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Hydroballochory in two Texas species of skullcap (Scutellaria Drummodii, S. Wrightii; Lamiaceae).

Abstract.--Scutellaria drummondii var. edwardsiana (Drummond's skullcap) and S. wrightii (Wright's skullcap) use hydroballochory to disperse nutlets following rains. The nutlets form in a fruiting calyx called a scutellum that expands in size and changes in color from green to yellow and finally brown. The scutellum has a cup-shaped upper portion and a scale-like lower portion. When a drop of rain hits the cup-shaped top of a yellow or brown scutellum, it dehisces and falls off the plant and the resulting mechanical energy from this event causes the scales to throw the nutlets away from the plant. Based on field and greenhouse experiments scutella were observed to disperse following precipitation events.

Although widespread among fungi, lichens, and bryophytes, fruit and/or seed dispersal mechanisms operated by rain (ombrohydrochory) are not common among angiosperms as a whole (Brodie 1952; Van der Pijl 1982; Pizo & Morellato 2002) but have been found in several angiosperm genera (Parolin 2006). Two general mechanisms are involved in fruit and/or seed dispersal by rains: the splash-cup or rain ballists (hydroballochory, Brodie 1951; Pizo & Morelanto 2002; Parolin 2006) and springboard or catapult (Brodie 1955). In splash-cup seed dispersal, raindrops are caught by cup-shaped capsules, and the seeds in the capsule are dispersed by the splashing water. In the springboard or catapult mechanism, the fruit or persistent calyx tube (or gemmae in the case of Kalanchoe tubiflora; Brodie 1955) is attached to the stem by a resilient pedicel, which is bent downward by falling raindrops and, as it recoils upward, seeds are dispersed.

The genus Scutellaria (skullcap) is named from the Latin scutella, a small dish or shield, in allusion to a dish-like protrusion of the calyx (Diggs et al. 1999). In this paper, hydroballochory utilizing the specialized calyx structure (the scutellum and under lip) is discussed. This dispersal mechanism, found in the genus Scutellaria (Lamiaceae) and described for S. altissima from Europe (Nordhagen 1936; Van Der Pijl 1982; Leins 2000), is examined in two southwestern species of North American skullcap (S. drummondii Benth. var. edwardsiana B. L. Turner and S. wrightii A. Gray) that are widespread in north central Texas (Diggs et al. 1999).

Nordhagen (1936) observed that the scutellum received a shock from rain drops causing it to be thrown off the under lip releasing the fruits called nutlets. The structures involved were photographed and explained by Leins (2000). Collins (1976) described calyx development in eastern species of Scutellaria. In these species, the calyx body expands to about 3-5 times its size at anthesis while the scutellum expands disproportionately more, becoming 4-6 times its original size at anthesis. Typically four nutlets develop per calyx and time of nutlet maturation varies from 3-6 weeks in spring-blooming species to 2-4 weeks for fall-bloomers. Dehiscence of the fruiting calyx allows the release of the mature nutlets from the persistent scale (Collins 1976).

Van Der Pij1 (1982) used the Nordhagen (1936) description and illustrations in his book on fruit and seed dispersal but termed the underlip a kettle. In addition to the term "kettle", Miller (2001), when describing S. integrifolia, introduced the term "spoon" for the underlip. Correll & Johnston (1970) describe dehiscence of the scutellum and refer to the underlip, kettle, or spoon as a scale. This terminology is preferable because scale is commonly used in botanical descriptions.

The objectives of this study were to test hypotheses of hydroballochory in two common species of Scutellaria using quantitative data and qualitatively examine fruit maturation. Misconceptions regarding dispersal and anatomy of the scutellum are discussed.

MATERIALS AND METHODS

Scutellaria drummondii var. edwardsiana (Drummond's skull-cap), an annual species, was observed in fruit before and after a heavy rain on 24 May 2009 and again on 22 May 2010. On 22 May 2010, 40 plants were flagged and mature scutella counted before and after a heavy rain (Table 1).
Table 1. Mature scutella of 40 Drummond's skullcap prior to and
after a precipitation event during the field experiment.

Scutella  Scutella  Scutella  Scutella  Scutella  Scuteila
prior to    after   prior to    after   prior to    alter
event       event     event     event    event      event

9                3         3         0        40        11

12               4        11         0         3         0

5                3                   0         6         0

8                0        13         2         5         1

41              18         6         4        25         3

6                1         5         2         5         0

7                2         5         0         6         0

4                1        11         1         4         0

14               2         5         0        22         2

26               8         5         1         5         1

10               6         6         4         5         1

17               0                   0         3         0

8                1         7         0        20         3

37               6


In addition to these field observations, two live specimens of S. wrightii (Wright's skullcap), a perennial, were collected on 25 May 2009. Upon examination of the fruiting calyces, only green scutella and scales from scutella with fruits that had likely dispersed after the aforementioned rain were observed on these collected plants. The two plants were placed in pots and monitored daily for scutellar development. During precipitation events, pots were taken from the growing area and placed outside, directly in the rain (Table 2). After the precipitation event, the plants were placed back in the greenhouse and carefully examined for dispersed scutella.
Table 2. Precipitation events and number of mature seuiclla prior to
and after each rain during the greenhouse experiment using Wright's
skullcap.

