154 VEGETABLE PHYSIOLOGY 



Hyacinths which emerge from the soil in the early spring are 

 often colourless or pale yellow. The chloroplasts are found 

 to he 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 lormation 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 sub- 

 sequently the activity of the chlorophyll apparatus. The 

 illumination 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, 



