THE CHLOROPHYLL APPARATUS 158 
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 Huonymus europeeus, 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. 
Tf a green stem is withdrawn from the light, the chloro- 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 bemg 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 | 
