BEHAVIOR OF ROOTS DUE TO IRRITABILITY 89 



51. Osmosis in root hairs. The soil water, practically identi- 

 cal with ordinary spring or well water, is separated from the 

 more or less sugary or mucilaginous sap inside of the root hairs 

 only by their delicate cell walls, lined with a thin layer of pro- 

 toplasm. This soil water will pass rapidly into the plant, while 

 very little of the sap will come out. The selective action, 

 which causes the flow of liquid through the root hairs to be 

 almost wholly inward, is due to the living layer of protoplasm, 

 which covers the inner surface of the cell wall of the root hair. 

 Traveling by osmotic action from cell to cell, a current of water 

 derived from the root hairs is forced up through the roots and 

 into the stem, somewhat as the contents of the egg was forced 

 up into the tube shown in Fig. 28. 



But there is this important difference in the two cases, that 

 while the process in the tube was all due to the impulse received 

 at the start from the egg membrane, in the plant stem the origi- 

 nal pressure due to osmosis in the root hairs may be affected by 

 osmosis in countless thousands of cells higher up. 



52. Behavior of roots due to irritability. In Chapter iv a 

 little was said about the geotropism of roots, their tendency to 

 put themselves into the most favorable conditions as regards 

 moisture, heat, and light, and their manner of avoiding obstacles. 

 All these actions are manifestations of irritability. 



K 



The subject of geotropism of roots is a very complicated one, 

 but it seems pretty certain that gravity somehow acts as a stim- 

 ulus on the sensitive cells of the root tip, this stimulus is trans- 

 mitted to the cells of the most rapidly growing portion of the 

 root (a little farther back), unequal growth of the upper and 

 under cells of this portion follows, and so the root is bent, if its 

 position is not vertical in the beginning. 



I^f 



Moisture and heat (in the case of Indian corn up to 99.5 F. 

 or 37.5 C.) are favorable to the growth of roots, and so as stimuli 

 produce growth toward the source of moisture or heat, while 

 light is usually slightly unfavorable and therefore generally 

 results in growth of the root toward darkness. 



