194 PHYSIOLOGY OF NUTRITION 



acid). Asparagin (NH 2 COCH 2 CHNH 2 ) appears to be formed from these in most 

 plants, in the presence of oxygen, so that the amino acids do not accumulate except 

 when oxygen is absent. In the presence of oxygen asparagin appears as the main 

 nitrogenous waste of plants (somewhat as urea (NH 2 CONH 2 ) does in animals). But 

 this waste is not given off to the exterior of the plant; in green leaves, in sunlight 

 (apparently because of a plentiful supply of sugars), asparagin is combined with carbo- 

 hydrates, forming simple proteins, and thus returns to the metabolic system. Only in 

 very young seedlings (in the presence of oxygen) does asparagin accumulate consider- 

 ably, since it is used up in protein synthesis about as rapidly as it is formed; but it 

 becomes clearly evident in older plants kept for a time in darkness, where protein 

 formation is stopped because of lack of carbohydrates. — The decomposition of proteins, 

 and the products formed, are influenced by the chemical nature of the substances 

 supplied to the living cells. The mould Aspergillus forms oxalic acid when grown in 

 acid media, but when there is an excess of calcium carbonate it forms tyrosin and 

 leucin instead of oxalic acid. 



The compound proteins are also broken down in active tissues, and their decomposi- 

 tion products appear, especially in darkness and when the plants are starved. Appar- 

 ently the simple proteins are attacked first (and as long as the supply lasts) , and the 

 complex proteins are considerably decomposed only when the supply of simple pro- 

 teins is about exhausted. Compound proteins appear in many instances to be formed 

 at the expense of simple proteins. 



5. Nitrogenous Products of Protein Decomposition. — As has been said, asparagin 

 appears as the most important decomposition protuct of simple proteins. In some 

 plants its place is taken by a similar substance, glutamin. Tyrosin, leucin, and some 

 other amino-acids are also formed in plants, from simple proteins. The purin bases 

 (xanthin, hypoxanthin, adenin, guanin), as well as their derivatives (such as caff ein 

 — the main alkaloid of coffee and tea, and theobromin — the main alkaloid of 

 chocolate) result from the decomposition of the nucleo-proteins. 



6. Protein Synthesis in Plants. — The primary synthesis of simple proteins in 

 ordinary green plants appears to occur in the leaves, where carbohydrates (formed in 

 the chlorophyll-bearing tissues by photosynthesis) are combined with the nitrogen 

 of nitrates (which reach the leaves through the xylem vessels, from the absorbing 

 regions of the roots). Nitrates are usually found in leaves only in very small amounts, 

 and it appears that they are ordinarily used up as rapidly as they arrive. In 

 prolonged darkness, however, the supply of carbohydrates is stopped, and nitrates accu- 

 mulate in leaves to considerable amounts. Also, nitrates have been found to accumu- 

 late in the chlorotic parts of white-green variegated leaves, even in light. In these 

 parts no carbohydrates are formed. Light is apparently not directly necessary for the 

 synthesis of simple proteins from carbohydrates and nitrates, but it is of course neces- 

 sary for the photosynthesis of carbohydrates, and it is thus indirectly necessary for these 

 protein syntheses. 



Hydrocyanic acid (HCN) has been supposed to be an intermediate product in 

 simple protein synthesis from carbohydrates and nitrates. Many leaves ordinarily 

 contain this acid (combined with sugars) in detectable quantities. It disappears, 

 however, after prolonged darkness (being probably used up in protein formation), 

 but it reappears when the leaves are supplied with nitrate and sugar in darkness 

 or when they are supplied with nitrate in light. 



Decomposition of simple proteins occurs in germinating seeds, in darkness, as 

 has been said, while the later growth of the plantlets (in light and with photosynthesis 



