparati (Fig. 22) with deeply sunken guard cells and outer stomatal chambers almost completely covered by papillae extending from the subsidiary cells (Krings & Kerp, 1997). In addition, stomatal density is low in this taxon since the occurrence of stomata is restricted to the marginal parts of pinnules. However, the above-mentioned features of the stomatal apparatus are not exclusively connected with a vine- to liana-like growth habit but may also occur in non-scrambling/climbing taxa that need special protection from excessive transpiration.
[FIGURE 22 OMITTED]
A particularly conspicuous feature of many scrambling/climbing pteridosperms is marginal thickenings of the lateral pinnule veins (e.g., in Blanzyopteris praedentata, Lescuropteris genuina, Mariopteris occidentalis [Fig. 8, arrows], and Pseudomariopteris busquetii; cf. Krings & Kerp, 1997, 1999, 2000; Krings et al., 2001b). These structures are very similar to the thickened vein endings that occur in many extant ferns (e.g., Phyllitis or Polypodium species) where they consist of groups of short, large-diameter tracheids that end below the upper epidermis and often terminate in distinct water pits (Potonie, 1892; Pray, 1960). Such structures are regarded as functional analogues to hydathodes, which typically occur in angiosperms (Ziegenspeck, 1948) and may maintain a sufficient water flow (and nutrient supply) by guttating water, which, in turn, facilitates a constant xylem stream when transpiration is reduced (Ziegler, 1982). This may be of primary importance in young and developing leaves, in which stomata are not yet (fully) operational and nutrient demand is high (Hohn, 1950), especially for evergreen climbing plants in warm and humid climates that often possess rather large proportions of young and developing leaves (Hegarty et al., 1991). In some extant angiosperms, guttation occurs primarily in the young leaves, whereas older leaves do not consistently express guttation (e.g., Glenn & Takeda, 1989). At some level (e.g., due to drought caused by lack of rainfall and/or sustained wind), scrambling/climbing plants may incur problems with the water supply through their long, narrow stems. Haberlandt (1894) pointed out that hydathodes and their analogues may also absorb water from the atmosphere. This intake of water by organs other than roots may be of some significance in order to avoid irreversible desiccation of the leaves.
Plant trichomes are among the primary recipients of numerous abiotic and biotic environmental influences (Payne, 1978) and display considerable variation in morphology and spatial arrangement, often clearly related to function (Uphof, 1962; Werker, 2000). Thus, preserved trichomes and induments may he of particular interest in elucidating (aut-)ecological aspects of scrambling/climbing pteridosperms (Kerp, 1990; Krings, 1997; Krings et al., 2002). One of the most interesting examples the tendril-climber Blanzyopteris praedentata from the Stephanian of central France. Fronds and compound tendrils of this species produced several types of trichomes, two of which may have functioned in the defense against phytophagous arthropods.
Krings et al. (2003) documented dense bands of hooked, nonglandular trichomes on the adaxial side of tendrils, frond- and pinna axes (Figs. 23 & 24). Such trichomes are also known from extant plants where they create mechanical obstacles (Renner, 1909; Levin, 1973; Pillemer & Tingey, 1976). Other trichomes that are glandular (Figs. 25 & 26) occur on most parts of the foliage and tendrils and may represent a different defense mechanism against herbivores. These trichomes apparently opened a secretion-filled cell when touched. Krings et al. (2002, 2003) interpret the glandular trichomes of B. praedentata as being functionally similar to the so-called exploding trichomes (Explosionshaare sensu Zimmermann, 1922) of certain extant Cucurbitaceae (e.g., Momordica anigosantha, M. charantia, Sicana odorifera; Zimmermann, 1922; Inamdar et al., 1990) and Solanaceae (e.g., Solanum berthaultii, S. polyadenium. S. tarijense; Gibson, 1971). When touched and ruptured by a small arthropod (e.g., an aphid), "exploding" trichomes rapidly release a sticky exudate that precipitates on the arthropod's legs. A gradual accumulation of exudate on the limbs eventually disables the arthropod (Zimmermann, 1922; Levin, 1973; Kellogg et al., 2002).
