CONDITIONS AFFECTING PHOTOSYNTHESIS I49 



Time Required. — Engelmann's bacterium method shows that 

 photosynthesis begins instantly on exposure to light. It is found, 

 however, that it takes several minutes, and in some cases an 

 hour or more before starch appears in the chloroplasts; and this 

 is e\adence that this starch represents carbohydrate that is formed 

 faster than it is being carried away — a surplus that would hinder 

 the constructive process if allowed to remain in solution in the 

 cell-sap. And the starch in the chloroplasts might be of direct 

 benefit in increasing the chloroplastic surface and in refracting 

 and reflecting the light so that more of it would be retained within 

 the body of the chloroplast. 



Carbon Dioxide. — It is found that the very small percentage 

 of carbon dioxide in the atmosphere is not the optimum amount 

 for photosynthesis; for this function rises in activity as the COj 

 content is increased from .03 per cent, to 8 per cent., and with 

 this increase photosynthesis can go on even with closed stomata. 

 About one-half of the dry weight of a plant is carbon, and it is 

 of great importance that this so necessary element in nutrition 

 is in an extremely mobile condition and capable of distribution 

 by the unceasing currents of the atmosphere. 



Water. — The photosynthetic cells are like all others in being 

 able to perform their functions better when in a state of tur- 

 gidity. It is found, however, that isolated cells are able to 

 photosynthesize to a certain extent when in a flabby or even 

 plasmolyzed condition; and mosses and lichens are remarkable 

 for their power of photosynthesis even after they have begun 

 to dry up. But the higher plants, with their waterproof epi- 

 dermis, are unable to photosynthesize after the reduction in 

 the water content has caused the closure of the stomata, and 

 thus prevented the continued inflow of carbon dioxide. 



Temperature. — The temperature may rise too high or fall 

 too low for photosynthesis; but, as might be anticipated, plants 

 in different latitudes are not affected alike by the same tem- 

 peratures. For instance, in the tropics photosynthesis ceases 

 when the temperature becomes as low as 8° to 4° C, while in 

 the subtropics and temperate zones to produce a like result 



