Biology of Higher Plants — Anatomy and Physiology 235 



oxygen. Moonlight is sufficient to continue the process in certain algae. 

 While some photosynthesis can occur in all parts of the visible spec- 

 trum, not all parts of the spectrum are of equal value. In general, the 

 red end of the spectrum is twice as valuable as the blue end. The lowest 

 rate occurs in the green region; the highest rate, in the red end; infra- 

 red radiations (Fig. 368) are not used at all; ultraviolet radiations are 

 used to a limited extent. While the rate of photosynthesis is highest in 

 the red end because of its greater energy value in sunlight and stronger 

 absorption of light by chlorophyll in this region, the absence of blue- 

 violet light decreases the rate of photosynthesis. This may explain the 

 lower rates of photosynthesis under artificial lights, which may be de- 

 ficient in the blue-violet rays. 



Brown has shown that in bright sunshine a sunflower leaf receives 

 600,000 units (gram calories) of radiant energy per square meter per 

 hour with the formation of 0.8 Gm. of carbohydrate. Of all the light 

 which falls on a leaf, only about 3.5 per cent is absorbed by the chloro- 

 phyll proper. Raber states that the chlorophyll apparatus has an 

 efficiency rate of about 15 per cent. 



Up to a certain point the rate of photosynthesis increases as the in- 

 tensity of light increases. There are variations, but most plants require 

 light much below the intensity of strong sunlight at noon. Usually there 

 is more light available in nature than plants use, provided other factors 

 are normal. 



The duration or length of time a plant is in the light aflfects the 

 amount of carbohydrate produced. This is an important factor in 

 autumn and winter when light is available for shorter periods. Plants 

 need for maturation and growth a certain number of light energy units, 

 the unit being the product of the light intensity and the duration of 

 time. In general, if other factors are constant, a weak light acting for 

 a long time may have the same effect as a stronger light acting for 

 a shorter time. The growth of plants and the ripening of their products 

 can be speeded up by increasing the duration of light by using artificial 

 light. Differences in light intensities and duration in various parts of 

 the country influence the rate of photosynthesis in those different areas. 



3. Water Supply: Since carbon dioxide is combined photosyntheti- 

 cally with the constituents of water, the latter becomes a limiting factor, 

 especially if present in minimal quantities. However, increasing the 

 water supply will increase photosynthesis only up to a certain point. 

 If water is so deficient as to cause wilting of leaves, there probably is 

 insufficient water for photosynthesis. The wilting may close the stomata 