[FIGURE 23-26 OMITTED]
Although these finds suggest that trichomes may have played an important role in pteridosperm adaptation, it is almost impossible to elucidate whether the presence of certain trichome types or distribution patterns of trichomes is somehow related to (or was particularly advantageous for) the liana-like growth. However, Hegarty et al. (1991) hypothesize that vines as a life-form may have evolved higher levels of protection from herbivores than those of trees, based on a surface:volume ratio that is much higher in vines than in trees, because of a greater leaf biomass and leaf area when plotted against total biomass. In addition, evergreen climbers in warm and humid climates may possess a larger proportion of young stems and leaves that may require special protection during their development (e.g., from excessive transpiration, UV-B radiation, infestation by microorganisms or phytophagous animals). It therefore seems at least possible that the abundance of nonglandular and glandular trichomes in B. praedentata was correlated with the liana-like growth (Krings & Kerp, 1999; Krings et al., 2002).
Endogenous secretion (i.e., elimination of secondary metabolites into intercellular spaces) occurs in the form of secretory cavities (e.g., Fig. 27) in pinna axes and pinnules of a number of scrambling/climbing late Paleozoic pteridosperms (Stidd & Phillips, 1973; Rothwell, 1975; Krings, 1997, 2000; Krings & Kerp, 2000). Although it is difficult to assign a specific (secondary) ecological function to the endogenous secretion of secondary metabolites in fossil plants, the occurrence of secretory cavities among various pteridosperm taxa from the Stephanian of the Blanzy-Montceau Basin (central France) indicates that these structures may have been related to special demands relative to the protection of scrambling/climbing plants. Krings (2000) points out that, among the pteridosperms from Blanzy-Montceau, endogenous secretion occurs more commonly in vine- or liana-like forms than in non-scrambling/climbing species. Thus, the above-mentioned special demands for protection of scrambling/climbing plants (e.g., from infestation by herbivores or phytopathogenic microorganisms) have perhaps been fulfilled in part by endogenous secretion of certain secondary metabolites.
[FIGURE 27 OMITTED]
Heterophylly is a conspicuous feature of many extant climbing plants (Cremers, 1973, 1974; Madison, 1977; Hegarty, 1989; Lee & Richards, 1991) and has also been demonstrated for the hook-scrambling/climbing pteridosperm taxa Mariopteris nervosa from the Westphalian of Great Britain (Kidston, 1925) and Pseudomariopteris busquetii from the Stephanian and Early Permian of France and Germany (Krings & Kerp, 2000; Krings et al., 2001a) and for the tendril-climber Blanzyopteris praedentata from the Stephanian of France (Figs. 28 & 29; Krings & Kerp, 1999). However, the fact that heterophylly may arise by a number of different mechanisms (Steeves & Sussex, 1989) argues against assigning adaptative significance to within-individual variation of frond/leaf size and shape in fossil plants. For example, heterophylly in pteridosperms may be an expression of heteroblastic development in which the smaller fronds may have come from juvenile plants. The occurrence of heteroblastic development in pteridosperms has already been documented by Kerp (1988) for Autunia conferta. On the other hand, heterophylly in pteridosperms may be a mechanism of adaptation. Tropical vines/lianas may experience a wide range of light regimes during growth through the forest in which they seem to be able to adaptate to maintain a positive carbon balance (Holbrook & Putz, 1996). For example, Penalosa (1983) reports increased internode lengths or suppressed leaf expansion as responses to local shade. In addition, in favorable conditions, extant tropical vines/lianas may produce specialized shoots (Sucher sensu Goebel, 1923) that elongate rather quickly during their "search" for suitable support media (i.e., exploratory growth) and are often characterized by small and/or undifferentiated leaves (Raciborski, 1900; Goebel, 1923). As growth continues, larger and/or more fully differentiated leaves develop once suitable support is achieved. Thus, heterophylly in pteridosperms could also indicate that these plants were capable of adaptating frond morphology to environmental changes or that they produced specialized exploring shoots with small, undifferentiated fronds, perhaps in order to transition from one support tree into the foliage of a neighboring tree.
