728 PLANT RESPONSE 



doses of poison at such a temperature brings about, at least 

 temporarily, a renewal of resultant growth (p. 487). 



Another important point in the effect of temperature has 

 already been alluded to. It has been shown that a plant 

 below the optimum temperature, being in proportionately 

 sub-tonic condition, will to a very great extent, or even 

 entirely, hold the incident stimulus latent, thus increasing 

 its own latent energy. In this sub-tonic condition, then, 

 the stimulus induces little direct contractile effect, but is 

 utilised to induce the indirect acceleration of growth. At 

 the optimum temperature, however, almost the whole of 

 the incident stimulus finds expression in direct contractile 

 response, there being now little or no absorbed com- 

 ponent ; and beyond the optimum, the tissue not only 

 possesses little or no power of holding stimulus latent, but 

 its receptivity also appears to undergo great diminution 

 (p. 461). 



Responsive growth-curvature under unilateral stimu- 

 lation. — I have shown that the response of a growing is not 

 essentially different from that of a pulvinated organ. The 

 direct effect of unilateral stimulation gives rise in both cases 

 alike to negative turgidity-variation, with consequent con- 

 cavity of the side acted upon ; and the indirect effect, on 

 the other hand, consequent on the unilateral stimulation of a 

 distant point, gives rise, in both cases alike, to a positive 

 turgidity-variation, or convexity of the same side of the 

 responding region. This fact was demonstrated in the case 

 of Mimosa by applying stimulus : (i) near the motile organ, 

 in which case we obtained the direct effect by fall of the 

 leaf; and (2) at a considerable distance, when the indirect 

 effect gave rise to the erection of the leaf (p. 531). It was 

 found, however, that when the stimulus applied at a distance 

 was very strong and long-continued, true excitation was 

 ultimately transmitted by conduction, inducing excitatory 

 contraction, with fall of the leaf 



In growth-curvatures, similarly, we obtain responsive 

 movements appropriately due either to the direct or indirect 



