THE CHLOEOPHYLL APPAEATUS 153 



with steam, the colouring matter may be made to exude 

 from the framework in viscid drops, leaving the latter colour- 

 less. It then appears to have a reticular structure, but 

 how far this condition is brought about by the action of 

 the reagents is uncertain. The chlorophyll, however, is cer- 

 tainly not uniformly diffused through the body of the plastid. 



In the process of the formation of the chloroplast it is 

 not difficult to see that its two constituents are not in- 

 extricably connected. The plastids are not, as already 

 mentioned, differentiated out of the ordinary protoplasm of 

 the cell, but are formed by the division of other plastids. 

 In many cases they are found without the colouring matter, 

 as in underground organs such as the tubers of the potato. 

 They are then known as leucoplasts. Plants which are 

 grown in darkness have no green colouring matter in 

 their leaves, but the cells of their mesophyll contain the 

 plastids much as normal ones do. They are pale yellow 

 in colour, containing another pigment known as etiolin, 

 which is replaced by chlorophyll when the leaf is brought 

 into the presence of sunlight. Exposure to light is almost 

 universally a necessary condition for the formation of the 

 green pigment. Exceptions are known among the Ferns 

 and the Conifers, particularly the seedlings of Pinus ; also 

 in the seed of Euonymus europceus, the embryo of which is 

 green, though it is buried in the interior of the endosperm 

 and surrounded by a thick testa covered by an arillus. 

 If a green stem is withdrawn from the light, the chloro- 

 phyll slowly disappears, as is shown in the process of the 

 bleaching of celery. The disappearance is, however, very 

 gradual. It is probable that in the living chloroplast the 

 colouring matter is continually being decomposed and 

 reconstructed, and that the reason of the bleaching is that 

 the reconstruction cannot take place in darkness. Light of 

 too great intensity causes the destruction of the green colour. 



Chlorophyll can be developed only when the temperature 

 rises above a certain point, which varies with different plants. 

 It is a matter of common observation that the leaves of young 