[FIGURE 28-29 OMITTED]
V. Scrambling/Climbing Pteridosperms in Coal-Swamp Forest Ecosystems
Scrambling and climbing plants are a main feature of contemporary subtropical and tropical forest ecosystems. Although the vine/liana biomass in tropical forests generally represents less than 10% of total forest biomass, up to 40% of the foliage in a forest can consist of vine/liana leaves (Putz, 1984). The role of scrambling and climbing plants in the ecology of tropical forests is considerable. For example, vines/lianas compete with the trees for light, nutrients, and water (e.g., Schenck, 1892; Putz, 1980). There is circumstantial evidence for damage to trees due to attachment of vines/lianas (cf. Putz, 1984; Stevens, 1987). On the other hand, vines/lianas help to close the canopy, tie the tree crowns together, and thus facilitate the movements of (small) animals (Jacobs, 1976). Based on the significant role vines and lianas play in contemporary tropical forest ecosystems, it is possible to envisage that scrambling and/or climbing late Paleozoic pteridosperms perhaps had a similar role within Late Carboniferous-Early Permian coal-swamp forest ecosystems. Potonie (1898: 217-218) suggested that, in the late Paleozoic, scrambling and climbing pteridosperms and pteridophytes have played the same role angiospermous lianas play in contemporary forest ecosystems and may thus have contributed significantly to the tropical character of the Late Carboniferous-Early Permian coal-swamp forests.
Evidence for vine- or liana-like growth forms among pteridosperms can be documented throughout the late Paleozoic. Based on the information accumulated today, the number of taxa with scrambling/climbing growth habits appears to have increased during the Stephanian (Kerp & Krings, 1998). The ability to reach an optimal position in the vegetation (i.e., where sufficient light and free space are available) with a large, photosynthetically active surface area (often in a rather short growing time), but without producing much stem biomass, is the decisive advantage of the scrambling/climbing growth (Darwin, 1867; Schenck, 1892; Gessner, 1956; Lerch, 1991). In the extensive and stable Westphalian coal-swamp ecosystems, which had a rather open canopy, this advantage was utilized by only a few taxa since the selective pressure to develop specialized growth forms was less important. The Stephanian coal-swamp forests, however, covered smaller areas and were more dynamic ecosystems. Moreover, for the first time in geological history, larger tree ferns formed a relatively closed canopy (DiMichele & Hook, 1992). These two factors, more dynamic Stephanian coal-swamp forest ecosystems and relatively closed canopies, may have positively influenced the radiation of scrambling/ climbing forms among pteridosperms. Forest edges and temporary openings (e.g., tree-fall gaps) are preferred habitats of growth for vines/lianas because they provide sufficient light and ample suitable supports that enable quick ascent (in)to the canopy (Putz, 1984). The more dynamic Stephanian ecosystems certainly produced numerous such openings and thus may have provided ideal niches for scrambling and/or climbing taxa. Krings et al. (2001a) hypothesize that especially the mariopterid pteridosperms, based on their local abundance in the fossil record (e.g., DiMichele et al., 1984; Kerp & Fichter, 1985), locally may have contributed considerably to the structural complexity of the late Paleozoic coal-swamp forest vegetation. These forms appear to represent part of a rather vigorously growing, sprawling, scrambling, and/or climbing type of vegetation that may be structurally comparable to the vegetation that can be found prevailing at edges or in disturbed areas and tree-fall gaps of contemporary temperate and tropical forest ecosystems.
VI. Concluding Remarks
Scrambling/climbing growth habits have been demonstrated for a relatively large number of impression/compression pteridosperm taxa. The data here compiled underscore that these scrambling/climbing late Paleozoic pteridosperms developed a variety of modes of attachment, including climber hooks, leaflet tendrils, tendrils terminating in adhesive pads, and adventitious roots. These reports, and an increasing record of vine- to liana-like growth habits among permineralized taxa from localities elsewhere in Europe, Asia, and North America (e.g., Pigg et al., 1987; Robler, 2000), show that scrambling and/or climbing growth habits were much more widely distributed in this group of gymnospermous plants than commonly thought. Moreover, they demonstrate that the modes of attachment developed by pteridosperms in the late Paleozoic are very similar and perhaps analogous to those seen in extant angiosperms (cf. Schenck, 1892). In addition, the presence of various stem and foliage modifications and the various epidermal adaptations, along with the special physiological requirements of a scrambling/climbing growth form, underscore the fact that, in this group of ancient seed plants, the underlying genetic and developmental processes associated with growth form and physiological necessity were already in place approximately 300 million years ago. Pteridosperms, a once widespread but now extinct group of gymnospermous plants, presumably evolved vine- to liana-like lifeforms in the Early Carboniferous.
