118 TEXTBOOK OF BOTANY 



photosynthesis is often much too low for best results, for light intensity 

 in greenhouses may range as low as 1/100 of full sunlight and be below 

 the critical light intensity of many plants. 



Many investigations designed to determine the value of supplement- 

 ing natural light with electric light are in progress. This problem is fur- 

 ther complicated — and also rendered much more interesting — by the 

 fact that the length of dav to which a plant is exposed greatlv alters its 

 vegetative and reproductive processes. These length-of-day effects may 

 be brought about by light intensities much below those necessarv for 

 adequate photosynthesis. During the winter, exposure to lig;ht of an in- 

 tensity somewhere between 1/100 and 1/100,000 that of full sunlight 

 for a few additional hours each day greatly alters the behavior of certain 

 plants. Lengthening the day by this means, however, is not a substitute 

 for low light intensitv during dark days. These relations are discussed in 

 more detail in Chapter XXXI. 



Water and photosynthesis. The degree of saturation of the tissues of 

 a plant ma\' modif \' the rate of photosvnthesis in two wavs : through the 

 opening and closing of the stomates, and through its influence on the 

 rate of chemical processes in the chlorenchyma cells. The effects are 

 apparent in the daily periodicity of photosynthesis. In the morning 

 hours the plant tissues are nearlv saturated, the stomates are open, and 

 photosynthesis soon attains a maximum rate. Later in the day the rate 

 declines because of water loss from the mesophyll cells. Still later 

 photosynthesis is further reduced by the gradual closing of the stomates. 

 Of course, the temperature of the leaf is another factor which influences 

 the daily increase and decrease in sugar manufacture. When plants 

 begin to wilt, photosynthesis is sharplv decreased, first because the water 

 content of the cells is lower, and second through the closure of the 

 stomates. 



Structures and photosynthesis. Plant structures are the result of cer- 

 tain plant processes. These processes in turn are conditioned by both 

 heredity and environment. After a structure is formed it may in turn 

 influence certain processes in the plant. We may therefore expect the 

 rate of photosynthesis to be different in different kinds of plants, partly 

 because their structure-building processes and resultant structures are 

 different. 



Even a cursory observation of plants in one's surroundings is suffi- 

 cient to show that chlorenchvma is not restricted to leaves. Green stems, 

 green fruits, green parts of flowers, and even certain green seeds (peas 



