MOSSES AND THEIR WATER-SUPPLY. 483 



plants. I have within a sliort 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 dlterni- 

 foUum, 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 cen- 

 tral 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 analo- 

 gous with submerged phanerogamous plants, in the leaves and 

 stems of which the water-bearing system appears to have under- 

 gone 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 Polytrichacece, 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. 



