ABSORPTION OF MATERIALS IN GENERAL 



125 



Plants can absorb solid soil constituents but these must first be dissolved in 

 water. If a polished marble plate is placed in the bottom of a box in which 

 seedlings are grown, many of the roots come into close contact with the plate, 

 and if the latter is removed after a time the imprint of the roots may be seen on 

 the polished surface, etched by acid root excretion. The acid character of root 

 excretion may also be shown by the reddening of blue litmus paper against 

 which the roots are induced to grow. 



The following experiment illustrates the solution of soil particles and their 

 absorption after being dissolved. A broad glass tube with its lower end firmly 

 bound with animal bladder (Fig. 73) is filled with and inverted over a weak solu- 

 tion of hydrochloric acid, so that the cylinder remains filled. A piece of marble 



Fi c 72. Fig. 73. 



Fig. 72. — Cells of endosperm of Areca oleracea. a, thick cell wall; b, canals piercing cell 

 walls and containing protoplasmic strands. 



Fig. 73. A piece of calcium carbonate dissolving in hydrochloric acid as this diffuses 

 upward through the bladder membrane M. 



is placed upon the smooth surface of the bladder. The marble gradually becomes 

 smaller and smaller as it is dissolved by the acid imbibed in the membrane. 

 Calcium chloride is formed during the process and diffuses slowly through the 

 membrane into the solution below, where it can be identified with suitable 

 chemical reagents. 



Czapek 1 studied the nature of root excretions. He employed plates made 

 of a mixture of aluminium phosphate and plaster of Paris. These are soluble 

 in many acids (hydrochloric, nitric, sulphuric, phosphoric, formic, oxalic, suc- 

 cinic, lactic, malic, citric and tartaric) but they are insoluble in carbonic, 



1 Czapek, Friedrich, Zur Lehre von den Wurzelausscheidungen. Jahrb. wiss. Bot. 29: 321-390. 1896. 



