1022 



THE NATURE BOOK 



a retarding influence on growth, and it 

 lias been suggested that plants lean 

 towards the point of illumijiation, because 

 the shady side of the stem extends more 

 rapidly than the part which is strongly 

 lighted. This theory does not help us to 

 understand how it is that parts of the 

 plant avoid the light. One cannot but 

 feel that in all these phenomena there is 

 something: more than a mere mechanical 



\ 1 \. ^ t 1.1-TKAP, OPEN. 



adjustment, and that most of these 

 responses must be the outcome of inherent 

 tendencies which exist in the plant. \\'hilst 

 a little observation destroys the illusion 

 that the Sunflower always follows the 

 sun, there is no doubt that the plant, 

 in common with a large number of species, 

 does move its blossoms towards the 

 glf)wing orb. This is perhaps particularly 

 noticeable in the case of composite plants. 

 The Common Hawkweed, which grows 

 so abundantly in our fields, leans its 

 blossoms over very distinctly to the sun. 

 Thus an observer standing with his back 

 to the south sees the field as a blaze of 

 gold, but turning in an opposite direction 

 there is nothing to be noticed save the 

 backs of the flowers. Much the same 



kind of thing is to be seen in many species 

 of Anemone. On a brilliant day it is 

 obser\-ed that the copse appears to be 

 starred all over \\'ith the white flowers 

 of the Wood Anemone, but with a slight 

 change of position the flowers seem to 

 disappear, simply because the blossoms 

 are all fronting in one direction. 



The Sensitive Plant is a species with, 

 many interesting points. This curiosity 

 goes to sleep every 

 night, but, of course, 

 the most striking fea- 

 ture of the plant is its 

 extreme sensitiveness 

 to contact. As is so 

 well known, the light- 

 est touch is sufficient 

 to make the leaflets 

 close upon their sup- 

 port, the petiolules to 

 draw up together, and 

 tlie leaf-stalks to col- 

 lapse. The most sin- 

 gular feature is that the 

 shock may be com- 

 municated to other 

 parts of the plant, and 

 even, at times, to the 

 whole specimen. The 

 Sensitive Plant has 

 something which is 

 strangely like a rudi- 

 mentary nervous sys- 

 tem, in that it would 

 seem that there is a 

 continuity of suscep- 

 tible matter through- 

 out tlie whole specimen. There is little 

 doubt that the contractile power resides 

 in small C3'lindrical cushions — pulvini — 

 which occur at the points of insertion 

 of the leaf-stalk with the stem. 



A pulvinus is a process containing a 

 woody centre surrounded by spongy 

 cells rich in water. Now, when one 

 of the leaflets of the Sensitive Plant 

 is touched, the effect is transmitted, 

 })robably l")y the threads of protoplasm 

 l>assing through the cell wall, to the 

 pulvinus. 



The outcome of the reception of the 

 shock is that the water passes from 

 the cells on the lower side of the process 

 to those in the upper part, and, as a 

 consequence, the former portion becomes 



