72 PLANT PHYSIOLOGY 



the summit of the series of constructive processes carried on 

 by the plant, into the living protoplasm, results in making 

 them parts of the living substance. These molecules, the 

 buUding up of which we have tried to trace, do not them- 

 selves necessarily become alive when they are made parts of 

 the living substance, and yet these molecules recently formed 

 and others similar to them constitute the physical basis of 

 life, the living protoplasm. The new molecules incorporated 

 into and made parts of the living protoplasm are not given 

 any new or peculiar vital force, vis liaturse, or any other 

 occult power, but are so distributed in space, have such rate 

 and amplitude of movement, that under the conditions pre- 

 vailing in the cell— the conditions which make life possible 

 anywhere — they behave as do the old and are as the old. 



The majority of chlorophyll-containing plants depend, 

 as we haA'e seen, upon nitrates for their supply of nitrogen. 

 In the laboratory, ammonia salts and even ammonia 

 vapor may be made to serve as the source of nitrogen, but 

 in nature they are not the immediate source for green 

 plants. Besides the nitrates and ammonia, animals and 

 plants may obtain needed nitrogen from three sources, 

 namely, {a) in the uncombined state from the air, ( b ) 

 from its compounds in excrementitious matter of animals, 

 and in dead bodies of animals and plants, ( c ) from its com- 

 pounds in living bodies. The last two sources {h and c) 

 are drawn upon by dependent organisms — animals and 

 saprophytic and parasitic plants. The first is used by bac- 

 teria, living either by themselves in the soil or associated 

 with higher plants in special outgrowths of their roots. 



ROOT-TUBERCLE PLANTS 



For centuries it has been the profitable practice of farmers 

 to use leguminous crops to enrich impoverished soils. A 

 worn-out field will more rapidlj^ recover its fertility if sown 

 to clover than if allowed to lie fallow. Sowing to clover and 

 plowing the crop under will evidently enrich the soil more 

 than mowing it and plowing the stubble under, but even the 

 latter is better for the soil than sowing to grass and plow- 



