§1] UPON TPIE RATE OF GROWTH 313 



land animals are in a favorable position for making use of it, 

 since it penetrates with the oxygen to all parts of the body. 

 It is found in the blood of mammals, but still as free nitrogen. 

 Whether it eventually becomes fixed in the body is entirely 

 unknown. The a priori argument against such fixation — the 

 argument of. inertness — has lost much of its force since the 

 discovery of the nitrifying organisms. 



Phosphorus. — Tliis element is of constant occurrence in 

 ■organisms. It has been found in yeast, mucors of the most 

 diverse kinds, seeds, plant tissues, and animal tissues of all 

 kinds. It is indeed one of the first among the mineral ele- 

 ments of most organisms, as shown on pages 296 and 297. Of 

 the dry substance of a fish, about 7% is phosphoric acid, and 

 the dry substance of many seeds yields 15%. Phosphorus 

 occurs in organisms as phosphoric acid compounds. Of these 

 the most important organic compounds are nuclein, which has 

 albuminoid properties, and occurs chiefly in all nuclei and in 

 deutoplasm ; lecithin, of a fatty nature, occurring in yeast, 

 Plasmodium of ^Ethalium, seeds, milk, yolk of eggs, and 

 nervous tissue ; and glycerin-phosphoric acid, a product of 

 decomposition of lecithin and found wherever the latter occurs. 

 As examples of inorganic salts we have the sodium and potas- 

 sium phosphates of the blood and tissues and the calcium phos- 

 phate deposited in bone. 



So important an element as phosphorus would naturally form 

 an essential part of the food of all growing organisms. It is 

 supplied at first in the germ, — seed or egg, — but later must 

 come from without. Plants gain phosphorus from the disinte- 

 grating rocks. Animals derive it chiefly from plants, directly 

 or indirectly, or from the calcic phosphate of the sea ; in mam- 

 mals it is supplied to the developing young through the milk, 

 which, as we have seen on page 303, is rich in phosphates. 



The abundance of phosphorus in the body indicates that its 

 office in the organism is an important one, and its peculiar 

 abundance in seeds, yolk, and milk indicates that it is especially 

 important in growth. Experiments have been directed towards 

 this point. The fact has long been established that plant 

 growth cannot occur in the absence of phosphates, and this is 

 true not only for green plants, but also for molds and yeast 



