WORK OF LEAVES: PHOTOSYNTHESIS III 



176. Photosynthesis takes place only in chlorophyll- 

 bearing plants and in the chlorophyll-bearing parts. 



Plants which have been grown in the dark lack chlorophyll.* 

 When brought into the light photosynthesis does not take place 

 until chlorophyll has been formed. So in variegated leaves, 

 photosynthesis does not take place in the white portions because 

 chlorophyll is absent, but it does take place in the green parts of 

 such leaves or in such parts as have chlorophyll. Photosynthesis 

 does not, therefore, take place in the roots, not even in the roots 

 of green plants, nor in stems where chlorophyll is not present. 

 Experiments with chlorophyll-less plants like beech drops 

 (Epipkegus), the Indian pipe (Monotropa), and the fungi and 

 bacteria^ show that photosynthesis cannot take place in them. 

 They are dependent for their carbohydrate food on green plants. J 

 They obtain it either as parasites on living green plants, or by 

 growing on their dead or disintegrated remains as saprophytes; 

 or on other plants or animals which in their turn are dependent 

 on the green plants primarily for their carbohydrate food. 



177. Conditions favoring photosynthesis. From the pre- 

 ceding experiments and discussion it is clear that light and air, 

 the chlorophyll, and the living substance of the plant, are essential 

 to the formation of sugar and starch. But the rapidity of their 

 formation is influenced by the varying intensity of light, tempera- 

 ture, and the percentage of carbon dioxide in the air. 



* There are some few exceptions. The first leaves of pine seedlings have 

 chlorophyll leaves even grown in the dark. 



j" A few bacteria, however, are known to be able to form their own car- 

 bohydrates. The nitrite and nitrate bacteria which convert ammonia com- 

 pounds into nitrites and nitrates in the soil obtain energy from the chemical 

 process of making nitrites and nitrates, so that they can assimilate the CO 2 

 of the air. This is not photosynthesis however, since light does not supply 

 the energy. It is chemosynthesis. 



J In using the term green plants here, the chlorophyll-bearing plants 

 are understood. It should be remembered that there are green plants, 

 especially among the fungi, which do not possess chlorophyll. 



The products of photosynthesis increase, other things being equal, with an 

 increase of CO 2 from the normal (about .05 part in 100 of air) up to 4 to 10 

 per cent (4 to 10 parts to 100), but a larger increase in the CO 2 acts injuriously 



