THE CHLOROPHYLL APPARATUS 149 
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 
is supplied with a certain quantity of iron, but the relation 
of the latter to the pigment is not known. It does not 
enter into its composition. The influence of the metal can 
be ascertained by cultivating a seedling, by the method of 
water-culture, in a solution which is free from iron. The 
seedling assumes a sickly yellow appearance, not unlike that 
presented by a plant grown in darkness. It is said to be 
chlorotic. The addition of a very small quantity of an 
iron salt to the culture-medium causes the appearance of 
chlorophyll in the plastids. The presence of oxygen is 
also necessary for the formation of the pigment. 
The chlorophyll apparatus of a plant is primarily con- 
cerned with the production of carbohydrate bodies, such-as 
the various sugars which the plant contains, and it is to 
the formation of these that attention must first be given. 
It carries out this constructive process only under particular 
conditions, the most important of which is light. We have 
seen that a certain degree of illumination is necessary for 
the formation of the chlorophyll. The pigment once 
formed may continue to exist for a time in darkness, but it 
is quite incapable of exercising any constructive power 
unless light be admitted to it. Consequently the formation 
of carbohydrates is an intermittent process, being quite in 
abeyance during the night. The effect of light is thus 
twofold, its access causing the original formation and the 
subsequent activity of the chlorophyll apparatus. The 
illumination need not be very intense, though itis probable 
that the greatest activity is manifested in direct sunlight. 
Plants which grow even in deep shade are, however, capable 
of forming carbohydrates. It must be remembered, too, 
that the chloroplasts are situated some little distance within 
the leaf or stem, at any rate in phanerogamic plants, and 
there must be a certain loss of light as it penetrates through 
the epidermis. 
