Page:Popular Science Monthly Volume 33.djvu/495

 other cardinal points in Kant's doctrine; but am obliged to abandon the intention. A state of health such that the foregoing pages, commenced in the middle of March, I have been unable to complete till the first week in June, compels me now to desist.



HE interest with which botanists regard the mosses is, for various reasons, more lively and more diversified than lay-men might suppose so inconspicuous, unobtrusive a group could awaken. In more than one respect, they form a sharply marked point of departure for the morphological and phylogenetical study of the higher plants. The diversified forms of adaptation which these plants, and particularly the leafy mosses, exhibit in their outer and inner structure, are especially worthy of attention. From the fact that they exemplify so many different inclinations in respect to their local relations, and because, notwithstanding the variety in their forms, they are of relatively simple organization, the investigator's insight into their adaptive structure is made comparatively easy; and on more than one occasion their study has greatly aided the understanding of the adaptive phenomena of the more highly developed plants.

An instructive example of the way in which the observation of one order may be applied to facilitate the study of other orders, is afforded in the water-provision of the leafy mosses. Among the simplest in this category are those genera and species that grow on rocks, roofs, and tree-trunks, and are therefore most directly exposed to the rapid exhaustion of atmospheric precipitations. To these belong many Hypnaceæ—species of Gymnostomum, Barhula, Orthotrichum, etc. No special provisions for taking up water have yet been observed in these species. Their leaves all suck it in when it is abundant and swell out, and then completely dry up again as soon as the air has lost its moisture. It is not the taking in of water that interests us in these mosses, but their complete desiccation, which may occur again and again without harm to the vitality of the plant. In this is expressed a form of adaptation which is invisible to the investigator in microscopic anatomy, and which depends upon undetected properties of the protoplasm. It is evident that this kind of adaptation is most direct and effective, and is in exact conformity with the biological principle of economy of material, in that it makes special protective provisions for the prevention or retardation of the waste of water superfluous. The question arises. Why does this apparently advantageous 