LEAVES 



527 



intensity above which the amount of manufactured food decreases, each 

 plant having its optimum light relation. This optimum usually is much 

 lower for shade plants and water plants than for sun plants, but in all 

 cases a decrease of light below the optimum impairs synthesis much 

 more than does an increase of light above the optimum. 



It is not clear why increased light should ever cause reduced food production; 

 possibly it is because the enzyms or the chlorophyll itself are ill-adapted to with- 

 stand intense light, and it may be that the maximum activity of the chloroplasts is 

 impaired by their assumption of the profile position. In bright sunlight only a frac- 

 tion of the light available for synthesis is thus utilized. The depth at which green 

 plants are found in the sea is slight, doubtless because at a depth greater than 

 twenty meters the light generally is insufficient to initiate food-making. While green 

 algae (e.g. Halosphaera) have been brought up from great ocean depths, it is likely 

 that they have sunk from their place of development near the surface. Red algae 

 grow at greater depths than do most chlorophyll-bearing plants, and it has been 

 thought that their color makes this possible (see p. 529). Probably sulfur bacteria 

 and nitrifying bacteria are able to manufacture carbohydrates at great depths be- 

 cause of their independence of light. The chlorophyll layer beneath the bark of 

 trees is known to manufacture carbohydrates, although the light intensity must 

 be low. 



The absorption spectrum of chlorophyll shows that rays toward the red and the 

 blue ends of the spectrum are absorbed more completely than are the intermediate 

 rays, and usually it has been supposed that the red, orange, and yellow rays are 

 more efficient in carbohydrate synthesis than are the blue rays, the green rays 

 being the least efficient of all. 1 It has been held that the color of chlorophyll is of 

 adaptive significance, since it absorbs the more useful red rays rather than the less 

 useful green. This theory lacks adequate support ; the absorption spectrum of 

 blood is quite as remarkable as is that of chlorophyll, but the color has no adaptive 

 significance. The color of blue-green algae may be of significance, for it appears 

 that in many forms (e.g. in Oscillatoria) developing cells assume a color comple- 

 mentary to that of the incident light ; the red alga, Porphyra, is said to become 

 green if grown in red or yellow light. It is not known whether these chromatic 

 changes have any effect upon the synthesis of carbohydrates; some recent investi- 

 gators doubt the existence of such changes. 



Temperature; carbon dioxid; water. In the laurel cherry, synthesis 

 takes place only between 6 C. and 45 C., while the limits of effec- 

 tive synthesis are still narrower. The limits and the optima vary with 

 the species, being lowest in arctic and alpine forms, where effective syn- 

 thesis can take place below o C., though probably not so far below as 

 sometimes has been thought. The optimum temperatures usually are 



1 From recent research it seems probable that the green rays are not utilized at all, 

 and that the blue rays are equal in importance to the red rays, if of equal energy value. 



