COLOUR OF LEAVES. 807 
the contained plants from immediate contact with air impregnated 
with soot and other injurious substances ; for in consequence of 
the glass cover fitting closely to the trough in which the plants are 
placed, the external air in its passage has to pass through the 
very narrow crevices beneath the cover, and in so doing becomes 
_ filtered, as it were, in a great measure from its impurities, 
before it is brought into contact with them. 
Besides the use of these cases in growing plants luxuriantly 
in those places where, under ordinary circumstances, they would 
perish, or at all events grow but languidly, they havea still more 
important application, for they have now been most successfully 
employed in transporting plants from one country to another 
which under ordinary circumstances would have died in their 
transit, and whose seeds could not have been transported 
without losing their vitality. The action of the Wardian cases 
in this mode of transporting plants is twofold: in the first 
place, the plants are protected from the influence of salt breezes, 
which are in most instances very injurious to them; and, 
secondly, the atmosphere of such cases remains in a quiet state, 
and they are therefore also protected from rapid changes of 
temperature. 
(6) Colowr of Leaves.—The green colour of leaves is due to 
chlorophyll contained in the cells situated beneath the epidermis. 
Chlorophyll bodies may be formed in the dark, but remain 
yellow, only becoming green under the influence of light, and 
hence the leavesand other parts of plants grown in darkness are 
blanched or etiolated (page 804). To this rule there are some not- 
able exceptions—viz., the germinating seeds of many Coniferx 
and the fronds of Ferns, which will become green even in total 
darkness, provided that the temperature is sufficiently high. If 
plants with green leaves be withdrawn from the action of light 
and be placed in the dark, these leaves soon fall; and if others 
are produced, they have a whitish or yellowish colour. Again, 
if plants which have been grown in the dark be removed to the 
light, the leaves upon them soon lose their whitish hue and 
become green. The rapidity with which leaves become grcen, 
and the intensity of their colour, will be in proportion to the 
amount of light and heat (25°-30° C. being about the maximum) 
to which they have been exposed. It has also been shown that 
iron is necessary for the production of chlorophyll. (See also 
The Effect of the Electric Light on the Growth of Plants, &c., 
page 858.) 
The different rays of the spectrum have a varying influence 
in promoting the formation of chlorophyll. Some difference 
of opinion exists as to those rays which are most active in 
this respect, but the majority of experimenters agree that the 
illuminating or yellow rays—namely, those which, as we have 
already seen (page 800), have the greatest effect in promoting 
the decomposition of carbon dioxide—are those also which are 
the most active in the production of chlorophyll. 
