PLANT GROWTH. 5 



the function of the leaves. The decomposition of carbonic 

 acid does not proceed in darkness, or at a very low tem- 

 perature. The rays of light most active in effecting the 

 decomposition are the yellow and orange rays ; the blue, 

 violet, and dark red rays of the spectrum have scarcely 

 any influence. 



The oxygen gas given off by a green plant exposed to 

 light is equal in volume to the carbonic acid decomposed, 

 so that apparently the whole of the oxygen contained in the 

 carbonic acid is returned to the atmosphere ; the reaction 

 is, however, really more complicated, as water is probably 

 decomposed at the same time as the carbonic acid. 



The exact nature of the reaction which takes place 

 when carbonic acid is decomposed in the chlorophyll cells 

 is still unknown. Starch, composed of carbon and the 

 elements of water (CoH-ifi^), is undoubtedly among the 

 earliest products. Starch being an insoluble substance is 

 converted into sugar (glucose) for the nourishment of 

 distant parts of the plant, to which it is conveyed by the 

 movement of the sap. In parts where growth is taking 

 place, and new cells are being formed, the sugar of the 

 sap is converted into cellulose, the substance which forms 

 the cell walls, and of which the whole structure of the 

 plant primarily consists. The conversion of starch into 

 sugar and cellulose presents no chemical difficulties, as all 

 these substances are carbo-hydrates, that is they are 

 composed of carbon and the elements of water. 



The formation of albuminoids in the plant is not at 

 present understood ; we can onh^^ say that they are con- 

 stituted out of the carbo-hydrates and some of the simple 

 nitrogenous substances, most probably amides, present in 

 the sap. 



