500 SECRETORY AND EXCRETORY SYSTEMS 



water absorbed by night possesses a similar ecological significance, since 

 it must tend to counteract the heating effect of the sun. Most prob- 

 ably, however, the secretion of saline matter in such instances mainly 

 represents a device for obviating the accumulation of excessive 

 quantities of mineral matter within the plant-body. 243 



More widely distributed even than these "salt-glands" are the "chalk- 

 glands" which occur in a large number of families; here a varying amount 

 of calcium carbonate is secreted together with the water and remains as 

 a scaly deposit when the water evaporates. In certain species of Saxi- 

 fraya, the "chalk-glands" simply represent epithem-hydathodes situated 

 in depressions in which lime accumulates. In certain Ferns {Lorn aria 

 attenuata, species of Polypoclium and N&phrol&pis), and in many Plum- 

 baginaceae, the hydathodes similarly become encrusted with scales 

 of secreted lime. A remarkable feature is the occasional provision of 

 arrangements which prevent the scales of lime from falling off. In 

 Limoniastrum, for example, the glands are situated, according to Volkens, 

 at the bottom of a cavity, which is cruciform in section below and 

 contracts to a simple tubular form above. Here it is quite impossible, 

 in any circumstances, for the scale to fall out of its socket, just as a key 

 cannot be removed from a lock in which it has been partially turned. 

 In Statice pruinosa the epidermal cells adjoining the gland are produced 

 into inwardly direct hook-shaped processes, which grip the scale from 

 either side. Arrangements of this kind certainly suggest that these 

 chalky scales which are always present in considerable numbers 

 serve some useful purpose ; as the plants concerned are all desert- 

 plants, or at any rate pronounced xerophytes, Volkens is probably 

 right in supposing that these structures help to restrict transpiration. 244 



In many cases hydathodes may act secondarily as aerial water- 

 absorbing organs, which are made use of when the normal water-supply 

 is deficient. It is particularly the epidermal hydathodes {e.g. those of 

 Gonocaryum, Anamirta, Phaseolus and Machae,rium,(Q,i. above p. 488) that 

 tend to assume this twofold responsibility. Minden has, however, also 

 observed the entrance of water through epithem-hydathodes in various 

 seedlings. Prolonged wilting of the leaves will, however, cause the 

 intercellular spaces of the epithem to become full of air, whereupon 

 any further entrance of water is, of course, rendered impossible. 



In conclusion, we have to consider the question of the ecological 

 significance of hydathodes. There seems no doubt that these organs 

 play a very important part by regulating the water-content, and hence 

 the turgor, of the foliage leaves and the whole plant-body. The activity 

 of the hydathodes prevents that injection of the intercellular spaces with 

 water which tends to result from any sudden increase of exudation- 

 pressure ; such injection, though apparently not in itself injurious, would 



