148 CHAP. XVIII. THE TONIC FACTOR IN PHOTOSYNTHESIS 



stimulation, and also to the overloading of the cells with 

 starch-grains formed under strong light, of which there may 

 be as many as forty in each cell. In the analogous case 

 of charging a storage-cell, a fully charged battery exhibits 

 increased counter-electromotive force which opposes a 

 charging current. Similarly the overloading of a vegetable 

 cell with photosynthetic products may to a certain extent 

 inhibit further formation. This inference finds support in 

 the following experiment. 



Some of the sun-exposed plants which had exhibited 

 relatively feeble photosynthesis were kept for two days in 

 a semi-dark room. Microscopic examination showed that 

 the starch-grains which were present in such abundance had 

 practically disappeared. The specimens depleted of starch 

 were now found to exhibit considerable increase in the rate 

 of evolution of oxygen. 



Effect of Stimulus in modifying the Tonic Condition 



External stimulus is found to modify the physiological 

 tone of the plant in a definite manner. I have shown that 

 external stimulus brings about both the anabolic and 

 catabolic changes A and D, the resultant effect being that of 

 their algebraical summation. The up-change A is associated 

 with an increase of the potential energy of the system, while 

 the down-change D is attended by a run-down of energy. 

 The tonic level is raised when A is greater than D. 



The relative intensity of the A and D reactions is also 

 modified by the existing tonic condition of the plant : in a 

 condition above par, D is relatively predominant, whereas 

 in a condition below par, or of sub-tonicity, A is the more 

 pronounced. 



These conclusions are demonstrated by the following 

 typical results. 



(a) Effect of stimulus on mechanical response. — There are 

 three characteristic types of response which I have obtained 

 with Mimosa. 



