Page:Popular Science Monthly Volume 68.djvu/257

Rh of course, take its nourishment from the soil. It has long been believed, 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

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 alternative 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