154 VEGETABLE PHYSIOLOGY 
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 
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 apparently 
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 substances, 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 tormation 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 suh- 
sequently the activity of the chlorophyll apparatus. The 
illumimation need not be very intense, though it is probable 
that the greatest activity is manifested in a bright diffused 
light. Plants which grow even in deep shade are, however, 
capable of forming carbohydrates. It must be remembered, 