Precipitation event  Scutella prior to event  Scutella after event

7/21/09                                    3                     0
7/26/09                                   21                    14
7/27/09                                   15                    12
7/28/09                                   11                     8
7/29/09                                    8                     7
7/30/09                                    7                     6
7/31/09                                    6                     5
8/1/09                                     5                     5
8/2/09                                     5                     3


To test the hypothesis that rainfall played a significant role in fruit dispersal, statistical analyses were conducted using SigmaPlot 11 (Systat Software, Inc.). Numbers of scutella were counted on individual plants and these data analyzed for significant differences between pre- and post-precipitation events in the field and greenhouse study. A Mann Whitney Rank Sum test was conducted on the field data and a paired t-test was used for the greenhouse data.

In addition to testing hypotheses regarding fruit dispersal, scutella were classified as green, yellow, brown, or aborted based on size and color (Table 3). Aborted scutella could be recognized by their small size, which is about one fourth that of a mature scutellum. Mature scutella were large and began as a green color, turned yellow, and eventually became brown in color. Greenhouse plants were observed from 30 June to 3 August, 2009. Scutellar classifications began on 21 July and ended on 3 August, 2009.
Table 3. Classification of Wright's skullcap scutella during
precipitation events of the Greenhouse experiment.

Precipitation               Sculellar     Categories
Event                     Classification

               Aborted      Green         Yellow    Brown

7/21/09              3              23           0      3

7/26/09              7               1          17      4

7/27/09              7               0          11      4

7/28/09             II               0           7      1

7/29/09             11               0           s      1

7/30/09             11               0           5      1

7/31/09             11               0           4      1

8/1/09              11               0           4      1

8/2/09              11               0           3      0


Information presented in Texas floras and field guides was evaluated for accuracy regarding fruit dispersal mechanisms in the two species. Discrepancies of terminology, morphology, and function were noted in some of these works and are discussed within this manuscript.

Voucher specimens for S. drummondii var. edwardsiana (TAC 4373) and S. wrightii (TAC 4374) are deposited at the Tarleton State University Herbarium (TAC). Localities of collections are the Goetze property in Mills County (31.56080N, 98.68190W) for Drummond's skullcap and the Tarleton State University Agricultural Center (32.24820N, 98.20990W) for Wright's skullcap.

RESULTS

Prior to a heavy rain that occurred on 24 May 2009, specimens of Drummond's skullcap were being collected for a floristic study. While collecting specimens, numerous mature scutella were observed and when pressed, fruits were expelled into specimen papers. About 75-100 scutella were observed in the population of plants. Following the heavy rain, the plants were observed to have only scales present, indicating that fruit dispersal had occurred. During the following field season on 22 May 2010, the 40 flagged plants were observed to have dispersed 60 scutella (Table 1) and the numbers of scutella prior to and after the precipitation event were significantly different (P = <0.001).

In the greenhouse study using Wright's skullcap, 34 scutella were found to have dehisced leaving scales on the plants and scutella on the surface of the potting soil following nine precipitation events (Table 2). There was a significant difference prior to and after the precipitation events (P = 0.009).

Aborted scutella numbers remained the same after 28 July 2009. Scutella classified as aborted remained immature, were about one-fourth the size of mature scutella, and contained no nutlets. Other scutella quickly turned from green to yellow and more slowly turned brown in color (Table 3) and only yellow or brown scutella were observed dehiscing. Similar patterns of scutellar development were observed in the field study of Wright's skullcap.

DISCUSSION

Prior to these observations, dispersal by hydroballochory had only been described for Scutellaria altissima, which is only known from Massachusetts in North America. The observations from this investigation indicate that hydroballochory also occurs in Wright's and Drummond's skullcap. Although there are several qualitative descriptions of rain drops causing scutellar dispersal in S. altissima (Nordhagen 1936; Van Der Pij1 1982; Leins 2000), this analysis represents the only quantitative data available that indicates that scutella are dispersed after rain.