XI. Appendix 1: Demographic information, endangerment, and geographical distribution of cactus species with different life-forms. Pattern of distribution: 1 = random, 2 = regular, 3 = clumped; Recruitment: 1 = under the canopy of trees and shrubs, 2 = in spaces deprived of vegetation; Survivorship: 1 = mortality is higher in the last size/age categories than in the first ones, 2 = mortality remains constant with age/size, 3 = mortality is higher in the first age/size categories than in later ones; Fecundity: 1 = fecundity increases with size/age, 2 = fecundity increases until a certain size/age and then remains constant, 3 = fecundity increases with size/age, reaching a maximum, then decreases; Size structure: 1 = number of individuals decreases with size/age, 2 = number of individuals increases with size/age, 3 = number of individuals varies with size/age; Population dynamics: 1 = species analyzed with matrix models; CITES: I = species listed in appendix I; IUCN: e = endangered, v = vulnerable, r = rare, i = indeterminate; Geographical distribution: N = North America, S = South America, * = endemic to one country of the geographical region. Pattern of Recruit- Survivor- Species distribution ment ship Globose cacti Ancistrocactus tobuschii Ariocarpus trigonus Coryphantha robbin- sorum Mummillaria crucigera M. gaumeri 3 1 2/3 M. magnimamma 3 1 3 M. microcarpa 3 1 Neolloydia 3 2 pseudopectinata Barrel cacti Copiapoa cinerea 1/3 Echinocactus horizonthalonius Echinocereus 3 1 engelmanni E. triglochidiatus Echinomastus erectrocentrus Ferocactus 1 acanthodes F. cylindraceus F. histrix F. wislizeni Columnar cacti Carnegiea gigantea 3 1 3 Cephalocereus columnatrajani Escontria chiotilla Lophocereus schottii Neobuxbaumia macro- 1 3 cephala N. tetetzo 3 1 3 Pachycereus pringlei 1 Stenocereus thurberi Trichocereus pasacana 3 1 Opuntioid cacti Opuntia echios 3 O. rastrera 1 3 Size Population Species Fecundity structure dynamics Globose cacti Ancistrocactus 1 tobuschii Ariocarpus trigonus 3 Coryphantha robbin- 1 1 sorum Mummillaria 1 3 1 crucigera M. gaumeri M. magnimamma 1 3 1 M. microcarpa Neolloydia pseudopectinata Barrel cacti Copiapoa cinerea 1 Echinocactus 3 horizonthalonius Echinocereus engelmanni E. triglochidiatus 3 Echinomastus erectrocentrus Ferocactus 1 acanthodes F. cylindraceus 3 F. histrix 1 F. wislizeni 3 Columnar cacti Carnegiea gigantea 1 3 1 Cephalocereus 3 columnatrajani Escontria chiotilla 1 3 1 Lophocereus schottii 1 3 Neobuxbaumia macro- 1 3 1 cephala N. tetetzo 1 1 1 Pachycereus pringlei Stenocereus thurberi 1 Trichocereus pasacana 3 Opuntioid cacti Opuntia echios 1/3 O. rastrera 1 3 1 Endangered species Geographical Species CITES IUCN distribution Globose cacti Ancistrocactus I N * tobuschii Ariocarpus trigonus I v N * Coryphantha robbin- e N sorum Mummillaria r N * crucigera M. gaumeri N * M. magnimamma N * M. microcarpa N * Neolloydia I N * pseudopectinata Barrel cacti Copiapoa cinerea v S Echinocactus v N horizonthalonius Echinocereus e N engelmanni E. triglochidiatus i N Echinomastus e N erectrocentrus Ferocactus N acanthodes F. cylindraceus N F. histrix N * F. wislizeni N Columnar cacti Carnegiea gigantea N Cephalocereus columnatrajani N * Escontria chiotilla N * Lophocereus schottii N