68 THE FLOWERING PLANT. 
of its constructive function. A plant placed in darkness soon 
gets pale and unhealthy, and though it may grow considerably in 
length, gradually becomes less in weight, ultimately dying. Such 
a plant is said to be etiolated. Celery, and some other garden 
plants, are purposely reduced to this condition by heaping earth 
around them. They thus become very sickly, and the tissues 
not being vigorously developed, are very tender. Also, in the 
case of celery, the characteristic essential oil is too strong and 
too abundant under normal conditions to let the shoots be eaten. 
Chlorophyll cannot be formed, in most cases, unless light is pre- 
sent. If a bean-seed is germinated in the dark, a yellowish 
etiolated seedling is produced, which grows to a certain extent 
at the expense of the reserve materials stored up in the seed. 
The yellow colour is due to the development of etiolin, which is 
closely related to chlorophyll, and exists, like it, diffused through- 
out the substance of protoplasmic granules (cf p. 8). It is 
also to be noticed that the seedling weighs Jess than the seed 
from which it grew. A very feeble light suffices to convert 
the etiolin into chlorophyll, so that the granules in which it 
occurs become chlorophyll granules. If the now green seedling 
is placed in a fairly strong light, it will rapidly increase in size 
and weight, since its chlorophyll is able to build up organic com- 
pounds. ‘The roots of the seedling must of course be placed in a 
suitable soil or food solution, but this alone is useless if light is 
excluded. 
Since carbon dioxide is one principal item of food, it is obvious 
that chlorophyll cannot work unless supplied with it. This is 
readily proved by growing plants under a bell-jar to which air is 
freely admitted that has been deprived of its carbon dioxide by 
means of caustic potash. Under these circumstances the plant 
does not increase in weight, and starch cannot be detected in its 
leaves. The palisade parenchyma is the most important part of 
the leaf for the purposes of assimilation, and therefore, as we 
have seen, is developed on the side turned towards the light. 
Where, as in vertical leaves, the conditions of illumination are 
equalized, there may be such tissue on both sides. It is also 
interesting to notice that, in plants with dense foliage, such as 
trees, the leaves most exposed to light are thicker than the 
others, owing to the formation of extra palisade layers. 
As already explained (p. 10), the assimilatory process helped 
on by chlorophyll involves the liberation of a large quantity of 
oxygen, which passes off into the surrounding medium. This is 
easily proved by experiments, one of which consists in cutting off 
a vigorous shoot from a water plant, and, by means of a small 
weight, keeping this submerged, cut end up, in a vessel of fresh 
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