!q8 physiology of nutrition 



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 

 going on) shows protein synthesis. Light is here, again, apparently necessary only for 

 the formation of the necessary carbohydrates. 



As to the nucleins (of compound proteins), active growth is generally accompanied 

 by nuclein synthesis. The nucleo-proteins increase during the first stages of seed 

 germination, doubtless being formed at the expense of the simple proteins, which 

 decrease during these stages. Compound proteins are formed in leaves when sugars 

 are plentiful, and the process seems to be more rapid in light than in darkness, as 

 though light exerted a direct influence in this case. 



7 [8]. Lipoids and Phosphatides. — Lipoids are substances that can be dissolved out 

 of plant or animal cells by treatment with ether or similar solvents. Here belong not 

 only fats and fatty acids, but also phosphatides. The latter may be defined as lipoids 

 containing phosphorus; they are very active chemically and are present in all proto- 

 plasm. Lipoids apparently form labile compounds with the proteins, on the one hand 

 and with iron, calcium, etc., on the other hand. 



8 [9]. Carbohydrates. — The carbohydrates of plants are either water-soluble 

 (glucose, saccharose, inulin, etc.) or insoluble in water (starches, celluloses, etc.). 

 Starch and cellulose appear to be formed in plant cells by a sort of decomposition of 

 proteins. Protoplasm is necessary for their formation, and they form as solid masses 

 in leucoplasts (starch grains) or at the protoplasmic periphery (cell walls). There is 

 evidence that their formation is accompanied by the production of nitrogenous sub- 

 stances such as asparagin. (These nitrogenous products appear to be then combined 

 with sugar, forming more proteins, so that cellulose and starch are said to be formed by 

 a sort of condensation of sugar, although proteins seem to be involved in the process.) 

 Cellulose forms the main mechanical support of plant tissues, but the thickened cell 

 walls of some forms (as the endosperm of date seeds) are subsequently dissolved, by the 

 action of the enzyme cytase, and furnish sugar that is used in later growth. Ordinary 

 cell walls contain hemicelluloses (which can be extracted with hot 1 per cent, solution 

 of hydrochloric or sulphuric acid) and true celluloses (which cannot be extracted in 

 that way). The cell walls of many fungi (ordinary mushrooms) are composed of 

 fungus cellulose, which contains considerable nitrogen and is similar to the chitin of the 

 external skeleton of insects, crabs, etc. 



Grape sugar (dextrose or dextro-glucose) is a glucose sugar very generally present 

 in plant cells. It is one of the simplest of the carbohydrates, being represented by 

 the formula C 6 Hi 2 6 . As has been said, it is freely soluble in water and always occurs 

 in aqueous solution in the tissues. Saccharose (cane sugar) is a more complex water- 

 soluble carbohydrate, represented by the formula C12H22O11. It is very common in 

 plants (in fruits, roots, stems, etc., in large amounts) and forms dextrose (grape sugar) 

 and levulose (fruit sugar, fructose) upon partial decomposition, as by the enzyme 

 invertase. 



9 [10]. Glucosides. — Glucosides are chemical combinations of glucose, or other 

 sugars, with various organic substances, and they decompose into these constituents, 



