70 THE BIOLOGY OF FLOWERING PLANTS 



primarily important function is scarcely credible. The 

 water current from root to leaf in a water plant might be 

 thought a functional relic of a remote land ancestor. In 

 fact, it serves to promote the supply of salts. Snell (1908) 

 has shown that plants of Elodea and Potamogeton allowed 

 to root in the mud grow much more vigorously than plants 

 in the same vessel supported away from the substratum. 

 In one experiment shoots of Elodea showed an average 

 increase in length of 9*85 cm. when rooted in mud, of 

 4*4 cm. when free, and of 2'6 cm. when rooted in sand. 

 Similar results were obtained by Pond (1905) and by Brown 

 (1913), who, however, interprets them as due to an increase 

 in the supply of carbon dioxide (see pp. no, 201). The 

 mineral content of fresh water is generally very low, and 

 the advantage of rooting in the rich mud is evident. 

 Further, it has recently been shown by Mayr (191 5) that 

 in some submerged leaves, e.g. of Alisma plantago, only 

 certain cells or cell groups, called hydropotes, are easily 

 permeable by salts and water. 



§ 6. Exceptional Means of absorbing Water and Salts 



The soil is the normal habitat of the land plant and the 

 root is the normal organ of absorption. The great majority 

 of plants make good their requirements of water and salts 

 by this means ; they are efficient, as we have seen, not only 

 in ordinary cases but in extreme conditions ; the submerged 

 water plant thrives best rooted in the mud, and the plants 

 of the most arid deserts draw an exiguous but sufficient 

 supply from an almost dust- dry soil. There are many 

 plants, however, which depend on specialised or peculiar 

 organs, drawing water from rain or even dew and fog, or 

 from reservoirs of their own. These plants are the excep- 

 tion ; but in certain stations they are prevalent, and the 

 peculiarities of their water supply have always excited 

 interest. 



Epiphytes. — The biological class which is most con- 

 spicuous in this respect is that of the epiphytes — plants living 



