424 



THE GUIDE TO NATURE. 



other as it appears when pressed be- 

 tween two slips of glass. 



AY hen I spoke above about stimu- 

 lating a certain portion of the petiole, 

 and having that act followed by a fall- 

 ing" of the leaf and the two leaflets, I 

 was referring to that special portion 

 which botanists call the pulvinus. 

 This region is a thick, cylindrical swel- 

 ling" at the bases of the petiole and the 

 stalks of the leaflets. It is at or 

 through these pulvini that the results 

 of the stimulation are exhibited, for it 

 is at these points that the flexure takes 

 place. The mechanism of this is un- 

 derstood, but not the method by which 

 the transmission of the influence is 

 accomplished. This remains as much 

 of the mystery as it was when first 

 observed. The leaves and their parts 

 fall because a sudden change takes 

 place in the water-pressure within the 

 cells that form the pulvini. When the 

 cells on one side of the pulvinus give 

 off water, which passes into the spaces 

 between the cells, a curvature toward 

 this side results from the unchanged 

 pressure of the turgid cells of the op- 

 posite side. The sinking" of a leaf upon 

 its petiole is due to the relaxation of 

 the cells of the lower side of 

 the pulvinus. When these cells 

 reabsorb the water from the intercel- 

 lular spaces, their former size and shape 

 are slowly regained, and the leaf is re- 

 turned to its former position. 



The manner by which the irritation, 

 or the stimulation, is conveyed from 

 one part to another, is not known. 

 Many and various theories have been 

 offered in explanation, but none is en- 

 tire!) satisfactor}'-, and none responds 

 properly to actual test. The stem may 

 be girdled, as has been clone, yet the 

 influence passes that part about as 

 successfully as it passed the region 

 when it was intact and uninjured, thus 

 proving thai the transmission Lakes 

 place through the wood. 



Several sections of tin stem of the 

 same plant have been killed by the ap- 

 plications of bandages kept saturated 

 foi several minutes with boiling water. 

 in such a case, too, the transmission 

 is ma hindered, thus proving that dead 

 wood is no barrier. 



The continuity of protoplasm is a 



beautiful structure, readily visible with 

 the microscope, in the tissues of vari- 

 ous plants, the colorless protoplasm 

 passing through the cell wall in the 

 form of delicate threads and processes, 

 to join similar filaments on the oppo- 

 site side of the same cell wall. One of 

 my friends who had become greatly 

 interested in this microscopic struc- 

 ture, imagined there might be a con- 

 tinuity of protoplasm in the cells ot 

 Mimosa, and as at that time he had no 

 Mimosa within his reach, and no 

 means of cultivating it, he sent to 

 Paris for a microscopical preparation 

 of a section of the stem, hoping, that 

 by a fortunate chance, he might see 

 what he thought might exist. But the 

 dealer had thought more of the beauty 

 and the neatness of the section, than 

 of its value as a scientific object. The 

 consequence was that every cell was 

 as empty as any other vacant space, 

 and the preparation was valueless. Ii 

 those readers who have access to a 

 number of sensitive plants, and have 

 the ability to cut and prepare micro- 

 scopical sections, would bear this in- 

 ference in mind, it would be interest- 

 ing to know whether or not such a 

 continuity exists. 



It is not believed nor imagined that 

 the motor influence is transmitted 

 through these continuous threads of 

 protoplasm, provided they are present, 

 but the thought is suggestive, and to 

 know whether or not such continuity 

 is present, would be an acceptable ad- 

 dition to our knowledge. 



The motor influence may. in vigor- 

 ous plants, be transmitted for a dis- 

 tance of about seventy centimeters (or 

 about twenty-eight inches), at the rate 

 of from one-third to one inch a second. 



The utilitarian purpose for which 

 the plant is supposed to have devel- 

 oped its sensitiveness to shock or to 

 touch, is said to be for protection, es- 

 peeialh from browsing animals. It 

 grows in great profusion in tropical 

 countries, especially on the plains of 

 "Brazil and of Venezuela, where it 

 forms immense patches. "A recent 

 writer says: "When a browsing ani- 

 mal approaches a clump of Mimosa 

 and agitates any part of it at all 

 strongly, the green appearance disap- 



