The Use of the Microscope in the Study of Mosses

This paper on mosses is not intended to be used for the identification of moss plants but to suggest some of the details in the structure of various parts of the moss plant whose study under the microscope is necessary in order to identify it as to genus and species.

Sphagnum or peat moss and the liverworts, as interesting as the leafy mosses,will not be discussed although the identification of the sphagnums demands very careful use of the microscope and the study of all types of mosses becomes a very interesting and time consuming hobby.

In order to identify any moss, it is necessary to learn many details of its structure. Some of these are macroscopic, and can be seen with the naked eye or with a hand lens. Others are so minute that the use of a compound microscope is essential for their study.

A moss would generally be defined as a small green plant with a leafy stem, growing on moist soil or rocks, the bark of trees or even in water in streams and springs. Their leaves are usually green, but in some mosses growing on rocks, they appear to be nearly black.

Moss stems may be upright and branch at an acute angle or they may be horizontal and branch pinnately, reminding one of a feather.

At certain seasons, mosses produce brownish capsules which may be sessile, or have very short stems or long stems.

In these capsules the spores are developed and the mechanism for their distribution is one of the characteristics of a genus and has been used in the keys for the identification of mosses.

Specimens of moss with capsules are more easily identified than sterile ones.

In the plant world mosses belong to a group known as Bryophytes. They are the simplest plants having stems and leaves.

This group includes the liverworts or hepatics and the sphagnums or peat mosses as well as our leafy mosses.

These bryophytes are placed just above the Thallophytes which means the fungi and algae and just below the pteridophytes which includes the ferns and fern allies.

The bryophytes and pteridophytes have two generations in their life cycle, an asexual one and a sexual one.

Among the bryophytes, the leafy plant is the sexual stage (gametophyte) and bears the sex organs.

The simple little capsule with its bare stem (seta) is the asexual stage (sporophyte) and produces the spores.

Among the pteridophytes, the sexual stage (gametophyte) bearing the sex organs (antheridia and archegonia) is an almost microscopical flat green leaf bearing these organs on its under surface and the asexual stage ( the sporophyte) is the green frond by which we recognize ferns.

To mention a few of the structural details which are made use of in identifying a moss, one might begin with the leafy stem. Observe its cross section under the microscope, note the pattern of the attachment of the leaves and their arrangement on the stem.

Note the shape and size of the cells that compose them and, as noted, the way in which they are attached to the stem. Note also the thickness of the cell walls and whether they are smooth or warted.

Observe whether the cells of the leaf are alike or are of different shapes, sizes, and colors, in different parts of the leaf. Note whether the leaf has a midrib (costa). It may have one, two, or none. If it has a costa, is it shorter than the leaf, or as long as it is, or does it extend beyond the tip as a spine? Does it have parallel ridges of green cells above the costa? How many parallel ridges, and how many cells high are they?

Do the leaves have a border of cells of a different shape? Is the edge smooth or toothed? Is it plane involute or revolute?

Is the leaf only one cell thick, or are there several layers of cells? Is it attached to the stem on a straight line, or are the sides decurrent on the stem? Are there any little threads similar to roots (radicles)on the stem?

The leaves at the top, middle, and base of the stem, or the branches, vary and the student should always choose one from the middle of the stem or branch for study.

These are a few of the things we must know, but there are other details to observe.

Does the leafy plant have both kinds of sex organs on it, or are they on separate plants. These organs (antheridia, male) (archegonia, female), grow in the tufts of leaves at the tip of the stem of the upright moss which bears the seta and capsule at its tip (acrocarpous moss) or on a short branch of a horizontal one (pleurocarpous moss).

If there is a capsule present, we know at once that the archegonia was there as the capsule develops in it, but to find these sex organs, if there is no capsule present, one must separate the terminal leaves and look for them there.

The spermatozoids developingin the antheridia can only be seen under the microscope. They are colorless cells spirally coiled and have two whiplash cilia at the tip by which, after ejection from the antheridia, they can swim in moisture (dew or rain). They are attracted, probably chemically, to the open tube of the archegonium, down which they swim, but only one spermatozoa fertilizes the ripe egg at its base. This fertilized egg, remaining at the base of the archegonium, develops into the sporophyte (capsule and seta). The archegonium remains attached to the gametophyte, obtaining the greater part of its nourishment from it. The spores develop in the capsule, from which they are scattered.

If a spore falls on moist ground or humus, it germinates, sending out small green threads which branch and finally buds appear on them. These buds develop into the gametophyte or leafy plant, thus completing the life cycle.

