THE LARGER AQUATIC VEGETATION 2OI 



more branches are formed in succession. The following experi- 

 ment seems to strongly indicate that light inhibits the formation 

 of lateral branches of the roots and that the roots of drifting plants 

 do not branch because of the exposure to the light. Five cuttings 

 of equal length were mounted in bottles so that about 3 nodes of 

 the stem were inside the bottle. Five similar cuttings were like- 

 wise mounted in bottles which were wrapped with black cloth so 

 that the roots which developed from the nodes of the cutting 

 inside the bottle were protected from the light. The darkened 

 plants developed in all 22 roots having a total length of 1048 centi- 

 meters and bearing in all 73 branches. The plants exposed to 

 light developed 28 roots, having a total length of 459 centimeters 

 and bearing not one single branch. The influence of light is very 

 evident. 



There are two possible reasons why soil may be necessary for 

 good growth. First, as a source of food and, second, as a substratum 

 into which the roots can penetrate to anchor the plant. If the 

 soil is not necessary as a source of food the ordinary water in which 

 the plant grows should furnish sufficient food. If the soil is neces- 

 sary only as a substratum to anchor the plant a clean washed 

 sand substratum ought to do equally as well. Experiments along 

 this line have been completed by the author and it has been found 

 that soil is necessary for the good growth of every one of the plants 

 tested. Clean washed sand cannot be substituted for soil without 

 sacrifice of growth to the plant. It is reasonably certain that not 

 one of our common water plants which naturally grows well rooted 

 in a good soil bottom could live through an entire growing season 

 if clean washed sand should be substituted for its ordinary soil 

 substratum. 



The behavior of Ranunculus aquatilis is interesting as indicating 

 (figuratively speaking) an effort on the part of the plant to secure 

 a soil attachment. This species lives wholly submerged in shallow, 

 slowly flowing water. The leaves are finely divided and collapse 

 when the plant is taken from the water. The stem branches 

 freely, any branch being able to continue the growth of the plant 

 if the main stem be removed. Roots may arise at any exposed 

 node, except, perhaps, the terminal one. If a fragment a few inter- 



