THE FOOD OF PLANTS 81 



CHAPTER IX 



THE FOOD OF PLANTS 



WE have in our introductory chapter considered in out- 

 line the important question of the nutrition of plants, a 

 subject which has been treated of also in Chapter VI. of 

 the primer of Biology. It is necessary now to return to 

 this subject and examine it a little more fully. 



The substances that are taken in by a plant are the 

 carbon dioxide which is present in the air, and the water 

 and dissolved mineral matters which the roots obtain 

 from the soil. We must again emphasise the fact that 

 these materials are not capable of serving as food in the 

 condition in which they are absorbed, but that a great 

 deal of work has to be carried out to convert them into 

 nutritive material. It is only the green plant which can 

 build them up into such compounds as the living substance 

 can incorporate into itself, the work being effected by 

 the chloroplasts, the little ovoid bodies which are the 

 seat of the green colouring matter. Further, the chloro- 

 plasts can only work when they are properly illuminated. 



The carbon dioxide exists in very small proportion in 

 the air, not more than about 3 parts being present in 

 10,000. The gas enters the plant by way of the stomata, 

 and so makes its way into the intercellular spaces 

 whence it obtains access to the cells in which the chloro- 

 plasts are present. The water from the soil is conducted 

 in the way we have described to the same cells, continu- 

 ally replenishing the supply in their vacuoles. We 

 have thus present in the cells of the parenchyma of the 

 leaf a supply of carbon dioxide, water, and the chloro- 

 plasts themselves. When sunlight shines upon the 

 leaves in appropriate intensity the constructive action 

 commences. The stages are not yet fully understood, 

 but there appears to be no doubt that in some way the 

 chloroplasts cause a certain chemical action to be set 



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