Neobuxbaumia macro- i N * cephala N. tetetzo N * Pachycereus pringlei N * Stenocereus thurberi N Trichocereus pasacana e S Opuntioid cacti Opuntia echios i S * O. rastrera N * Species Source Globose cacti Ancistrocactus Lockwood, 1995 tobuschii Ariocarpus trigonus Martinez et al., 1993 Coryphantha robbin- Schmalzel et al., 1995 sorum Mummillaria Contreras & Valverde, 2002 crucigera M. gaumeri Leitrana-Alcocer & Parra-Tabla, 1999 M. magnimamma Valverde et al., 1999; Ruedas et al., 2000; Valverde et al., in press M. microcarpa McAuliffe, 1984b Neolloydia Martinez et al., 1994 pseudopectinata Barrel cacti Copiapoa cinerea Gulmon et al., 1979 Echinocactus Reid et al., 1983 horizonthalonius Echinocereus McAuliffe, 19846 engelmanni E. triglochidiatus Reid et al., 1983 Echinomastus Johnson, 1992; Johnson et erectrocentrus al., 1992 Ferocactus Jordan & Nobel, 1981; acanthodes Franco & Nobel, 1989 F. cylindraceus Bowers, 19976 F. histrix Huerta & Escobar, 1998 F. wislizeni Reid et al., 1983 Columnar cacti Carnegiea gigantea Niering et al., 1963; Turner et al., 1966; Steenbergh & Lowe, 1969; Hutto et al., 1986; Franco & Nobel, 1989; Parker, 1993; Silvertown et al., 1993; Pierson & Turner, 1998 Cephalocereus Zavala-Hurtado & Diaz- columnatrajani Solis, 1995 Escontria chiotilla Ortega-Baes, 2001 Lophocereus schottii Parker, 1989 Neobuxbaumia macro- Esparza-Olguin et al., 2002 cephala N. tetetzo Valiente-Banuet & Ezcurra, 1991; Valiente-Banuet et al., 1991a, 1991b; Godinez-Alvarez et al., 1999 Pachycereus pringlei Carrillo-Garcia et al., 2000 Stenocereus thurberi Parker, 1987, 1993 Trichocereus pasacana De Viana, 1996-1997 Opuntioid cacti Opuntia echios Hicks & Mauchamp, 2000; Hamann, 2001 O. rastrera Mandujano et al., 1998, 2001
Parts of the original research here referred to were supported by the German Science Foundation (DFG grants Ke 584/2-1 and 584/2-2 to H.K. and DFG Habilitation scholarship KR 2125/1-1 to M.K.), the Alexander von Humboldt Foundation(Feodor Lynen Research Fellowship to M.K.), and the National Science Foundation (NSF grant OPP-9614847 to E.L.T. and T.N.T.). The authors thank all colleagues who contributed material, in particular Dr. Manfred Barthel and Dr. Stephan Schultka (Berlin, Germany), Dr. W. A. DiMichele (Washington, DC, U.S.A.), Dr. J. Langiaux (Gourdon, France), Dr. G. Pacaud (formerly Autun, France), Dr. A. Prieur (Lyon, Villeurbanne, France), A. and H. Stapf (Nierstein, Germany), and Dr. J. van der Burgh (Utrecht, Netherlands).
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MICHAEL KRINGS, (1) HANS KERP, (2) THOMAS N. TAYLOR, (3) AND EDITH L. TAYLOR (3)
(1) Bayersche Staatssammlung fur Palaontologie and Geologie-Palaontologischen Institut
D-80333 Munich, Germany
(2) Forschungsstelle fur Palaobotanik am Geologisch-Palaontologischen Institut
Westfalische Wilhelms-Universitat Munster
(3) Department of Ecology and Evolutionary Biology
and Natural History Museum and Biodiversity Research Center
University of Kansas
Lawrence, KS 66045-7534, U.S.A.
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|Author:||Krings, Michael; Kerp, Hans; Taylor, Thomas N.; Taylor, Edith L.|
|Publication:||The Botanical Review|
|Date:||Apr 1, 2003|
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