148 VEGETABLE PHYSIOLOGY 
In the process of the formation of the chloroplast 
it is not difficult to see that its two constituents are not 
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 their 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 Huonymus ewropeus, 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If a green stem is withdrawn from the light, the chloro- 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 indarkness. 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 
