What is a leaf? An online tutorial and tests.
A leaf is a fundamental unit in botany. Understanding what constitutes an individual leaf is essential when reading the botanical literature, attempting to identify plants and communicating botanical information with other people. It is an important understanding at many levels of endeavour--from high school and university biology students keying out unknown plant specimens, to a restoration worker recognising which plants to spray with herbicide, or a farmer describing a plant they suspect of poisoning stock. I had assumed that my plant identification students, after a standard introductory leaf morphology practical session, would be able to consistently and confidently recognise and describe characters such as leaf arrangement (alternate, opposite of whorled), and leaf simple or compound (trifoliolate, palmate, pinnate, bipinnate or tripinnate).
The problem In many identification keys some of the first (and thus most important) encountered questions are:
i) leaves simple or compound?, and
ii) if compound how was the blade or lamina divided?
I noticed that many students were frequently making incorrect choices in these initial questions, before getting to the potentially even more difficult questions pertaining to the petals, anthers and ovaries. It appeared the students had a very specific form of 'plant blindness' (Wandersee & Schussler 1999) in that they could see the leaves but frequently did not perceive their correct arrangement and/ or construction. To determine where the misunderstandings were arising I prepared a simple test. I set up ten stations around the laboratory each with plant specimens, with a wide range of leaf morphologies. The students were supplied with a sheet of paper on which they were required to circle the correct alternative for each specimen for:
* leaves: alternate, opposite or whorled
* leaves: simple, trifoliolate, palmate, pinnate, bipinnate or tripinnate.
The students obtained reasonably satisfactory results when assessing leaf arrangement, with the greatest confusion arising with specimens with a basal rosette of alternate leaves that were frequently considered whorled and with species with numerous small closely-arranged alternate leaves that were often judged to be opposite of whorled. Determining if leaves were simple or compound was generally handled well, but if leaves were compound less than half of the responses correctly identified the type of division.
Clearly there was a problem in recognising what an individual leaf was and in assessing fundamental variations in leaf structure and arrangement. Showing students a wider range of material in the introductory session would help, but in many parts of the world this is not always possible due to cold or dry season deciduousness. In addition, at Charles Sturt University we have a large number of distance education students. It is an advantage for them if they can undertake preparatory tasks before coming to the short but intensive residential schools.
The online tutorial
In response to the various challenges outlined above I prepared an online leaf morphology tutorial and associated tests. Leaves of approximately 80 species were scanned with a standard flatbed scanner. Species were selected for showing a wide range and different combinations of the various characters and should have world-wide applicability. When scanning it is useful to make sure that both upper and lower leaf surfaces are evident (Figure 1) and it is useful to include a scale (Figures 1-5). The tutorial (http://www.csu.edu.au/ herbarium/H RT202/intro/intro.htm)is divided into three main sections. In each section there are explanations, excellent line drawings from Name that flower by Ian Clarke and Helen Lee (permission to use these has been obtained and is acknowledged on the web site) and an extensive array of scanned leaf specimens.
The first section examines what a leaf is, based on the presence or absence of an axillary bud. This section shows that individual leaves and the leaflets of a single leaf can appear very similar and the location of the axillary buds is needed to determine which is which (Figure 1 ). Also illustrated is that axillary buds can develop into branches and this can potentially complicate the shoot's appearance, especially if the branches are short with numerous leaves (Figure 2).
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The second section looks at leaf arrangement (alternate, opposite or whorled) and illustrates an area that often causes problems--the difference between leaves whorled and leaves in a basal rosette (where the internodes are short and the leaves are usually alternate) (Figure 3).
As mentioned, once a student can recognise what a leaf is they can usually tell the difference between a simple and compound leaf, but can have difficulty determining the type of compound leaf. Again line drawings and numerous scans are provided to help distinguish between trifoliolate, palmate, pinnate, bipinnate and tripinnate leaves (Fig. 4). I had always used the term 'trifoliate' to describe clover, medic, etc. leaves, but the research for this project showed the correct term is 'trifoliolate'. Surveys of many texts, botanical dictionaries and web sites revealed that the term 'trifoliate' is often misapplied.
Critical components of my objective of improving leaf morphology recognition were two tests of 10 and 12 species respectively. Images of these 22 species were not used in the tutorial and thus students are required to apply the knowledge they gained from the tutorial to a new set of information. The tests were constructed in ToolBook (SumTotal Systems), an e-learning development application that allows for interactive content. Upon answering each question a correct/ incorrect response is given that is supplemented with additional explanatory text information (Figure 5). One the main benefits of the ToolBook-style of test is that after an answer is given additional explanatory labelling can be superimposed on the image (Figure 5).
I have conducted several evaluations of the tutorial and tests. In one evaluation in the first hour of the first practical session of a 13-week plant identification subject I randomly split the class into two groups. These students had had no previous botany-specific training. One group independently went through the tutorial and tests using internet-ready laptops, while I gave the other group my 'standard' leaf morphology explanation and demonstration session. The students were then tested as above. Both groups obtained similarly positive results, which shows the effectiveness of the application, especially for distance education students or for a pre-practical exercise for on-campus students.
There are various web sites with leaf morphology line drawings and/or images but usually there is only a single image of drawing to illustrate each feature. To the best of my knowledge the present application is the only online tutorial of leaf morphology where numerous samples are used to illustrate each characteristic. I am reasonably certain that the leaf morphology test is unique. Stern (2006) mentions an online leaf morphology tutorial on the Peabody Museum (Yale University) web site but this seems very different to the application described here.
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This approach is readily adaptable to a wide range of other topics. I am currently developing a gynoecium web page that will examine the problem areas of ovary superior/inferior, number of carpels, number of loculi, etc.
I wish to thank Kylie Kent for the web page design and Scott Black for the ToolBook programming.
Clarke, I. and Lee, H. (2003). Name that flower. Second Edition. Melbourne University Press.
Stern, D. (2006). Yale leaf morphology digitization and network project. Science and Technology Libraries 26:137-155.
Wandersee, J.H. and Schussler, E.E. (1999). Preventing plant blindness. American Biology Teacher 61:84-86.
Dr Geoff Burrows is a Senior Lecturer at the School of Agriculture and Veterinary Science, Charles Sturt University, NSW.
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|Title Annotation:||hands on|
|Date:||Jun 1, 2008|
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