164 FUNDAMENTALS OF FRUIT PRODUCTION 



of 50 per cent, carbon dioxide have a narcotic effect and depress assimi- 

 lation. Changes in the rate of assimilation in so far as they depend on 

 carbon dioxide supply are affected only by those factors that determine the 

 rate of intake. This is increased by movement of the air, by the degree 

 of stomatal opening and by any factors increasing the rate of utilization. 



Water. — The water supply of plants is treated in a preceding section 

 and no further discussion need be added here. However, it must not be 

 forgotten that water is one of the materials out of which carbohydrates 

 are made. 



Light. — In the absence of limiting factors and particularly of high 

 temperatures and extremely high light intensities, carbon assimilation 

 increases with the intensity of light. Under such circumstances equal 

 areas of different plants, equally illuminated, produce the same amounts 

 of carbohydrates. There is evidence that at the intensities of the different 

 wave lengths in the solar spectrum, red light is the most and green the 

 least effective for photosynthesis. 



Light acts indirectly on carbon assimilation by raising the tempera- 

 ture of the leaf and by stimulating the guard cells of the stomata to 

 open, thus increasing the absorption of carbon dioxide. ^^ 



Leaf Pigments. — The chloroplasts of all green plants contain four 

 pigments, two green and two yellow. They are: 



L Chlorophyll a, blue-black in the solid state, green-blue in solution. 



2. Chlorophyll b, green-black in the solid state, pure green in 

 solution. 



3. Carotin, forming orange-red crystals. 



4. Xanthophyll, forming yellow crystals. 



In the fresh nettle leaf, these four pigments occur in the following 

 quantities, chlorophyll a, 24 parts in 12,000; chlorophyll b, 9; carotin 2 

 and xanthophyll 4.'"=' 



In the chloroplast these pigments occur in a colloidal mixture with fats, 

 waxes and salts of fatty acids. The chlorophyll content of leaves varies from 9.6 

 to 1.2 per cent, of the dry weight. Shade leaves have a higher chlorophyll 

 content than sun leaves in terms of dry weight, but not in proportion to leaf 

 surface. The yellow pigments comprise 0.1 to 1.2 per cent, of the dry weight and 

 there is no higher percentage in shade leaves than in sun leaves. There is no 

 diurnal fluctuation in the amounts of the pigments, the mean ratio of chloro- 

 phyll a to chlorophyll b being 2.85:1. On the whole, shade leaves con- 

 tain less chlorophyll a than other leaves, their ratio of chloroph.ylls being 

 2.93:1. Less difference in this ratio is found in real shade plants like the beech 

 than in plants that are ill adapted to growth in the shade. The mean ratio of 

 carotin to xanthophyll for ordinary leaves is 0.603 : 1 and for shade leaves 

 0.421 : 1. Xanthophyll is relatively more abundant in shade leaves. 



Variation with Age. — The chlorophyll content of leaves increases 

 with age; so also the assimilatory power of the leaf, though not in the 



