Popular Science Monthly/Volume 33/August 1888/Mosses and their Water-Supply
|MOSSES AND THEIR WATER-SUPPLY.|
THE 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 protective property of adaptation to desiccation so rarely appear among the higher plants?—why have the plants of the steppes and deserts, for example, to protect themselves against the perils of drought by so various anatomical features, of thick skins, corky bark, waxy and hairy envelopes, receptacles for water, etc., instead of simply drying up and reviving again in the rainy periods? The answer to the question is not hard to find. The maxim, "one thing is not suitable for all," is valid in the biological domain. That which works well in the little mosses is for various reasons not available to the larger phanerogams. First, the larger plants must continue to vegetate actively for longer periods, in order to prepare the amount of food required for the proper growth of their organs. Ever-recurring interruptions of their feeding by drying out would so retard the whole process of their growth, that in spite of their vital tenacity they would be at a great disadvantage in the struggle for existence. To this is added another not less weighty reason, that the mosses are so simple in their anatomical structure that the mechanical shrinking of the drying tissue involves no danger; the collaborated cells easily resume their original form and size on the accession of a new water supply. It is very different with the organs, far more complicated in their structure and composed of tissue of diversified kinds, of the more highly developed plants. In them extensive shrinkage would result in damaging tensions and distortions, and even cracks, for the limitation of which various mechanical protective adaptations would be required. Besides this, the mechanical structure of the tissues would have to have a proportionately enormous development, else the dry, brittle leaves and branches would be broken up by every gust of wind. A careful regard to the consequences of such an adaptation to complete desiccation should be sufficient to convince any one that it would be too dearly purchased. But in the case of many of the humbler plants insensibility to continuous desiccation is a life-question, and accordingly they have practically acquired that property. It is of equal interest from a physiological and a biological point of view that the protoplasm of the young individual should, by further development, gradually suffer the complete loss of so pregnant a property as that of reviving after it has been dried up.
The power of the mosses to endure repeated desiccation has recently been experimentally treated by G. Schröder, who obtained the interesting result that many of these plants can not only resist months of dryness without any harm, but also that they do not perish even under the strongest desiccation carried on in a drier with the aid of sulphuric acid. Plants of Barbula muralis, which were exposed for eighteen months in the drier, after a few wettings resumed growth in all their parts. Other species of Barbula behaved similarly. A curious experiment was performed with Grimmia pulvinata, in which a stock which had been cultivated for some time in a moist atmosphere under a bell-glass was suddenly exposed to a warm and perfectly dry current of air. It became so dry in a short time that it could be pulverized. Then it lay in a drier for ninety-five weeks. But the quickening moisture was still competent to awaken it to renewed life. The most rapid drying which could be performed in the laboratory could not destroy the plant. It even showed greater power of resistance than would correspond with its real necessities, for so speedy and complete a drying out as was effected in the experiments never occurs in Nature. The fact that a property acquired by adaptation is so plainly manifested in excess is sometimes otherwise demonstrable, and is a hard problem for the theory of selection.
Those mosses which are not capable of drawing water in considerable quantities from the soil, are yet able to make the best use of the smaller quantities with which they are moistened. For this object their stems are furnished with provisions for the capillary distribution of the local water with which they are in contact. This capillary "outer water-conduit" was perceived several years ago in various mosses by C. Schimper; and it has recently been more closely studied with the aid of colored solutions by Fr. Oltmann.
The capillary spaces in which the water rises or, more generally speaking, diffuses itself, exist in different forms. In the simplest cases the leaves supply them; and of this kind there are, according to Oltmann's comparisons, several types. Thus in Hylocomium loreum, Hypnum purum, and similar forms, the leaves are so shaped and arranged with their opposite sides in close contact as to form a hollow cylinder around the stem, which is composed in its interior of a system of connected chambers. When enough water is present, the capillary space between the stem and the leaf is quite full; in other cases the water ascends only between the overlapping leaf-edges. In Plageothecium undulatum, Neckera crispa, etc., the leaves lap like shingles; in other cases they are small and thickly packed, so that a whole system of narrow capillary spaces is generated between them. The frequently observed phenomenon of the drying leaves erecting themselves and lying close to the stem, with wrinklings and curlings, involves, as Oltmann has remarked, an increase of capillary space. By these means the water, when a wetting takes place, is diffused more readily and completely over the surface of the plant.
In another series of cases, the capillary apparatus is formed by a felting of hairs encompassing the stem, in which water rises as in a piece of filtering paper. Dicramim, undulatum, Climacium dendroides, and Hylocomium splendens are among the species thus furnished. These hair formations commonly resemble the root hairs, and might eventually he designated as of that class; but in single cases, as in Thuidium iamariscinum, they exhibit a peculiar construction. The hairs are undoubtedly adapted to taking up the water with which they come in contact.
