170 PLANT-LIFE. [LESSON 27. 



the particles of matter, arranges them, and shapes the fabric in a 

 manner which we cannot at all explain by any mechanical laws. 

 The organs are not shaped by any external forces ; they shape 

 themselves, and take such forms and positions as the nature of 

 each part, or the kind of plant, requires. 



490. Special Movements, Besides growing, and quite independent 

 of it, plants not only assume particular positions, but move or bend 

 one part upon another to do so. Almost every species does this, as 

 well as what are called sensitive plants. In springing from the seed, 

 the radicle or stem of the embryo, if not in the proper position 

 already, bends itself round so as to direct its root-end downwards, 



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and the stern-end or plumule upwards. It does the same when 

 covered so deeply by the soil that no light can affect it, or when 

 growing in a perfectly dark cellar. But after reaching the light, 

 the stem bends towards that, as every one know r s ; and bends 

 towards the stronger light, when the two sides are unequally ex- 

 posed to the sun. It is now r known that the shoot is bent by the 

 F^ortening of the cells on the more illuminated side ; for if we split 

 the bending shoot in two, that side curves over still more, while the 

 opposite side inclines to fly back. But how the light causes the 

 cells to shorten on that side, \ve can no more explain, than we can 

 tell how the will, acting through the nerves, causes the contraction 

 of the fibres of the muscles by which a man bends his arm. We 

 are sure that the bending of the shoot has nothing to do with 

 growth, because it takes place after a shoot is grown ; and the del- 

 icate stem of a young seedling will bend a thousand times faster 

 than it grows. Also because it is yellow light that most favors 

 growth and the formation of vegetable fabric, while the blue and 

 violet rays produce the bending. Leaves also move, even more 

 freely than stems. They constantly present their upper face to the 

 light ; and when turned upside down, they twist on their stalks, or 

 curve round to recover their original position. 



491. Many leaves make other and quicker movements, as is seen 

 in what has been called the sleep of plants. That is, they change their 

 position as night draws on, and in different ways, according to the 

 species, the Locust and Wood-Sorrel turning down their leaflets, 

 the Honey Locust raising them upright, the Sensitive Plant turning 

 them forwards one over another ; and the next morning they resume 

 their diurnal position. One fact, among others, showing that the 

 changes are not caused by the light, but by some power in the plant 



