AQUATIC PLANTS AND FOOD FISHES 



2 53 



of course, take its nourishment from the soil. It has long been be- 

 lieved, and the statement is current in our latest and best text-books, 

 that the submerged and rooting aquatics take their nourishment from 

 the water which bathes them, that their roots are not for absorption, 

 but merely for anchoring. An examination of the literature convinced 

 me that so far as experimental evidence is concerned there is none 

 which can be accepted as demonstrating this conclusion or a contrary 

 one. The necessity of exact knowledge in this particular is apparent 

 when we consider that if such plants, as hitherto supposed, feed from 

 the water, they then during the entire growing season are diminishing 

 the quantity of food for the phytoplankton. On the other hand, if 



Fig. 3. VaUisneria spiialis alter f>% weeks' Fig. 4. Vallisneria spiralis after 5J£ weeks' 

 growth in loamy soil. growth in sandy soil. 



this food comes from the soil they not only do not reduce the plankton 

 food supply, but actually become important contributors to it in that 

 the mineral food taken by them from the soil finally through their 

 decay becomes available to the phytoplankton. That the latter alter- 

 native is the true one may be seen from the following glimpse of an 

 experimental study. 



The special organs of absorption in terrestrial plants are the delicate 

 root hairs which occur on the young roots in a narrow zone situated 

 just back of the growing tip. The quantity of such hairs can, in the 

 laboratory, be made to vary greatly by regulating conditions, showing 

 that they are sensitive structures and not likely to be produced unless 

 needed by the plant. Some land plants are known which do not have 

 root hairs, but they are exceptions. On the other hand, aquatic plants 



