THE CHLOEOPHYLL APPAEATUS 163 



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 il- 

 lumination need not be very intense, though it is probable 

 that the greatest activity is manifested in direct sunlight. 

 Plants which grow in deep shade are however capable of 

 decomposing carbon dioxide. 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. 



The activity of the chlorophyll apparatus is also con- 

 siderably influenced by variations of temperature. The 

 lower limit beyond which no carbohydrates are constructed 



