THE PLANT AND THE ANIMAL 261 



ture : thus, for instance, the movements of protoplasm 

 can be accelerated or arrested at will by subjecting the 

 cells under investigation to heat or cold. 



We must differentiate two kinds of motion when we 

 speak of movement in the organs of the highest plants. 

 Some of them are slow and gradual ; like growth they 

 can be observed only by their results, and they generally 

 depend on the influence of variable external conditions. 

 Others are rapid and abrupt like the movements of 

 animals ; and, as in animals, they are either provoked by 

 external irritation, or take place without any stimulus 

 whatever, apparently quite spontaneously. 



Phenomena known under the name of sleep movements 

 belong to the first class. We mean by these the changes 

 in position of leaves and parts of flowers at different 

 hours of the day and night which are manifested by 

 nearly all plants, but most obviously by some of them. 

 If you look at a field of pink clover, you will get a different 

 impression of it according to the time of day. In the 

 day-time its surface will be more uniform, because the 

 leaflets are almost horizontal, and catch the light falling 

 right down upon them with the whole of their surface 

 turned up to the sky. In the twilight, on the contrary, 

 the surface of the field will look dishevelled, and if we 

 examine more closely the separate leaflets, we shall 

 notice that all the three blades of the leaf are raised ; 

 they are now turning their edges instead of their 

 surface up to the sky ; the two side blades are folded 

 together, while the third is pressed against their common 

 edge. Other plants have the lamina of their leaves 

 bent down in the nocturnal position, and so appear as 

 if withered ; in this case of the clover, on the con- 

 trary, they are raised, and so it is clear that we have 

 to do with quite a peculiar mechanical phenomenon. 



Sleep movements in flower organs are even more 

 distinctly seen. Thus, for instance, in the early morning 

 or in the twilight we do not see any of the yellow heads 



