2 3 2 THE POPULAR SCIENCE MONTHLY. 



The manner of action of the motor cells (Fig. 5, c, e) has not 

 been clearly made out. By different investigators the curvature 

 has been successively attributed to an accelerated growth of the 

 convex (upper) side of the tendril, a retarded growth of the con- 

 cave (lower) side, an increased turgor in the region of curvature 

 with an increased extensibility of the walls of the convex side, 

 and an increased turgor of the convex side only, coupled with an 

 increased extensibility of the walls of the cells on this side. But 

 recently the hypothesis that the curvature is due to the increased 

 turgor of the concave side, with an increased extensibility of the 

 walls of this side in one direction only, has been offered. Whether 

 the last offers the true solution of the problem or not remains to 

 be proved. However, it accounts for certain features of the curv- 

 ature irreconcilable with previous explanations. The conditions 

 of the motor cells after curvature may be seen in Fig. 5. 



In plants provided with cushions of tissue pulvini for the 

 purpose of rapidly displacing leaf stalks and other organs, the 

 movement is effected by a direct contraction of the cells on the 

 side toward which the organ is curved. It will be quite remark- 

 able if it should be found that the rapid movements of the " sensi- 

 tive plants " are effected by the contraction of the motor cells, 

 and the slower movements of tendrils and other organs by the 

 expansion of these cells. 



Of the ultimate molecular changes ensuing in the cells of the 

 motor or perceptive zones, as well as in the transmitting tissues, 

 nothing is known, except that in such cells the metabolic action is 

 necessarily very rapid. In this the physiologist confronts a prob- 

 lem which may not reach its final solution until the ultimate or- 

 ganization of protoplasm is at least approximately ascertained. 



The delicacy of the mechanism of irritability in certain in- 

 stances is such that the amount and intensity of the stimulus 

 necessary to secure a reaction are extremely small. It has been 

 found that tendrils would respond to the contact of a weight not 

 greater than one five-thousandth of a milligramme, and that a 

 plant in a dark chamber would curve toward the light afforded by a 

 single spark from a small condenser coil, or to a light too diffuse 

 to cast a shadow perceptible to the human eye. Some of the lower 

 free-swimming forms have been found susceptible to the presence 

 of an amount of oxygen so small as the one-trillionth part of a 

 gramme, which is very nearly the atomic limit of this substance. 

 This delicacy of perception is doubtless beyond that of the senses 

 of the higher animals. 



At all times the amount of a stimulus necessary to produce a 

 response increases with the amount previously acting upon the 

 plant. Thus a plant in total darkness will react to an extremely 

 small amount of light, as has been pointed out ; but, if placed in 



