SENSITIVITY IN PLANTS AND ANIMALS 61 



means always lies midway between the minimum 

 and maximum. 



Another important point which has to be noted 

 is that only very rarely does one stimulus act alone ; 

 it is generally accompanied and affected by other 

 stimuli, which may render the organism more or less 

 sensitive to the special stimulus under consideration, 

 or may affect the primary stimulus itself, either by 

 diminishing or increasing its intensity. 



The moment of application of a stimulus of any 

 kind is followed by a latent or quiescent period 

 during which no visible response can be detected. Latent 

 Doubtless, however, during this period (which may period, 

 be of longer or shorter duration) certain molecular 

 rearrangements and other changes are going on in 

 the stimulated organ in preparation for the ultimate 

 visible response. 



The sensitive body in plant or animal is in all 

 cases protoplasm that mysterious substance whose 

 analysis has as yet defied the ingenuity of chemists proto- 

 and biologists. We know only that it is an exceed- P lasm - 

 ingly complex mixture or aggregate of chemical 

 compounds, whose relationships to each other in the 

 living organism are but little known though these 

 constituent compounds must be arranged in an infinite 

 variety of ways, as may be deduced from the varied 

 behaviour of protoplasm under different conditions, 

 from an analysis of dead protoplasm from different 

 situations, from its microscopic appearance at diffe- 

 rent times, and from the mere fact that in one situation 

 it constructs a bone, in another a nerve, in another 

 a green cell of a leaf, in another a hair, and in yet 

 another an enzyme. 



Let us now turn our attention to plant Sensitivity 

 sensitivity more especially. The gradual origin o f inplants ' 