With the phanerogams, the plenteous absorption of water by organs above ground is a rare phenomenon of adaptation, and is limited to a number of epiphytes (Bromelaciæ) and desert-plants. In these, again, different forms of hair-growth assist the reception of water. Volkens has recently shown that many of those desert species whose leaves are furnished with a hair-felting absorb rain and dew in this manner. But he has never observed the reception of the water going on over the whole surface of the hair, but only in specific cells at the base of the hair which act as the absorbing element; while the dead cells composing the felt fulfill the purpose of retaining the water, covering the surface of the leaf, and in that way facilitating absorption.
The capillary apparatus of the peat-mosses is peculiar and without any analogies with the more highly developed plants. The leaves of the Sphagnaceæ consist of two kinds of elements; of long-drawn, chlorophyll-bearing cells woven into a net-work, and of dead, colorless capillary cells, which form the meshes of the net. The walls of the capillary cells are furnished with large, usually round pores, the points at which the water is admitted, the situation and arrangement of which in many species materially facilitate the passage of the water from one cell to another. The edges of the pores are usually hemmed with a thicker fibrous ring, the office of which is evidently mechanical, or to prevent tearing. The walls of the cells are also stiffened with spirally arranged fibrous structures, like the duct-walls of the more highly developed plants. The stems of the peat-mosses have also a "bark-envelope" from two to four cells thick, which serves as a reservoir and a medium for the circulation of water.
The stems, fruit-stalks, and leaves of numerous mosses possess a water-bearing tissue-cord occupying an axillary position. This cord, which consists of narrow, thin-walled, and elongated cells, has been described by W. Ph. Schimper and Fr. Unger, to whom we owe some excellent researches on the anatomy of the mosses. Its precise physiological function has until very recently not been made clear. While it was not doubted that it had some connection with the circulation, it was not certain whether or not it presented any analogies with the vascular system of the higher plants. I have within a short time obtained evidence that the typical tissue-cord represents a water-bearing structure. With a solution of sulphate of lithium I found that the average velocity of the circulation through the central cord was not very far behind that which took place in the stems of phanerogamous plants; and that the solution was very quickly transferred from the central cord through the leaves. Experiments in transpiration further showed that the water-bearing capacity of the central cord, where it is well developed, is amply sufficient to supply the water lost by transpiration.
It is a point of interest with respect to the relations between the structure of the central cord and the local conditions of the habitat of the plant, that only those mosses that grow on more or less moist ground have this cord well developed. It is easy to perceive that the cord can be of advantage only where a steady supply and circulation of water for a relatively considerable length of time is possible. To classes fulfilling these conditions belong chiefly the longer leaved and therefore more actively transpiring plants of Mnium, Bryum, Bartramia, Funaria, Fissidens, and Splachnum. On the other hand, the systematic position of the moss appears to be a matter of no account. Archidium dlternifolium, which grows in moist fields, and which is phylogenetically regarded as one of the lowest of the leaf-mosses, has a typically developed central cord. It is, therefore, plain that the central cord indicating the formation of a water-bearing tissue in the leaf-mosses is in no way a sign of higher phylogenetic structure, but is wholly a mark of adaptation.
The mosses living in dry places form another biological group. Their stems possess either no or only very weakly developed central cords, which seem to have suffered degeneration. They are apparently the predecessors of the mosses of which we have just spoken as inhabiting moist situations, and which are furnished with typically constructed central cords. Mosses growing in water, likewise, for reasons easily to be understood, possess no or strongly degenerated central cords, and in this respect are analogous with submerged phanerogamous plants, in the leaves and stems of which the water-bearing system appears to have undergone a more or less extensive atrophy. Finally, those mosses in which an external circulation of water occurs are unprovided with a central cord, or present it in a very reduced form.
In the most highly developed mosses, the Polytrichaceæ, the central conducting bundle of the stem consists no longer of water bearing tissues only; just as in the conducting bundles of the ferns and phanerogams, the vascular tissues for carrying plastic growth-food are combined with the water-ducts into a single system.
When we survey the different kinds of water-provision among the mosses, we are struck with the variety of ways of adaptation that have been developed for the attainment of the same end. In so small plants, the demands upon the efficiency of the water-bearing apparatus are likewise quite small, and the choice among them is only slightly limited. The larger the plant-forms become, the greater the demands made upon the water-bearing system, the more plainly appears the diversified efficiency of the adaptation henceforth possible, till at last there remains to Nature a single available model, which she applies with certain variations everywhere that large, stately plant-forms are produced.—Translated for the Popular Science Monthly from Humboldt.
- "Ueber die Austrocknungsfähigkeit der Pflanzen." Untersuchen aus dem botanischen Institut zu Tübingen. Published by Pfeffer. Vol. ii, part i, 1886.
- "The Flora of the Egypto-Arabian Desert." Berlin, 1887, p. 32.