ii6 



PHYSIOLOGY OF NUTRITION 



tance. Since diffusion progresses rapidly in gelatine and agar as well as m water, 

 these substances may be employed in difiusion experiments, being poured mto 

 a glass cylinder while hot and then covered after cooling, with a solution of the 

 substance to be studied (e.g., indigo). Intercellular protoplasmic connections, 

 like thin threads reaching through the cell waUs, are now known to be of common 

 occurrence in plants (Fig. 72)- How these structures may influence exchange 

 of materials between the cells is still unknown, however. 



Plants can absorb soUd soil constituents but these must first be dissolved m 

 water. If a poHshed 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. 



Fig. 72. Fig. 73. 



Fig. 72. — Cells of endosperm oi Areca oleracea. a, thick cell wall; 6, 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. 



and if the latter ij 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 

 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 



