162 VEGETABLE PHYSIOLOGY 



inextricably 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 the mesophyll contain the 

 plastids much as normal ones do. They are pale yellow 

 in colour, containing another pigment known as etiolin, 

 which appears to be an antecedent of chlorophyll, and to. 

 be transformed into the latter when brought into the 

 presence of sunlight. Exposure to light is almost uni- 

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 If a green stem is withdrawn from the light, the chloro- 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

 Hyacinths which emerge from the soil in the early spring are 

 often colourless or pale yellow. The chloroplasts are found 

 to be present in such leaves, but they are yellow, owing to 

 the presence of etiolin instead of chlorophyll. The leaves 

 which are produced later, when the temperature of the air is 

 higher, have the normal green appearance. 



Chlorophyll is not developed in a plant unless the latter 



