MOSSJSS AND THEIR WATER-SUPPLY. 481 



muralis, whicli were exposed for eighteen montlis 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 pulvinaia, 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 quicken- 

 ing moisture was still competent to awaken it to renewed life. 

 The most rapid drying which could be performed in the labora- 

 tory 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 experi- 

 ments 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 solu- 

 tions by Fr. Oltmann. 



The capillary spaces in which the water rises or, more gener- 

 ally speaking, diffuses itself, exist in different forms. In the sim- 

 plest cases the leaves supply them ; and of this kind there are, 

 according to Oltmann's comparisons, several types. Thus in 

 Hylocomium loreum, Hypnura 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 Plageotliecium un- 

 dulatum, 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 frequent- 

 ly 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 



VOL. XXXIII. — 31 



