SENSITIVE MECHANISMS 99 



fact that the movements of the Mimosa and of the Tamarind 

 are less well-marked at a temperature lower than that in which 

 the plants have been reared. Hill considered that " This is 

 probably due to the juices stagnating in the clusters of fibres, 

 and to the contraction of the bark by cold." His explanation 

 of the response to the contact stimulus is of course quite wrong ; 

 it may, however, be quoted as an illustration of the view, current 

 at that time, that such motion was due to the fibres which acted 

 like those of muscle. " The vibration of the parts is that which 

 keeps the leaves of the sensitive plant in their expanded and 

 elevated state : this is owing to a delicate motion continued 

 through every fibre of them. When we touch the leaf, we give 

 it another motion more violent than the first : this overcomes 

 the first : the vibration is stopped by the rude shock : and the 

 leaves close, and their foot stalks fall, because that vibrating 

 motion is destroyed, which kept them elevated and expanded.... 

 That the power of motion in the sensitive plant depends upon 

 the effect of light on the expanded surface of the leaves, is 

 certain; for till they are expanded, they have no such power. 

 The young leaves, even when grown to half an inch in length 

 have no motion on the touch, tho' rough and sudden." 



Hill fully appreciated the importance of comparative observa- 

 tions ; he compared the movements, in response to light, of Abrus 

 and Mimosa^ which plants he placed side by side so that the 

 conditions of the experiment might be the same for each. He 

 found that " In these and in all others, the degree of elevation 

 or expansion in the lobes, is exactly proportional to the quality 

 of the light : and is solely dependent upon it." 



Reference also may be made to Hill's views on reproduction 1 ; 

 he considered that the pollen grain contained the embryo which 

 was set free by the bursting of the grain after it had been 

 deposited upon the stigma. The stigmatic hairs or papillae 

 were supposed to be the ends of tubes into which the embryos 

 entered, made their way into the placenta, and thus arrived into 

 the " shells of the seeds " (the ovules). It is unnecessary to point 

 out the absurdities of these ideas, but it may be mentioned that 

 Hill's interpretations of his observations were at fault rather 



1 Outlines of a System of Vegetable Generation^ London, 1758. 



72 



