FORMATION AND ACTION OF CHLOROPHYLL 109 



insufficient for the vigorous growth of most seed plants except- 

 ing those which, in a wild condition, flourish in the shade. The 

 rate of photosynthesis for most of the higher plants increases 

 with the illumination up to a light intensity equal to that of 

 full sunlight. 



131. Conditions for formation of chlorophyll; its mode of 

 action. Chlorophyll is usually produced only in plants grown 

 in the light. Seedlings which have been sprouted in total dark- 

 ness almost always have a white or very pale yellow color, and 

 blanched celery affords a familiar example of the appearance of 

 leaves grown in comparative darkness. Microscopical examina- 

 tion of thoroughly blanched plants shows them to be destitute 

 of any decidedly green chloroplasts, and alcohol fails to extract 

 from them the green chlorophyll solution which is readily ob- 

 tained from ordinary leaves. 



Iron must be present in the soil in order to enable the plant 

 to form chlorophyll, and plants developed in water cultures abso- 

 lutely free from iron remain yellow and grow feebly. 



Chlorophyll appears to act by intercepting a considerable 

 portion of the light rays which strike the leaf, thus compelling 

 them to expend their energy on the chloroplasts and so to pro- 

 duce photosynthesis. If light traverses a substance with great 

 ease, as it does pure dry air, for example, comparatively little 

 effect is produced. On the other hand, when it strikes a sub- 

 stance which readily absorbs it, heating or chemical effects or 

 both are produced, as is evident when a rough sheet of iron, a 

 sensitized photographic dry plate, or blue-print paper is exposed 

 to sunlight. Chlorophyll cannot itself do the work of photo- 

 synthesis, but it causes the light rays to act on the chloro- 

 plasts so that their protoplasm carries on the manufacture of 

 carbohydrates from the raw materials. 



132. Rate of starch making. The amount of starch manu- 

 factured daily by a given area of foliage must depend on the 

 kind of leaves, the temperature of the air, the intensity of the 

 sunlight, and some other conditions. Sunflower leaves and 



