164 



BOTANY: PRINCIPLES AND PROBLEMS 



region of low content to one of higher. Such a response is known 

 as an hydrotropism and results in the pursuit of moisture by roots 

 for considerable distances, notably when the surface layers of the 

 soil are drying out and the water table is descending. 



It is in responses of structural change rather than those of 

 movement, however, that the effect of moisture is most often 

 manifest. Plants which have access to abundant water supply 



Fig. 81. — "Amphibious" plant. A shoot of mermaid weed (Proserjnnaca 

 palustris). Submersed leaves are finely divided, aerial leaves undivided, and 

 leaves between, intermediate in form. 



grow luxuriantly, for the most part, with large leaves and rather 

 soft tissues. Cuticle and epidermis are thin, woody tissues some- 

 what weak, and parenchyma abundant. A similar plant grown 

 where water is scanty becomes stunted throughout and has much 

 smaller leaves. Its tissues, particularly the epidermis and cuticle, 

 are much tougher, and the woody elements stronger and more 

 abundant. In some instances even more profound structural 

 changes are produced. The water buttercup, to cite a notable 

 example, when growing on the shore produces normal buttercup 

 leaves, but when submersed in water develops leaves which are 

 dissected into fine capillary segments and are thus particularly 

 well fitted for aquatic life. Other "amphibious" plants exhibit 

 similar structural changes (Fig. 81). 



