220 Causes and CorRSE of Organic Evolution 



w-ise in the same position or relation, is more instructive. Here 

 the gro^-ing stem apex, in elongating further, has responded 

 apogeotropically by curving upward, but not vertically so, 

 for heliotropism has also been at work, so the resultant direc- 

 tion seen in this figure has been taken. The varying angular 

 positions, assumed by the leaf stalks as well as the primary 

 and secondary leaflets, are most instructive, as exhibiting 

 every phase of resultant response between diageotropism and 

 diaheliotropism, in which however the more important latter 

 phase for the leaf surpasses the less important former one. 



But on bright hot days a third factor may be made evident 

 in resultant movements of the leaflets, namely parahelio- 

 tropism, or the effort on the part of leaflets to so tilt upward 

 that the intense illumination by blue-violet rays may be min- 

 imized. Thus when the sky is clear and the light intensity — 

 as roughly estimated by shade temperature — accompanies a 

 temperature of 32-35° C. the secondary leaflets become slightly 

 moved upward. With increasing light of blue- violet quality 

 the upward angle increases until a resultant between diageo- 

 tropism, diaheliotropism, and extreme paraheliotropism is 

 attained, and in which the leaflets have risen through an angle 

 of 40-45°. Then a temperature of 38-40° C. is usually re- 

 corded. 



Again the same leaflets show varying degrees of response 

 to thermic, mechanic, and chemic stimuli, in which each may 

 act separately, or one may, as in Jennings's results, overpower 

 another, or all by skillful adjustment to degree-intensity of 

 stimulation may form a resultant response. 



As the author also pointed out eighteen years ago (81)y 

 and has greatly extended since in his lectures on "Plant Irri- 

 tability," the species of each genus of sensitive plant exhibit 

 graded specific differences in perception of and in response to 

 environal stimuli, that cause corresponding differences in the 

 resultant reactions. Thus Mimosa Spegazzmi is the most 

 sensitive plant known to him at present. M. pudica is slightly 

 less sensitive, 31. Iwpulina, M. latispina, M. sensitiva or alhida 

 is each less sensitive than its predecessor in the list, till through 

 a series of other species we reach M. argentea that would in 

 no way be regarded as a sensitive plant in ordinary parlance. 

 Similar graded lines of sensitivity have been constructed by 

 the writer for the species of Cassia, of Desmodium, of Biophy- 

 tum, of Oxalis and other genera. Comparative studies of 

 the relative sensitivity of such genera will form a valuable 

 physiological and possibly phylogenetic contribution for the 

 future. 



