122 TEXTBOOK OF BOTANY 



in plants or parts of plants without chlorophyll. Rudimentary stomates 

 in all types of subterranean organs have also been reported. 



Although the presence and behavior of the stomatal mechanism ma\' 

 greatlv influence the rate of photosynthesis, it is evident that one should 

 be cautious about drawing inferences which imply that stomates behave 

 as they do in order to facilitate photosvnthesis. 



Chloroplasts and photosynthesis. In almost all green plants chlorophvll 

 is confined within the colloidal matrix of definite protoplasmic bodies, 

 the chloroplasts, which also contain carotene and xanthophyll. Certain 

 primitive algae do not have well-defined chloroplasts. In most of the 

 green plants, therefore, photosynthesis occurs only within the chloro- 

 plasts. As a result of changes in the viscositv and streaming of the proto- 

 plasm in which these plastids are located, their arrangement within the 

 cell mav vary from time to time. The number, distribution, and surface 

 area of chloroplasts constitute another structural mechanism that influ- 

 ences the relative rates of photosynthesis in different plants. 



Many facts about plants are so variable that an exact determination 

 of them is phvsicallv impossible and of no great consequence. On the 

 other hand, a close approximation to the truth based upon a limited 

 number of facts is often very helpful in checking one's concepts. As an 

 example, a few estimates of the number and area of chloroplasts in 

 plants may be cited. 



In the castor bean leaf there are about 495,000 chloroplasts in the 

 mesophvll beneath each square millimeter of leaf surface. About 82 per 

 cent of them are in the palisade cells, and 18 per cent in the cells of the 

 spongy mesophyll. In the sunflower, nasturtium, broad bean, and elm 

 the corresponding numbers of chloroplasts per square millimeter of leaf 

 surface are 465,000, 383,000, 283,000, and 400,000. 



These figures mav be used as a basis for further estimates. Beneath 

 each square inch of elm leaf there are about 250,000,000 chloroplasts. 

 The combined surface of these plastids is approximately 20 square 

 inches, if we consider them as spheres 4 microns in diameter. An average 

 elm leaf (5 square inches) has 100 square inches of chloroplast surface. 

 A large elm tree with 1,000,000 leaves — estimates have been as high as 

 7,000,000— has a leaf surface of about 0.8 acre, and a chloroplast surface 

 of about 16 acres, or 1/40 square mile. To appreciate the importance of 

 this enonnous plastid surface in photosynthesis, one must remember that 

 most of the chlorophyll is concentrated near the surface of the chloro- 

 plast, and that CO-.- and H2O enter the chloroplast through this surface. 



