268 CHAP. XXVIII. GENERAL REVIEW 



Effect of chemical agents. — Excess of carbon dioxide 

 arrests the pulsation of the Desmodium leaflet and the 

 activity of growth ; it also depresses the photosynthetic 

 activity. Ether induces a depression of response in 

 Mimosa, and diminution in the rate of photosynthesis. 

 Removal of the anaesthetic is attended in both by restora- 

 tion of normal activity. 



Solution of copper sulphate depresses and abolishes 

 all physiological activity: a solution of this substance, 

 ten parts in a million, arrests photosynthesis. A solution 

 of formaldehyde abolishes the pulsations of the Desmodium 

 leaflet, the ascent of sap, and also the photosynthetic 

 activity in Hydrilla. A minute dose of a poison often 

 enhances various activities of the plant ; photosynthetic 

 activity is increased in a remarkable degree by traces of 

 certain substances, ordinarily toxic in their action. 



Modification of tonic condition by age, season and unfavour- 

 able environment. — In Mimosa moderately young leaves are 

 found to be the most irritable ; the old leaves are insensitive. 

 Similar relation between age and activity is to be found 

 in the pulsating leaflets of Desmodium, and in the photo- 

 synthetic activity of the leaves of Hydrilla. 



The physiological vigour of the plant is greatly lowered 

 after flowering, as seen in the depressed response of Mimosa, 

 in the feeble pulsation of Desmodium leaflet, and in the very 

 low rate of photosynthesis in Hydrilla. 



The different activities of the plant are higher in spring 

 than in winter. In Hydrilla the photosynthetic activity 

 in spring is nearly double that in winter. 



Under unfavourable external conditions, the tonic condi- 

 tion of the plant falls below par, with resulting depression 

 of physiological response. In such sub-tonic specimens, 

 the immediate effect of stimulus is to confer an enhanced 

 activity to the organism, so that the stimulus which was 

 formerly ineffective now becomes effective. Thus the 

 activity of sub-tonic specimens of Hydrilla becomes en- 

 hanced on stimulation. 