Asexual reproduction of moss occurs in other ways - a small piece of a leafy stem will grow into another leafy plant if it has proper soil and moisture just as a slip of geranium grows. Even small bits of crushed parts of dried specimens may grow if given proper growing conditions of moisture, temperature, etc.

Some mosses produce groups of cells (gemmae) in cups of leaves at the tips of stems or in a ball at the tip of a leafless stem, or groups of cells, similar to spores of some fungi, develop on the leaf surface. These also can produce new plants.

Certain details about the sporophyte should be noted. As it develops in the archegonium and elongates, the tube of the archegonium is ruptured and its tip is carried up as a covering at the top of the capsule. It is called the calyptra. Its shape and character vary in the different genera. It may be conical, and if split on one side is termed culcullate, or it may be shaped like a beret and may have a long or short beak at the center. This type is said to be mitrate. The material of which the calyptra is composed may be a smooth membrane or may consist of fine silky fibres, running from the tip to the bottom of the cone, as in the hairy cap mosses, or stiff fibres may project from the calyptra at varying angles.

As the spores ripen, the calyptra falls off, exposing the capsule, whose shape varies from cylindrical to short or long elliptical or even cubical or globose, and its surface is usually smooth. At maturity, in a few genera, it splits open regularly, or with irregular fissures, through which the spores escape.

In most genera, however, the falling of the calyptra exposes a cap ( the operculum) at the tip of the capsule, and between this and the rim of the cup of the capsule, there is in some species a more or less elastic ring (the annulus) which may fall away when the operculum does. The pattern of the cells in the annulus is characteristic. The operculum, like the calyptra, may be beaked or not. When it is removed, there is exposed a more or less open space at the end of the capsule. The opening of the capsule does not have a smooth rim but bears a circle of teeth (the peristome). These teeth vary as to size, shape, and sculpturing. The number of teeth varies in different genera from 4 to 64, but they always occur in multiples of 4. These teeth may be short or long, free at the tips, attached to each other at the tips, or to a membranous diaphragm, or to the membrane at the end of a plug of material in the center of the capsule (the columella).

The spores are developed in the cavity between this columella and the outer wall of the capsule. Their distribution is regulated by the opening and closing of the apertures between the teeth, which are hygroscopic. In dry weather they separate, allowing the spores to be carried away by air currents, but they come together when the air is moist, keeping the spores dry.

The teeth, as noted above, vary as to size, arrangement, color, and sculpturing. Each tooth may stand by itself, they may be in pairs, or they may be split for part of their length. They may form in one circle, or two circles, and may have appendages.

To study the capsule in detail, one must use a microscope. The spores are green or brown and usually globose. Their surfaces may be smooth or rough. Their diameter varies from 10 - 20 microns.

The identification of specimens would be easier if, at the beginning of his study, the student would examine carefully some of the books about them. It is important that he should read the introduction carefully and observe the carefully drawn illustrations, which will show the differences in structure that have been mentioned here.

There are many books about the mosses to be found in libraries and after looking them over, the student can decide which book is best for him.

When specimens are collected, they should be kept separate, and records kept which state the place of collection, date and habitat. This information is necessary for the final labels, together with the name of the collector and identifier. Remember to get specimens with fruit, if possible.

One advantage of studying mosses is that specimens may be dried and kept for study, as they regain their size and color if placed in water, especially hot water.

The illustrations accompanying this article are not drawn to any scale and are merely to suggest what the student may find when studying a moss.

Among the more easily understood books on mosses for a student, I would suggest the following:

I. Mosses with a Hand Lens by A. J. Grout.

II. Mosses with a Hand Lens and Microscope by A. J. Grout

III. How to Know the Mosses, a popular guide to the mosses of the North Eastern United States by Elizabeth Marie Dunham, Published by Houghton Mifflin Company. The introduction is good, also the description of genera and species. Genera are listed by numerals which are used in the keys. These keys are based on habitat as well as leaves and capsules.

IV. How to Know the Mosses by Henry S. Conard. This book can be obtained in paper covers with spiral binder and has a black and white picture of the moss in the key. Published by E. E. Jaques, Mt. Pleasant, Iowa.

V. The Student's Handbook of British Mosses by H. N. Dixon. Illustrated by H. G. Jamieson. This has an excellent introduction and good illustrations. VI. Mosses and Lichens by Nina L. Marshall. This belongs in TheNature Library, published by Doubleday, Page & Co. It deals only with the commoner mosses.