Page:Popular Science Monthly Volume 33.djvu/499

Rh 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.