The amazing design of a moss leaf

Transcription

1 The amazing design of a moss leaf James R. Shevock California Academy of Sciences, Department of Botany, 55 Music Concourse Drive, Golden Gate Park, San Francisco, California USA Have you ever wondered why moss leaves are designed in so many different and intricate ways? Among plants, mosses have many unique structures and features. The design of a moss leaf (technically called a lamina) is simply a remarkable feature of architecture. Moss leaves come in a wide variety of shapes and sizes (Fig. 1) and basically in the majority of mosses the lamina (leaf surface) is only one cell thick. It is indeed remarkable that moss leaves can maintain their shape and form with so little structural support. Under a compound microscope, the cell arrangement of a moss leaf appears like a stained glass window. With careful observation one can begin to understand the relationships between groups of mosses by studying the overall cellular design and architecture of the entire leaf. The cellular arrangement across a moss leaf is not random, but rather, an intricate design and blueprint that is genetically programmed. Understanding and recognizing the different designs and cellular organization occurring in moss leaves will offer insights into relationships among related species, genera, and even families. By closely examining under the compound microscope how a moss leaf is constructed, one can learn to recognize many moss groups quickly. 9

2 Fig. 1. Shape of Leaves. A. Orbicular, concave. B. Ovate-acute. C. Obovate. D. Oval. E. Oblong, mucronate. F. Ligulate. G. Ligulate-apiculate. H. Spatulate. I. Linear. J. Lanceolate, narrowly acute. K. Lanceolate-acuminate base. L. Elongate-triangular. M. Deltoid, base excavate. N. Subulate from an ovate base. O. Falcate. P. Circinate-plicate. Q Cucullate, concave. R. Surface undulate. S. Canaliculate. T. Tubulose. U. Keeled, margin revolute. V. Conduplicate, clasping at the base. W. Ovate-lanceolate, with decurrent auricles at the base, margin serrate. X. Ovate-acuminate, base cordate. Y. Abruptly subulate and divergent from an oblong, clasping base. When we observe a moss (the green plant) we are observing the gametophyte life cycle (haploid stage). Because mosses are generally small plants that lack roots, flowers, and seeds, bryologists have focused on the intricate cellular design of leaves to aid in their identification. And because these structures are tiny, bryologists have developed very specific terminology used to describe these subtle differences. So if you want to learn more about mosses and to name them to species then you have to understand the meaning of these various bryological terms and where on the moss leaf to look for each specific feature. Fortunately, there is a wonderful colored illustrated glossary that has nearly every term used in bryology (Malcolm & Malcolm 2006). In my opinion this book is an essential reference and every bryologist should have one next to their 10

3 microscope. The first step in the examination of a moss leaf under the microscope is to determine how the cells are organized, their size and shape and if the cells are uniform across the leaf surface or if they differ. If they differ, how and where on the leaf are the cells different? The better you are at accurately describing the cellular features of your moss leaf the greater success you will have in using identification keys. Let s now begin a survey of the various features and locations to examine carefully on a moss leaf. Leaf arrangement Perhaps the very first step before even making a microscope slide of your moss leaves is to examine how the leaves are arranged on the stem (Fig. 2). Most leaves are spirally attached to the stem but there are a few distinct exceptions. Leaves that look like a feather formed in two distinct rows is called distichous and some mosses are arranged in one plane as if flattened along the stem. Leaves attached like this are called complanate. Besides the leaf arrangement, it is important to determine if your moss leaves are more or less flat or are they keeled or concave? Is the margin straight or wavy? Do the leaves contort or twist when dry? Are the edges of the leaves incurved or recurved? In some mosses you will see what appears to be longitudinal folds or creases across the lamina. These folds of the leaf are called plications. Another way moss leaves can appear as if they have wrinkles or ripples across the surface. This feature is termed rugose. When you are ready to examine your leaves under the scope, one only needs to hydrate them in a drop of water. Place a small stem on the glass slide and then remove the leaves with fine forceps and ensure that you have some leaves face up and a few face down. Some features are best observed when you can compare between the top (adaxial) and bottom (abaxial) of the moss leaf. In addition, in some mosses the branch leaves and the stem leaves can be different in size or shape. If the leaves on the stem and branches are not the same, how to they differ? You may also see smaller leaf scales or leaf-like projections along the stem. These are likely to be paraphyllia and structures resembling paraphyllia but are clustered around branch buds are called pseudoparaphyllia. You will come across these terms in moss keys so it is always good to determine if your specimen has them. Sometimes to see these features clearly you will need to make a slide of the stem with a few leaves removed. 11

