344 FRUITS AND SEEDS 



of the carbohydrate class, and that, by use of these products, 

 and the addition of other substances, all other foods are 

 built up. You learned something of the nature of car- 

 bohydrates, starch and sugar being described as examples 

 of this class of substances. (See page 230.) 



Further consideration of the classes of foods was post- 

 poned till now, for the reason that it is in seeds that the 

 most conspicuous accumulation of foods occurs. Foods 

 are present in all living parts of plants, and often they are 

 conspicuously stored in stems and roots, but it is the seeds 

 that are more generally filled with food than any other 

 plant organ. 



Life is best nourished by the use of all three kinds of 

 foods. This you have probably already learned in your 

 study of the human body. Both carbohydrates and pro- 

 teins are essential to life, and, under certain conditions, 

 fats are also necessary. Without proteins, protoplasm, the 

 living stuff, cannot be manufactured. 



Proteins, unlike the fats and carbohydrates, contain 

 nitrogen. Nitrogen is absolutely essential to their forma- 

 tion. Evidently, then, nitrogen is an element of great 

 importance both to plants and animals ; their life is impos- 

 sible without it. Nitrogen is abundant. Four fifths of 

 the air is composed of it. But very few forms of life, only 

 some of the smaUest and simplest plants, are able to make 

 use of this atmospheric nitrogen. Green plants obtain 

 their nitrogen, not from the air, but from the soil. They 

 get from the soil certain soluble compounds called nitrates. 

 Molecules of nitrates are absorbed by root-hairs along with 

 the water which they take in. Atoms of nitrogen are 

 present in the molecules of nitrates. It is largely in order 

 to maintain the supply of nitrates in the soil that farmers 



