200 FRESH-WATER BIOLOGY 



for absorption.! j^ jg possible by means of very simple apparatus 

 to actually measure the water which a root absorbs in a given time. 

 In one experiment of the writer's a small cutting 20 centimeters 

 long bearing a root 14 centimeters long was used and this un- 

 branched single root absorbed 5 cubic centimeters of water in 

 24 hours. 



Another method can be used to demonstrate that roots are 

 organs of absorption. A certain substance, namely, lithium nitrate, 

 which is soluble in water, is offered in solution to the roots. The 

 lithium in this compound burns with a rose red flame and very 

 small traces of this substance in plant tissue may be detected by 

 burning portions of the tissue and observing the color given to the 

 flame, and by using the spectroscope the test becomes very delicate. 

 In this method it is only necessary to enclose the root in a bottle 

 containing the hthium nitrate solution by means of a flexible 

 stopper made by saturating cotton in melted vaselin. After a 

 time portions of the stem which could not possibly contain hthium, 

 unless it had passed to it from the roots, are burned and the flame 

 observed with the spectroscope. Such tests have been made re- 

 peatedly and the presence of the lithium may always be traced 

 through the plant to whatever distance the tissue used in the 

 test allowed it to travel in the plant. There can be no doubt 

 then but that the roots are organs for absorption and that sub- 

 stances absorbed by them are conducted upward into the stem and 

 leaves. The distance that the lithium travels in a given tissue 

 probably does not represent exactly the rate of ascent of the up- 

 ward current but indicates a rate of ascent which is less than that 

 of the water passage through the plant; that is, water travels 

 upward a little faster than the lithium which it holds in solution. 



Mention has been made of the fact that when cuttings of Ranun- 

 culus aquatilis are left to drift in the water, new roots arise from 

 the stem at the nodes. These roots grow directly downward and do 

 not branch until after penetrating the soil, when they then commence 

 to branch repeatedly, and as the main root pushes through the soil 



' It is not strictly correct to spealc of roots as organs for absorption. The struc- 

 ture of roots is such that solutions can pass into them. However, the term is so 

 commonly employed as to make it impracticable to use other phraseology here. 