4 Fig. 2. Arrangement of Leaves. A. Erect, appressed or imbricated. B. Julaceous, imbricated. C. Tumid or turgid. D-E. Erect-patent. F. Patent or open. G. Spreading. H. Squarrose-spreading. I. Squarrose-recurved. J. Falcate. K. Complanate. Costa In most mosses, a costa (midrib) is seen. Usually the costa is in the middle of the leaf but in a few mosses it can be off center. In acrocarpous mosses (those that grow in tufts and are erect), the costa is generally easily visable with a hand-lens and is thickened. In pleurocarpous mosses (those that are highly branched and creep over the substrate), the costa is generally faint, weak, not thickened, or in some cases, the costa can be forked, prominent and located near the leaf margin. In other pleurocarpous mosses the costa can be basically absent which is called ecostate, or it is very short and forked near the point of attachment to the stem. Even if you see a faint forked costa at the base of the leaf, you will most likely need to take the ecostate route or choice of a key couplet in order to correctly identify your unknown moss. So the costa is a very important first step in learning more about the mosses and how they are designed. The costa length is also important. Does the costa go half-way up the leaf, less, or more? Does it stop below the apex (subpercurrent), at the apex (percurrent), or continue beyond it forming a hair-point (excurrent)? Is the costa straight or is it flexuose or wavy? Study the costa carefully. Are the cells of the costa uniform in shape and size or are they 12

5 different between the base and tip? Is the costa continuous in length or does the costa disappear part way in the leaf then reappear again near the leaf apex? The costa can also vary greatly in width. In some mosses the costa can fill a large portion of the lamina and in other mosses be only a few cells wide. A leaf cross section will display many different ways the costa is designed and constructed (Fig. 3). Fig. 3. Cross-section costal structures. A. Homogeneous costa of Grimmia. B L. Heterogeneous costae. B. Eucladium verticillatum, costa with medial guides, dorsal and ventral stereid bands. C. Trichostomum tenuirostre, costa with medial guides, dorsal and ventral stereid bands and larger ventral cells. D E. Phascum cuspidatum with subventral guides and large ventral cells. Crumia latifolia, with two rows of ventral guides. G. Crossidium aberrans, with ventral superficial guides bearing costal filaments. H. Plagiomnium medium, with medial guides in several irregular rows and begl, Begleiter cells and a small arc of stereids. I. Mnium marginatum, with medial guides and begl. Begleiter cells and small thick-walled marginal cells, 2 3 stratose. J. 13

6 Paraleucobryum enerve, with a row of adaxial hyalocysts, a row of medium chlorocysts and a row of abaxial hyalocysts. K. Barbula convoluta, with ventral cells vertically elongated. L. Stegonia latifolia with two ventral guides and small dorsal stereid band. Cell shapes Moss cells come in a wide variety of shapes and sizes from small circular to quadrate to elongate (Fig. 4). In some mosses the cell wall can be thickened and in other mosses the cell walls are thin. Are the cells the same shape and size throughout the leaf surface or is there more than one cell shape or size present? Where on the leaf does this change occur? Many keys will have couplets that will ask about the cell width to length ratio for example like cells greater or less than 6:1. Some leaves will have what seems like hundreds of rounded cells that are densely packed and other moss leaves will have much fewer enlarged rectangular shaped cells. Take a few moments and study how the moss leaf you are observing is designed. Understanding clearly how the cells are organized by their shape, size, and locations on the leaf being examined is essential. Careful observation will then make the keying process easier and offer a much higher degree of identification success. Fig. 4. Shapes of leaf cells. A. Quadrate. B. Rectangular. C. Oblong. D. Fusiform. E. Rhomboidal. F. Hexagonal, walls pitted. G. Linear. H. Linear, flexuous or vermicular. I. Quadrate alar cells of Brachytheciastrum collinum. J. Enlarged, inflated alar cells of Brachythecium rivulare. 14

