480 MANUAL OF BOTANY 



plant. Their food materials reach them dissolved in the water 

 in which they live, and hence they have no need of the com- 

 phcated root system with its absorbent root-hairs, which is so 

 characteristic of a plant growing in ordinary soil. Gaseous 

 absorption takes place also through the general epidermis as well 

 as from the cells of the lacunar passages. 



It is noteworthy ua this connection that the absence of the 

 transpiration current is associated with a comparatively small 

 development of the general plant body. Aquatic phanero- 

 gams are consequently never of large size. 



The difference between the two groups spoken of may be well 

 seen in such plants as Cabomha, which bears both submerged 

 and floating leaves. These show respectively the characteristics 

 described in each case. 



Another class of plants which show a definite response in 

 their structure to the conditions in which they live is that to 

 which the so-called xeropJdlous plants belong. These are 

 plants which grow in sandy deserts, exposed to great heat,' and 

 frequently undergoing long periods of drought. Those which 

 are woody in habit show considerable tendency to diminish their 

 leaf surface, probably to diminish evaporation and conserve their 

 stock of water. Thus they often have many of their branches 

 transformed into thorns or spines. Others which contain but 

 little wood are succulent, and their surfaces are covered with a 

 very thick and tough epidermis, which is strongly cuticularised. 

 Many of them have leaves which show special absorbing struc- 

 tures that are often incrusted with chalk or a cake of salt. 

 This becoming wetted by the dew gives up its captured water to 

 the absorbing organ of the leaf, thus enabling the latter to 

 make use of what in the absence of this mechanism would be 

 lost to it. Many trees growing under the same conditions secrete 

 a kind of resinous balsam, which coats the surface of their leaves 

 and subserves a similar purpose. The water absorbed in this 

 way is very rarely pure, but contains traces of sulphuric acid 

 and ammonia, which, though trifling in amount, are no doubt 

 of value in the nutritive processes. The adaptation to their 

 environment which these plants exhibit is thus chiefly in the 

 direction of economising a limited water supply. 



The influence of the environment on the form of the plant 

 body can be seen equally well in the case of such plants as grow 

 in Alpine regions where the cold is usually intense, and the atmo- 

 sphere for long periods so humid that transpiration is only pos- 

 sible at times, and where, consequently, the absorption of food 