Following pollination and subsequent fertilization, scutella are green and enlarge in size, turn yellow, and if are not dispersed, eventually turn brown. Yellow scutella will disperse but, based on mechanical manipulation with a pencil, require more force to dehisce than brown scutella. Green scutella turn yellow relatively quickly, but the brown coloring, likely resulting from drying and dehydration, found in the more commonly dispersed scutella occurs more slowly. Immature scutella likely formed from flowers that were unpollinated. At the end of the experiment, no fruits were found in the 11 immature scutella that persisted and none had dehisced. All 11 of the immature scutella turned brown and five of the 11 had been bent downward from the force of the rain but did not dehisce. Fruits were observed in all the yellow and brown scutella during the course of the experiment.

After observing the plants, it appears that the scutella swell after fertilization and turn from green to yellow to brown. Once the scutella are yellow or brown, they may dehisce. When a raindrop hits the cup-shaped surface of the scutellum it inverts and falls off the scale and the mechanical force of this event causes the scale to throw the mature fruits. The mature fruits or nutlets are then likely carried by water currents some distance from the plant depending on the amount of run-off occurring during the precipitation event. There were significant differences between the numbers of scutella prior to and after precipitation events.

Even though this dispersal mechanism was described over 70 years ago (Nordhagen 1936), there is confusion regarding dispersal mechanisms in these widespread Texas plants. Some floras and field guides only mention the scutellum or crest as an identification character without any indication of its function (Loughmiller & Loughmiller 1992; Diggs et al. 1999; Tull & Miller 1999; Loughmiller et al. 2006; Nieland & Finley 2009). Correll and Johnston (1970) describe dehiscence accurately but do not mention rainfall as a mechanism for fruit dispersal.

Others have not described the morphology accurately. Niehaus et al. (1984) stated that the seeds of Scutellaria resemble skullcaps. The seeds are within oval-shaped nutlets enclosed by the scutellum and scale and the skullcap-shaped structure would correctly be identified as the scutellum.

Ajilvsgi (1991) and Tveten & Tveten (1993) stated that the vernacular and scientific names of the genus Scutellaria are derived from the small cap-like structure which covers the seed during the fruiting period and remains on the plant long after the seed is gone.The scutellum covers the fruit and is not persistent once dispersal occurs. Only the scale is persistent, not the cap-like scutellum.

Bowers et al. (2009) indicate that each tube-shaped flower has a fuzzy bulbous upper petal (lip) that has a cap shape that resembles a monk's headgear. This description is erroneous because the petal does not form the scutellum, it is an outgrowth of the calyx.

ACKNOWLEDGEMENTS

We thank Dr. Chris Higgins for advice on statistical analyses and Matthew Nelson for field assistance. We also thank Barney Lipscomb and Bob Lonard for improving the manuscript with thorough reviews.

LITERATURE CITED

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Loughmiller, C., L. Loughmiller & D. Waitt. 2006. Texas Wildflowers. Austin, Texas: University of Texas Press, 278 pp.

Miller, K. E. 2001. Scutellaria integrifolia L. (Hyssop Skullcap) New England Plant Conservation Program Conservation and Research Plan for New England. New England Wild Flower Society, Framingham, Massachusetts, USA. http://www.newfs.org

Niehaus, T. F., C. L. Ripper & V. Savage. 1984. Southwestem and Texas Wildflowers. Boston, MA: Houghton Mifflin Company, 449 pp.

Nieland, L. J. & W. A. Finley. 2009. Lone Star Wildflowers: A Guide to Texas Flowering Plants. Lubbock: Texas Tech University Press, 321 pp.

Nordhagen, R. 1936. Ober dorsiventrale ubd transversal tangentballisten. Svensk Botanisk Tidskrift, 30(3):443-473.

Pizo M. A. & P. C. Morellato. 2002. A new rain-operated seed dispersal mechanism in Bertolonia mosenii (Melastomataceae), a neotropical rainforest herb. Amer. J. Bot., 89(1):169-171.

Parolin, P. 2006. Ombrohydrochory: Rain-operated seed dispersal in plants-with special regard to jet-action dispersal in Aizoaceae. Flora 201(7):511-518.

Tveten, J. & G. Tveten. 1993. Wildflowers of Houston and Southeast Texas. Austin: University of Texas Press, 309 pp.

Tull, D. & Miller G. 0. 1999. Wildflowers, trees, and shrubs of Texas, revised edition.New York: Taylor Trade Publishing, 347 pp.

Van der Pijl, L. 1982. Principles of seed dispersal in higher plants, 3rd ed. Springer-Verlag, Berlin, Germany, 215 pp.

ADN at: nelson@tarletonsedu

Allan D. Nelson & Jim R. Goetze

Department of Biological Sciences, Box T-0100, Tarleton State University, Stephenville, Texas 76402 and Natural Sciences Department, Laredo Community College Laredo, Texas 78040
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Author:Nelson, Allan D.; Goetze, Jim R.
Publication:The Texas Journal of Science
Article Type:Report
Geographic Code:1USA
Date:Nov 1, 2010
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