7 Alar region Near the base of the leaf where it is attached to the stem is called the alar region. In some mosses, the cells in the alar region are markedly different compared to the adjacent cells of the leaf. In some mosses, the alar region will have highly enlarged or inflated cells. Some of these alar cells may also be colored. In other mosses the alar region will not be as pronounced or appear very different from adjacent cells. The very base of the leaf along the margin may have a few other features such as auricles that appear as enlarged curved basal lobes of the leaf. Care is required in removing leaves from the stem to ensure you have complete leaves to examine. Take time to understand how the cells in this region appear. Highly enlarged and inflated cells can be up to 100 µm. Leaf margin Another place on the leaf where cells can be very different from adjacent cells is along the leaf margin. If one sees a series of cells along the margin of a leaf that are very different in size and shape compared to adjacent cells toward the costa then this is referred to as a border. Generally, a border is fairly obvious, especially if it is 3 or more rows of cells wide, but some borders can be formed by a single row of usually elongate cells. If a border is present, is it uniform in shape from base to apex or does it change at either end? Also the edge of the moss leaf can be smooth or the margin can be variously toothed from serrulate to dentate. Some mosses have prominent teeth-like cells located along the margin. If your moss leaf margin is not smooth throughout, where does it differ? Are the margins the same from the base to apex or are the serrations (or teeth) from mid-leaf to apex or only around the apex? Are the cells the same size throughout of do they change in shape or size? Again, careful examination of the features present is critical in making the correct choice when using identification keys. Some mosses have what appear to be thickened margins and by doing a leaf crosssection one can see if the border is made up of two or more cells. A leaf margin or border can be unistratose, bistratose or multistratose. This can only be determined for sure by making a leaf cross-section. As a general rule you are more likely going to need to make cross-sections for acrocarpous mosses compared to pleurocarpous ones. For some families all members will require a cross section for proper identification. For example, the Dicranaceae, Grimmiaceae, Leucobryaceae and Pottiaceae are four families where cross-sections are basically essential. If the key asks if the margins or entire leaf surface is bistratose, you will need to make a cross-section of the leaf to make the correct choice. 15

8 Leaf apex Sometimes the cells near the apex can differ too. In many mosses it is here where the costa disappears. In some mosses, there is an abrupt constriction near the apex that elongates the leaf at this location. This area of the leaf is called the acumen and it generally will have concave margins too. There are many ways the apex can be different. In some mosses, the costa reaches the apex or extends beyond it. The costa extension is generally referred as a hair-point or arista. Hair-points can be filamentous of a series of single cells or they can be thickened and rather stiff. Hair-points can also be smooth to highly spinose. Juxtacostal area In many mosses the region between the leaf margin and the costa can be very informative. Are the cells here fairly uniform in size or shape or are they similar but not identical? Do they differ from the base and alar region or the apex? Some keys will ask if the cells near the costa are uniform or not. Many cells in this region of the leaf can look irregularly shaped. The cell length or the cell width can vary markedly too. In many mosses the juxtacostal region can differ greatly from the leaf base to the apex. Cell walls One of the features of moss cells is to determine if there is any type of ornamentation or various types of markings on the cell wall. So in identification keys, you will be asked to determine if the cells are smooth or if they have some type of ornamentation on them (Fig. 5). The most common ornamentation is called papillae. These can look like small volcanoes on the cell wall. There can be a single papilla per cell or it could be several papillae per cell. The papillae can be centrally located or they can be found at either end of the cell. Papillae can also be forked or C shaped. In a few mosses, a distinct single row of papillae are formed on the cell wall. Other cells will be bulging with a nipple-like tip which is called mammillae. One has to examine the leaf surface carefully under the microscope because some of the cells may be smooth in one area of the leaf but be papillose in other regions. In a few groups of pleurocarpous mosses the papillae are hard to see but if one slowly uses the fine focus knob of the compound scope on a portion of a leaf that is rounded under the cover slip the papillae will come into focus on the cell wall if they are present. Always look over more than one leaf to be sure. Sometimes, one can believe papillae are present (generally at the ends of a cell) but this is a place where the ends of two cell cell walls slightly overlap. This term is called prorate. Again, a cross-section will be important 16

