IRRITABILITY; POLARITY AND CORRELATION 351 



plant should turn in one direction more than in another, and, 

 while the members of the same species usually twine in the same 

 direction, closely allied species may twine in the reverse fashion. 

 There are some species which twine in either direction, and, in a 

 few cases, the individual plant may change its direction during 

 the course of its development. 



If the plant is rotated on a clinostat, the twining ceases, show- 

 ing that the growth of one side more than the other, which pro- 

 duces the twining, is in some way related to the stimulus afforded 

 by gravity, although the progress of this growing region about 

 the stem, now on this side and now on the other, is regulaced in- 

 ternally. There are details in the mechanism of twining which 

 still remain to be solved, although negative and lateral geotropism 

 are known to play a very important role. 



The Sensitive Plant. — Mimosa pudica, called the sensitive 

 plant, is incorrectly named because it is not more sensitive than 

 other plants. Every plant is a sensitive plant; if it were not it 

 would be dead. The difference lies in the fact that Mimosa 

 seems to have an exceptionally rapid transmission mechanism and 

 an end response which may be much more clearly seen than 

 in most plants. In this member of the Leguminosse the leaf is 

 twice compound. The petioles and stems contain an abnormally 

 large cortex as compared with the wood and pith, and at the 

 base of each petiole and petiolule, outside the vascular tissue, is 

 an extra layer of parenchymatous tissue which makes a swollen 

 place called the pulvinus (Fig. 25). These pulvinus cells are 

 more than ordinarily large, are rich in water, and are interspersed 

 with abundant intercellular spaces. 



If the leaflets are stimulated by pressure, heat, anaesthetics, 

 etc., they quickly droop. This stimulus from the leaflets is rapidly 

 transmitted at the rate of 1-3 cm. per second to the large pul- 

 vinus at the base of the petiole, when the entire leaf then droops. 

 The stimulus may even travel both up and down the stem to the 

 adjacent leaves. After a time the normal position is resumed. 



The drooping is associated with changes in the tissue of the 

 pulvinus, the cells of the lower half of which are thinner walled 

 and have more intercellular spaces than those in the upper half. 

 If the lower half is cut through as far as the vascular tissue, the 

 leaf falls never to rise again; while if the upper half is cut, the 

 leaf falls but rises higher than ever, which shows that the lower 



