Fig. 59.—Longitudinal section through the summit of a small male plant of Funaria. a, aʹ, antheridia. p, paraphysis. L, section of a leaf, × 150.
This moss is diœcious. The male plants are smaller than the female, and may be recognized by the bright red antheridia which are formed at the end of the stem in considerable numbers, and surrounded by a circle of leaves so that the whole looks something like a flower. (This is still more evident in some other mosses. See [Figure 65], E, F.)
The leaves when magnified are seen to be composed of a single layer of cells, except the midrib, which is made up of several thicknesses of elongated cells. Where the leaf is one cell thick, the cells are oblong in form, becoming narrower as they approach the midrib and the margin. They contain numerous chloroplasts imbedded in the layer of protoplasm that lines the wall. The nucleus ([Fig. 63], C, n) may usually be seen without difficulty, especially if the leaf is treated with iodine. This plant is one of the best for studying the division of the chloroplasts, which may usually be found in all stages of division ([Fig. 63], D). In the chloroplasts, especially if the plant has been exposed to light for several hours, will be found numerous small granules, that assume a bluish tint on the application of iodine, showing them to be starch grains. If the plant is kept in the dark for a day or two, these will be absent, having been used up; but if exposed to the light again, new ones will be formed, showing that they are formed only under the action of light.
Fig. 60.—A, B, young antheridia of Funaria, optical section, × 150. C, two sperm cells of Atrichum. D, spermatozoids of Sphagnum, × 600.
Starch is composed of carbon, hydrogen, and oxygen, and so far as is known is only produced by chlorophyll-bearing cells, under the influence of light. The carbon used in the manufacture of starch is taken from the atmosphere in the form of carbonic acid, so that green plants serve to purify the atmosphere by the removal of this substance, which is deleterious to animal life, while at the same time the carbon, an essential part of all living matter, is combined in such form as to make it available for the food of other organisms.
The marginal cells of the leaf are narrow, and some of them prolonged into teeth.
A cross-section of the stem (63, E) shows on the outside a single row of epidermal cells, then larger chlorophyll-bearing cells, and in the centre a group of very delicate, small, colorless cells, which in longitudinal section are seen to be elongated, and similar to those forming the midrib of the leaf. These cells probably serve for conducting fluids, much as the similar but more perfectly developed bundles of cells (fibro-vascular bundles) found in the stems and leaves of the higher plants.
The root hairs, fastening the plant to the ground, are rows of cells with brown walls and oblique partitions. They often merge insensibly into the green filaments (protonema) already noticed. These latter have usually colorless walls, and more numerous chloroplasts, looking very much like a delicate specimen of Cladophora or some similar alga. If a sufficient number of these filaments is examined, some of them will probably show young moss plants growing from them ([Fig. 63], A, k), and with a little patience the leafy plant can be traced back to a little bud originating as a branch of the filament. Its diameter is at first scarcely greater than that of the filament, but a series of walls, close together, are formed, so placed as to cut off a pyramidal cell at the top, forming the apical cell of the young moss plant. This apical cell has the form of a three-sided pyramid with the base upward. From it are developed three series of cells, cut off in succession from the three sides, and from these cells are derived all the tissues of the plant which soon becomes of sufficient size to be easily recognizable.
The protonemal filaments may be made to grow from almost any part of the plant by keeping it moist, but grow most abundantly from the base of the stem.
The sexual organs are much like those of the liverworts and are borne at the apex of the stems.
The antheridia (Figs. [59], [60]) are club-shaped bodies with a short stalk. The upper part consists of a single layer of large chlorophyll-bearing cells, enclosing a mass of very small, nearly cubical, colorless, sperm cells each of which contains an excessively small spermatozoid.
The young antheridium has an apical cell giving rise to two series of segments ([Fig. 60], A), which in the earlier stages are very plainly marked.
When ripe the chlorophyll in the outer cells changes color, becoming red, and if a few such antheridia from a plant that has been kept rather dry for a day or two, are teased out in a drop of water, they will quickly open at the apex, the whole mass of sperm cells being discharged at once.
Among the antheridia are borne peculiar hairs ([Fig. 59], p) tipped by a large globular cell.