CHEMICAL AND PHYSIOLOGICAL STUDIES. 169 



in the Chloroplasts have been determined to be the fol- 

 lowing : — 



i. The action of light. In darkness, green plants become etiolated — 

 that is, the chloroplasts lose their green tint, a yellowish one being sub- 

 stituted, which is due to the formation of etiolin. 



ii. The presence of oxygen. 



iii. The presence of traces of an iron-salt in the soil. Without this iron, 

 the chlorophyll is not formed. Tlie influence of iron upon the colouring of 

 petals of Hydrangea is well known, and seems to point to the necessity of 

 the same conditions for the formation of other pigments than chlorophyll. 



Etiolated plants will, when again exijosed to light, develop 

 chlorophyll, provided the otlier conditions of its formation be 

 present; and it has been shown that photosynthesis can proceed 

 to a limited extent in chloroplasts in which only etiolin is present. 



iii. The Formation of Elaborated Nitrogenous Food {Proteids). 



The assimilation of nitrogen which takes place chief!}' in the 

 leaf-cells and other green parts of a plant is a subject which is 

 difBcult to deal with from an elementary point of view, seeing 

 that it involves complicated synthetic and analytic reaction.s 

 between organic and inorganic compounds in a cell. The 

 nitrogen is obtained from nitrites (or nitric acid) and nitrates, 

 as well as ammonia at times (c/. absorption of ammonia b}- 

 capitate hairs). In many cases reserve proteids are present in 

 the cell-sap, which, when acted upon by enzymes, are converted 

 into albumoses and peptones, and these are then gradually built 

 up into pro'toplasm by the further agency of the living substance. 

 The conversion of proteids into albumoses and peptones is mainly 

 a process of hydrolysis, the elements of water entering into the 

 reactions, a fresh compound being then formed by the splitting up 

 or reconstitution of the previous one. The formation of proteids 

 in a cell involves synthesis of a high order. During this pi'ocess 

 various waste-products are formed, notably oxalic acid, and in 

 some cases this acts upon calcium nitrate in the cell-sap, forming 

 calcium oxalate, the released nitric acid being again assimilated. 

 Speaking generally, in the synthesis of protein, the following 

 substances are involved in the reactions : — 



Nitrites, nitrates, phosphates, sulphates, and chlorides of K, Ca, Mg, Fe, 

 and at times other metals ; water, carbon-compounds (starch, &c. ), ammonia, 

 asparagin, and nitric acid. The actual chemistry of the process is a subject 



