CHAPTER XV 

 THE NUTRITION OF THE PLANT 



1. A Plant absorbs (feeds itself). — If healthy seeds be sown 

 under suitable conditions, they germinate, and finally give rise 

 to plants much larger than themselves. A small acorn de- 

 velops into a huge oak-tree : a tiny turnip-seed produces a large 

 turnip-plant. The plants obviously weigh more than the seeds 

 from which they have developed, and they must therefore con- 

 tain more matter or substance than was originally possessed by 

 the seeds. This substance gained by the plant during its 

 growth has not been created out of nothing ; it must have been 

 derived from matter previously present outside the plant. Thus 

 it is certain that the plant takes in, or absorbs, substance from 

 the outside world — that is, takes food from the soil or from the 

 atmosphere, or from both. 



2. What does a Plant absorb ? A seed, or a whole plant, 

 is composed of solid substance and water ; in addition it con- 

 tains gases, which we will not discuss for the present. If we dry 

 a seed, or a plant, at a temperature slightly higher than the 

 boiling-point of water (ioo° Centigrade), the water will be 

 driven off in the form of steam, and only solid substance 

 will remain. An ordinary seed, though it appears to be quite 

 dry, contains at least one-tenth of its whole weight of water, 

 whilst there is still more water in growing plants ; for instance, 

 nine-tenths of the weight of a turnip - plant is due to the 

 water. If we weigh separately a plant and one of its seeds, 

 both before and after drying them, we find that there is more 

 water and more solid substance in the plant than in the seed. 

 We thus see that as a plant grows it absorbs not only water 

 but also other substances. 



3. Chemical Composition of a Plant. — In order to learn 

 what substances, in addition to water, are taken into the plant, 

 we must find out what simple chemical substances {elements) 

 axe present in the plant. If we still further heat a completely 

 dried seed or plant there will be an additional loss in its 

 weight, because some of the substances composing the plant 



