8o PLANT RESPONSE 



the opening of the flower, the excitatory contraction is also 

 at its maximum. 



In order next to determine the effect of season on the 

 excitatory contraction, we shall compare the mean percent- 

 age of contraction in winter with that obtained in spring, in 

 the month of April. The average contraction in winter may 

 be taken as J'6 per cent. But in spring I obtained (i) with 

 a specimen 12 mm. long, a contraction of 17 mm., i.e. 14 per 

 cent. ; and (2) with a second specimen 10 mm. long, a con- 

 traction of 2 mm., i.e. of 20 per cent. The mean of these, 

 17 per cent, representing the contraction in spring, is thus 

 seen to be about two and a half times that obtained in 

 winter. 



We have now ascertained the universal occurrence of 

 longitudinal contraction in the organs of plants, and we have 

 seen that lateral response cannot take place under diffuse 

 stimulation in a strictly radial organ, owing to the antago- 

 nistic character of the equal and simultaneous responsive 

 contractions on diametrically opposite sides. 



Lateral response, however, as we have seen, will take place 

 in a radial organ when stimulus is not diffuse, but unilateral. 

 Such lateral response is only possible under diffuse stimulus, 

 when the excitability of the two opposite halves is different, 

 that is to say, when the organ is anisotropic. In such cases, 

 as in the petioles of leaves, for example, the very striking 

 lateral movement is simply the result of differential longi- 

 tudinal contraction. The differentiation of the upper and 

 lower halves is anatomically evident in the case of dorsi- 

 ventral organs. But the anisotropy may often be undistin- 

 guishable to the eye. For an organ, originally radial, may 

 become molecularly bilateral, owing to the unequal action of 

 external forces on diametrically opposite sides. It will be 

 shown in the next chapter that such molecular differentiation 

 gives rise to physiological differentiation, and there I shall 

 be able to trace a continuity between the longitudinal 

 response of radial organs and the lateral response of dorsi- 

 ventral organs through intermediate types. 



