Nervous System. 551 



Equally remarkable is the action of the rootlets of growing plants. 

 The radicle of the pea in pushing its way into the ground does not pen- 

 etrate directly but bores down spirally, a process which is brought about 

 by the growth of cells progressively around the point of the advancing 



; FIG. 268. Pulvinus of OxaHs Rosea ( a wood sorrel ). 



A longitudinal section of the petiole or leaf stalk of a 

 cotyledon or first formed leaf. 



p. The Petiole. 



/.Bundle of fibres and vessels. 



&. Beginning of the blade of the cotyledon. 



c. The cells constituting the Pulvinus or joint, the motor 

 organ of the leaf. ( Darwin.) 



root. The tips of the rootlets are very sensitive, 

 and when one encounters a little stronger press- 

 ure on one side than the other, it transmits a 

 stimulation upward to a part of the root some 

 little way above. At this place an expansion of 

 one side, or contraction of the other, bends the 

 r , radicle so as to move the tip away from the ob- 

 FIG. 268. struction which causes the pressure. When the 



stimulating agent is moisture, however, the tip is made to turn toward 

 it by the proper bending of the stem above. The expansion of the cells 

 is accompanied, perhaps caused, by an extra amount of sap forced into 

 them, and this results from the nervous stimulation sent up from the 

 tip of the root. Experiments made by sprouting and growing beans in 

 a moist air, suspended above the ground, showed that the radicle always 

 turned toward the earth. If, after a radicle had grown to some length, 

 the bean were fastened so as to place the radicle in a horizontal posi- 

 tion, as it continued to grow it would take a downward direction, mak- 

 ing a right angle turn. So that this tip is not only sensitive to pressure 

 and moisture, but to the direction down. This last faculty must have 

 some big name, so it is called geotropism. The sensitiveness of the tip 

 of the root, is confined to a length of root not more than of an inch. 



FIG. 269. Extremity of root of growing Plantlet, greatly enlarged. 

 B.lts " Brains" at the tip. 



When that much was cut off, the sensitiveness of the root 

 and its disposition to bend upon being stimulated, was de- 

 stroyed, and was not restored for about 24 hours. The 

 complete regeneration of the tip, in the case of beans, re- 

 quires about three days. The tips were cut off 29 hori- 

 zontally extended radicles of common beans ( in a moist 

 air), and they did not bend toward the earth for 22 to 23 

 hours, whilst unmutilated radicles bowed downwards in 

 eight or nine hours. ( Darwin, Movement of Plants.) 

 FIG. 269. Darwin says : ' It is hardly an exaggeration to say that 



the Tip of the Radicle thus endowed, and having the power of directing 

 the movements of the adjoining parts, acts like the brain of one of the 



