AIR AND SOIL IN RELATION TO SPROUTING 31 



This shows not only whether a given element is found to be 

 necessary or not, but also in what particular way it is related 

 to the life of the plant. 



49. The composition of plants. Another method used for 

 determining what there is in the soil that the plant depends 

 upon for its activities has been to analyze the plant to find out 

 of what it is composed. Such an analysis shows that certain 

 elements are present in the plant body, and we know that some 

 of these elements are present also in the soil. It is therefore 

 reasonable to suppose that the plant derives these elements 

 from the soil. It does not follow, however, that everything 

 taken by the plant from the soil is of use to the plant. The 

 most common elements found in plants are the following : 



Carbon Sulfur Potassium 



Oxygen Phosphorus Sodium 



Hydrogen Calcium Iron 



Nitrogen Magnesium Chlorin 



(Compare with the composition of the human body, Fig. I, 

 p. 1 6, to see how much we are like the plants.) 



Other elements may also be found in some plants, as silicon 

 and iodin ; but it is doubtful whether these are essential to 

 the life of the plant. Indeed, not all of those given in the 

 above list may be absolutely necessary, but most of them cer- 

 tainly are. Since a large part of the plant's life consists of 

 growing activity, the material for growth or for building up 

 the body must be a first condition of life. 



The materials taken from the soil by the growing plant are some- 

 times called plant food. Strictly speaking, these are not food, as we 

 shall see later (see p. 50) ; they are merely some of the materials 

 out of which plants manufacture their food. 



