228 Plant Biology 



In higher plants the carbon dioxide is taken from the air by the leaf and 

 the water is supplied by the roots. The carbon dioxide enters the leaf 

 through the stomata and intercellular air spaces (Figs. 53, 57, and 60) 

 and diffuses into the chlorophyll-bearing mesophyll cells. The chloro- 

 phyll combines this gas with the water brought to the mesophyll tissues 

 by the xylem tissues of the root and stem. Less than 4 per cent of the 

 light energy falling on a leaf is used in the photosynthetic process; part 

 of the additional energy absorbed by the leaf increases its temperature 

 and part is eliminated with the water during transpiration. 



The green color of the chloroplasts in plants is due to a mixture of 

 four pigments: two green ones, chlorophyll A (C55H7205N4Mg) and 

 chlorophyll B (C55H7o06N4Mg), and two yellow ones, carotene (C40H56) 

 and xanthophyll (C40H56O2). The term chlorophyll is often used for 

 the mixture of these four pigments. Light is essential for the develop- 

 ment of chlorophyll as is shown by the pale color of leaves grown in dark- 

 ness. We do not know how chlorophyll unites carbon dioxide and water, 

 but since it does not contribute to the product formed and is not itself 

 used up in the process, it is surmised that it acts as a catalytic agent. The 

 process may be illustrated by the following chemical equation : 



6 molecules 6 molecules Light Carbohydrate ^ Oxygen 



water (H2O) carbon dioxide (CO2) energy (CeH^Oe) (6O2) 



It will be noted that oxygen is a by-product of the process. Part of 

 this oxygen may be used by the plant for metabolic purposes and the 

 remainder eliminated. Part of the carbohydrates is oxidized, thus lib- 

 erating usable energy for plant use, but a much larger part is chemically 

 transformed by the plant into (1) components of living protoplasm, (2) 

 reserve foods, such as sugars, starches, proteins, and fats, and (3) other 

 substances, such as oils, resins, pigments, enzymes, vitamins, etc. Plants 

 form proteins by adding to the carbohydrates such elements as nitrogen 

 and sulfur and, in some cases, phosphorus. The living plant oxidizes 

 many of its substances, thus liberating energy for its various metabolic 

 activities. 



General Consideration of Photosynthesis. — Photosynthesis is one of 

 the most important of living processes because, directly or indirectly, it 

 provides most of the foods, fuels, clothing, and shelter for living organ- 

 isms. Photosynthesis (Gr. photos^, light; synthesis, put together) is a 

 constructive or anabolic process. Chlorophyll (Gr. chloros, green; phyl- 

 lon, leaf) serves as an absorber of the light and no doubt has much to 

 do with the physical and chemical reactions necessary to transform the 



