178 THE FORCE INCLUDED IN PLANTS. [MEMOIR XI. 



olive-oil ; indirectly, as in the case of animal oils and fats. 

 These have been collected by the animals from which 

 we obtain them out of their vegetable food. Even fats 

 derived from the carnivora have been procured from the 

 herbivora, and came originally from plants. 



This brings us therefore to a consideration of the 

 chemical facts connected with the life of plants. 



If a seed be planted in moist earth, the air having ac- 

 cess and the temperature that of a pleasant spring day, 

 germination in the course of a few hours will take place. 

 Should the process be conducted in total darkness, as in 

 a closet, the young plant shoots upward, pale, or at most 

 of a faint tawny tint. We can easily verify this state- 

 ment by placing a few turnip seeds in a flower-pot con- 

 taining earth, put into a closet or drawer from which 

 light has been carefully excluded. 



A sickly-looking plant thus springs from a seed in the 

 dark. It is etiolated, as botanists say. If we examine 

 it carefully, making allowance for the water it contains, 

 we shall find that no matter how tall it may be, its 

 weight has not increased beyond the original weight of 

 the seed from which it came. It has been developing at 

 the expense of the seed, the substance of which has been 

 suffering exhaustion for its supply of nourishment. We 

 cannot continue this development in the dark indefinite- 

 ly, for the seed-supply is soon exhausted, and then the 

 shoot dies. 



But if instead of exposing the seed which is the sub- 

 ject of our experiment to darkness, we cause the germi- 

 nation to take place in the open day, a very different 

 train of consequences ensues. There is no longer that 

 immoderate extension of a sickly etiolated stem upward, 

 but the parts emerging into the light turn green. Very 

 soon, to use a significant expression, they are w r eaned 

 from the seed ; they no longer use the material collected 



