102 GLIMPSES OF NATURE. 



Within them also is the living matter which con- 

 stitutes the sum and substance of the plant's activities. 

 A living plant cell is in a state of constant change. It 

 is perpetually receiving, and as constantly giving off, 

 water ; and to the alteration of the cells of a leaf, a 

 tendril, a flower-stalk, a stamen, or a leaf-stalk are 

 due all the movements of plants. Now, in producing 

 these alterations of cells, and in favouring cell-changes, 

 light plays a first part. 



Thus it is that the " heliotropism " of the botanist 

 comes to be regarded as a powerful factor in inducing 

 change in the vegetable world, and in carrying out 

 the characteristic habits of each species. Left to 

 themselves, in darkness, for instance, cells breed and 

 multiply. There is no light to interfere with this 

 dead level of cell-duty, as it were. That is why most 

 plants really grow most rapidly in the night-time, and 

 this even though the fall of the temperature is against 

 quick increase. 



The effect of light may be viewed as an interference 

 with the process of growth in the cells of plants. If 

 we return to our old friend the sunflower, we may be 

 able now to explain why it follows the sun in his daily 

 course. The effect of the light, which pours its rays 

 against one side of the plant, in contradistinction to 

 the other side, is to cause changes in the cells of the 

 ormer side. There is set up an alteration of vital 

 activity : growth recedes, as a power, into the back- 

 ground of things, and the result is to produce a 

 curvature towards the light. 



Thus, between light and darkness, our plants allow 

 their complex lives to swing. But the instincts of the 

 light may be paralleled by those of the darkness as 

 regards plants. The young root, emerging from the 



