202 FRESH-WATER BIOLOGY 



nodes in length be detached and left floating, roots will spring 

 forth from the nodes or joints of the stem in from 6 to 10 days. 

 While the roots are lengthening toward the bottom the stem does 

 not increase in length but quickly resumes its growth after the 

 root has penetrated the soil. More roots then arise from the newer 

 nodes of the stem and as they also enter the soil the plant is drawn 

 farther down and finally becomes firmly anchored. The roots of 

 such fragments do not branch before reaching the soil but do 

 so very shortly after the substratum is penetrated. Numerous 

 lateral roots then arise and form in succession as the main root 

 advances. 



The roots are well supplied with hairs; those arising from the 

 floating fragments are often covered almost their entire length 

 with root-hairs. In one instance a root was found to be clothed 

 with root-hairs for a distance of 45 centimeters, which was practi- 

 cally its entire length. Since this plant ordinarily grows rooted 

 in the soil whatever growth it makes under natural conditions 

 must be regarded as the normal growth and there is no escape 

 from the conclusion that the growth becomes abnormal when 

 sand is substituted for soil. 



How is the superior growth of plants rooted in the soil to be 

 accounted for? Is it possible that the soil furnishes plants rooted 

 in it with food materials that are not available to plants suspended 

 in the water standing over it? In 1850 it was discovered that 

 liquid manure loses its color if drained through a layer of soil 

 sufficiently thick. Not only does the solution lose color but the 

 organic and inorganic matter originally in it is considerably re- 

 duced after filtering through the soil. This property or capacity 

 of soil to withdraw salts from solution is now well established 

 although not well understood. For a time authors were not 

 agreed as to whether the retention or fixation of salts by the soil 

 is a chemical or a physical process. Now it is generally under- 

 stood that both chemical and physical processes operate to this 

 end. Some substances seem to be held much more firmly by the 

 soil than others. Various investigations go to show that most 

 soils absorb the oxids, salts of the alkalis and alkaline earths of 

 potassium, ammonium, magnesium, sodium, and calcium in the 



