52 BIOLOGY 



size, found only in the superficial parenchyma cells of 

 stem and foliage. The chloroplasts are embedded 

 within the cytoplasm, just inside the outer layer, and are 

 coloured green. Their form is subject to change due 

 to internal causes, and they are often distorted, showing 

 that they are soft and plastic. The body of the chloro- 

 plast seems to be like the cytoplasm, but coloured with 

 green matter. In fact the exact relation of colouring 

 matter to the body of the chloroplast is only a matter 

 of conjecture. There are various varieties of colouring 

 matter, the chief of which is chlorophyllin. 



While the chemical composition of chlorophyllin is 

 unknown, its chief physical feature is of great import- 

 ance. This feature is its capacity to absorb radiant 

 energy. The chlorophyll is so constituted that it can 

 absorb light-waves of a certain length, and it is from 

 these absorbed light- waves that the energy is obtained 

 that drives the machinery of photosynthesis. If a plant 

 be kept in the dark no carbohydrates are formed ; 

 further, it is only the chloroplast that is directly illumi- 

 nated that receives this energy. Now although a leaf 

 may absorb 40 to 70 per cent, of the sunlight and 95 

 per cent, of diffuse light, it is only able to store up in 

 the carbohydrates as potential energy about J to 3 

 per cent. 



The first products of photosynthesis are not exactly 

 known, but the later product of the synthesis which is 

 best known is starch. Now the fact that no starch can 

 be detected in a leaf under certain conditions is no test 

 that photosynthesis has not taken place, but only that 

 it has taken place at a rate just rapid enough to supply 



