CONDITIONS FAVOURABLE TO MECHANICAL RESPONSE 47 



is not a passive but an active process ; for when the tissue is 

 killed it remains flaccid. Throughout the phenomenon of 

 response, the essential factor is the variation of turgidity 

 from, and its return to, the normal. This variation, as 

 generally seen, consists of a fall below, and recovery to, the 

 original level of turgescence. And this, as we have seen, is 

 accompanied by the sequence of the fall and rise of the leaf. 

 But theoretically it should be quite possible to bring about a 

 responsive movement by means of the counter-variation, 

 namely, an increase, followed by diminution of turgescence. 

 In such a case the concomitant movement would be a rise 

 and fall, instead of the opposite. Something of this kind will 

 be observed in studying the daily periodic movements of the 

 Mimosa leaf, where the rise and fall of the leaf will be found 

 to synchronise with alternating increase and diminution of 

 hydrostatic pressure. 



Abnormal hydrostatic and true excitatory effects. — 

 Certain effects due to the variation of turgor above and back 

 to the normal, will be seen in growth-responses, to be treated 

 later. For the present we may find an instance in the 

 abnormal erectile twitch, which has already been noted, in 

 certain responses of BiopJiytum (fig. 17). In that case, as 

 was explained, the pulse of increased pressure, due to the 

 expulsion of water from the distant stimulated point, was 

 the first to reach the motile organ, causing erection. The 

 true excitatory effect reached the same organ later, with the 

 normal effect of depression. The abnormal erectile effect 

 may be produced artificially, say by sudden forcing in of 

 water. But, in the case mentioned, the pulse of increased 

 pressure which brought about this effect was due to stimula- 

 tion of a distant point. I shall henceforth, as stated before, 

 distinguish the two effects as (a) the direct and (&) the 

 indirect effects of stimulation. When a tissue is directly 

 stimulated, there is produced a negative turgidity-variation ; 

 normal negative mechanical response, or fall of the motile 

 leaf; and normal electric response of galvanometric nega- 

 tivity. The velocity of transmission of this excitation is 



