THE FOOD OF PLANTS 433 



which are stored away in the substance of the plant for the 

 nutrition of embryo, bud, or growing point. 



This may be made clearer by examining whether these 

 simple inorganic materials are capable of nourishing protoplasm 

 when freely supplied to it. If they are the true food, plants 

 everywhere should be able to make use of them. But if we 

 consider only one of them, the CO^ of the air, we find this is not 

 the case. The plants which are not green, that is, which con- 

 tain no chloroplasts, can do nothing with this gas. So long as 

 a seed is in the early stages of its germination it is surrounded 

 by COo which is given off by its own protoplasm. Eut it can 

 make no use of it, and if the store of nourishment provided for 

 it in the endosperm or cotyledons is out off, it inevitably dies 

 of starvation. 



If a green plant, which is in good health and endowed with 

 ample vigour, is removed from light to darkness, though 00.^ 

 be supplied in appropriate quantity, it can make no use of it. 



So with other constituents of the materials from which the 

 true food of the living substance is elaborated. They are 

 ■ absorbed in quantity, but they are not food until a con- 

 siderable amount of work has been done upon them by the 

 plant itself. 



In connection with the nutrition of plants we have thus to 

 deal with the absorption of the crude food materials and to study 

 the changes which they undergo after such absorption. But 

 this is not all ; the food which is laid up in seeds, tubers, bulbs, 

 &e., is not deposited there in exactly the condition in which the 

 living substance requires it, bo that there remains for us to con- 

 sider the processes which these materials also undergo for the 

 purpose of feeding the living protoplasm. The first process is 

 one of building up complex bodies from simple materials ; the 

 second is comparable with the digestion which is so marked a 

 feature of animal alimentation, and is one of breaking down 

 very complex bodies into simpler ones. The nutrition of the 

 protoplasm shows two similar phases ; the absorption of the 

 ultimate constituents of the food, or its assimilation, is a con- 

 structive process ; it is ui turn associated with a destructive 

 - one, by which, from the protoplasm itself and by its own activity, 

 simpler bodies are produced. The whole round of changes 

 which embraces all these operations is called metabolism, the 

 constructive processes being grouped together under the name 

 of anaboUsm, the destructive ones under that of catabolism. 



The absence of well-differentiated organs set apart for the 



VOL. 11. F F 