9 in determining if the cell walls actually have ornamentations on them. The thickness of the cell wall can also be an important diagnostic feature in some mosses. Fig. 5. Types of cell ornamentations as papillae or cuticular outgrowths of the upper medial cells. A. Multipapillose cells of Trichostomum tenuirostre. B. Multipapillose cells of Tortula cernua. C. Unipapillose cells of Dichodontium pellucidum. D. Bifurcate papillae of Orthotrichum alpestre. E. C-shaped papillae of Syntrichia ruralis. F. Tall hollow papillae with branches, antler-like tips of Syntrichia papillosissima. G. Papillose by the projections of the lower ends of the cells of Philonotis fontana. H. Longitudinal cuticular ridges extending over the surface of the leaf of Dicranoweisia crispula. Gemmae Gemmae are various types of asexual reproductive propagules that can be common on some moss leaves. Some of these gemmae can be spherical while others being rod shaped. Others form bulbils or are tuber-like in shape. Sometimes the gemmae are found in the leaf bases adjacent to the stem, in other mosses they are located directly on the costa, in others near the leaf apex, and in a few mosses are elevated on stocks. Examine your moss carefully to determine if gemmae occur. If they do, identify their shape, color and location on the moss leaf. 17

10 Cell measurements It is common that keys will ask for the various sizes of moss cells and this is nearly always reported in microns symbolized as µm. The only way to accurately measure a moss cell is to have an optical micrometer in the eye piece of your compound scope. This will be a small scale bar with divisions. However, optical micrometers come in many different designs so one will need to calibrate the type of micrometer installed in the eyepiece of the compound scope. Once the calibration is determined (what value one division represents for each objective on your scope), then this information should be recorded and made into a small card and attached to the base of the microscope for easy reference. It is then basic math to measure a cell. One counts the number of divisions seen across the cell, and then based on the objective used (generally 4x, 10x, 20x, 40x), calculates the leaf width and length. For example on my scope, one division at 10x equals mm and at 40x one division represents mm. So the number of divisions observed is then multiplied by the value of a division at the objective being used. A division therefore represents a fraction of a millimeter, and in bryology we reference all cell measurements in microns (µm). When one measures a cell it should be the cell surface (lumen) and only one of the side walls. This accuracy of this measurement can be very important when cells are thick walled. Moss cells, depending on their shape, generally range from 5 30 µm wide and from 5 80 µm long. In some mosses with very long and narrow cells the ratio can be as great as 20 times as long as wide. Identification keys Now that you have examined your moss leaf carefully and know how your moss leaf is designed and built, you are now ready to begin the identification process. Once you have used the key and have come to a possible name, comparing your specimen to an illustration of that species is a good first step. For Taiwan, the recently completed 8 volume Moss Flora of China Editorial Committee, eds is an excellent reference. Be advised, however, that illustrations generally are an artist s interpretation of what was seen under the microscope. Some slight variation may be evident but if your moss deviates markedly in any cell features illustrated across the leaf then you can be fairly confident that it is not the species you have. If the illustration appears to be a good match, then the next best thing is to compare your specimen against a species in the herbarium with that name. Making a microscope slide of a leaf of that species and comparing it to your slide will be the final decision step. If your leaf matches that species in every way, then you have identified your unknown correctly. Again, keep in 18

11 mind that the design of a moss leaf is genetically controlled and not random. Everything has to match perfectly with the species description for you to have named your unknown correctly! As with any other scientific endeavor, one learns through practice. After a while you will recognize many common species quickly, both in the lab and in the field. You may also encounter a specimen that cannot be named in any moss flora. There is always the potential that you have discovered a moss new to science. Making detailed and accurate observations and measurements of moss leaves is the first step to becoming a good bryologist. Acknowledgments Permission to use several of the wonderful illustrations presented in Flowers (1973) throughout this paper was granted by the publisher, Brigham Young University Press, and is greatly appreciated. References FLOWERS, S Mosses: Utah and the West. Brigham Young University Press, Provo, Utah. 56 pp. MALCOLM, B. AND N. MALCOLM Mosses and other bryophytes: an illustrated glossary. Second edition. Micro-Optics Press, Nelson, New Zealand. 336 pp. MOSS FLORA OF CHINA EDITORIAL COMMITTEE, EDS Moss Flora of China, English Version. 8 vols. Beijing and Saint Louis. Vol. 1, 1999; Vol. 2, 2001; Vol. 3, 2003; Vol. 4, 2007; Vol. 5, 2011; Vol. 6, 2002; Vol. 7, 2008; Vol. 8, 苔葉 - 令人驚奇的設計 詹姆士. 西華哥 美國加州科學院 19