63O PLANT RESPONSK 



former instances, I shall here refer once more to the 

 case, already mentioned, of the heliotropic response of the 

 terminal leaflet of Desmodium y which in winter is always 

 positive, but in a given experiment, under the conditions of 

 greater transverse conductivity which are brought on in 

 summer, exhibited, after two hours of continuous exposure to 

 vertical sunlight, a neutralisation of the previous positive 

 effect (p. 604). 



{a) Response of pulvinus of Mimosa. — The second type of 

 response will be the better understood if we first study in detail 

 all its characteristics. As I have already said, these negative 

 heliotropic responses result from the transverse diffusion of 

 stimulus across the tissue, which brings about the concavity of 

 the more excitable lower half of the organ. We can easily, 

 in the continuous response-record to be given presently, 

 detect this gradual process of the percolation of stimulus 

 through the tissue, when light is applied from above. As 

 there is in the case of Mimosa a considerable mass of inter- 

 vening tissue between the upper and lower surfaces of the 

 pulvinus, it follows that unless the stimulus applied be 

 excessive, there will be a certain interval of time required 

 for its passage. We should therefore expect that on the 

 application of light to the dorsal surface there would be a 

 local contraction and concavity of that side, raising the 

 leaf up, and causing a preliminary positive response ; but 

 this movement will be arrested, and gradually reversed, so 

 soon as the stimulus reaches the lower side, and begins to 

 induce antagonistic contraction there ; and after this, the 

 greater excitability of the lower will be manifested by its 

 greater contraction, as seen in the negative response, or 

 depression of the leaf. All this will be clearly understood 

 from the series of records given below. 



It is necessary, however, before describing the effect of 

 the continuous application of light from above, to analyse 

 the responsive sensibilities of the two sides of this organ ; 

 and this is the more desirable since, in the case of Mimosa, 

 it is commonly assumed that excitability characterises only 



