Biology of Higher Plants- — Anatomy and Physiology 229 



radiant (kinetic) energy into the potential energy of the sugar. Photo- 

 synthesis is also known as "carbon assimilation" or "carbon fixation." 



The photosynthetic apparatus is made of the chlorophyll-bearing chloro- 

 plasts which in higher plants are usually most abundant in the chlo- 

 renchyma (mesophyll) tissues of the leaves (Figs. 57 and 60), although 

 they may be present in any living plant tissues exposed to light. Recent 

 investigations with the electron microscope reveal that the chloroplasts 

 in plant cells contain tiny green bodies of chlorophyll known as grana 

 (Fig. 72). The latter increase in size until they divide to form two new 

 chloroplasts. Chlorophyll is also present in the mosses, ferns, and algae. 

 In certain algae the green chlorophyll may be masked by other pig- 

 ments which are considered in greater detail in other parts of the book. 

 In addition to chlorophyll, the chloroplasts may contain two other pig- 

 ments: a yellow pigment called xanthophyll (Gr. xanthos, yellow; 

 phyllon, leaf) and an orange pigment called carotene. The latter was 

 formerly called carotin and was named because of its abundance in car- 

 rots. The yellow pigments are more resistant than chlorophyll to low 

 temperatures, drought, minimum light, diseases, and injuries. Conse- 

 quently, in the fall when some or all of the above factors may be present 

 and the chlorophyll begins to disintegrate, the yellow pigments may play 

 important roles. Chlorophyll was first named by Caventou and Pelletier 

 (1819), but it was isolated and its chemical composition determined in 

 1912 by Willstatter and his associates. 



The chloroplasts in difTerent species of plants vary greatly, some func- 

 tioning in temperatures above 50° C. Inman found several species of 

 blue-green algae in Yellowstone National Park growing in a temperature 

 of 70° C. Other chloroplasts function in temperatures below -25° C. 

 These are extremes and undoubtedly most chlorophyll functions best in 

 less extreme temperatures. Some plants photosynthesize in full sunlight 

 while others function better in shaded environments. Diffuse light for 

 the necessary period of time is most favorable. It is believed that the 

 same type of chlorophyll is present in all species of plants. The total 

 quantity of chlorophyll in a plant averages about 1 per cent of the total 

 dry weight of that plant. The average plant makes about 1 Gm. of car- 

 bohydrate per square meter of leaf surface per hour under average 

 conditions. 



Most photosynthesis is accomplished in leaves because ( 1 ) their ar- 

 rangement and position permit them most effectively to receive air and 

 light; (2) in form they are relatively thin and broad to enable them to 



