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Leaves of Polytrichum

The leaves of a Polytrichum represent about the highest stage in the development of mosses. The mid-vein is broad, and only at the extreme margins is the leaf-blade one-layered.

The central tissue of the mid-ribs of the leaves continue so as to unite with the central axis of the stem in a manner quite analogous to that found in stems of higher plants. A cross section of a leaf shows that the marginal cells and a line of cells running through the central part are comparatively thin-walled and are empty water-conducting cells similar to the wood-ducts (tracheae) of a fibro-vascular bundle in a higher plant. The next layer is composed of similar but smaller cells containing starch. The rest are thick-walled cells (sclerenchyma). The outer cells contain more or less leaf-green (chlorophyll). When breathing pores occur they are on the spore-case walls.

The cells of the upper surfaces, have their walls exceedingly delicate, so that they can absorb gases and permit gases or water to leave them. The thin blades (lamellae) are undoubtedly the result of an effort on the part of the plant to increase to the highest degree its absorbing surface without widening the leaf-blade itself. By directing the growth of the delicate cells upward in thin blades, this end is accomplished.