PHOTOSYNTHESIS AND RESPIRATION. 89 



The optimum intensity is usually that of bright diffuse 

 daylight. 



It must, however, be remembered that the leaves are never 

 all equally illuminated, and even when a plant with close 

 foliage is exposed to bright sunlight, the average intensity of 

 the light received by the leaves as a whole may not be greater 

 than that of bright diffuse daylight. Hence the plant can 

 afford to risk slight injury to the more exposed leaves, in 

 order that the rest may be able to assimilate more actively. 

 Similarly, plants with thick fleshy leaves or green stems pre- 

 fer bright sunlight because otherwise the inner assimilating 

 layers only receive very weak light. 



The intensity of the light is also affected by the angle at 

 which the sun's rays strike the earth, being greatest when 

 the sun is overhead and least when it is on the horizon. 

 This effect is, of course, due to the absorption of the light- 

 rays by the earth's atmosphere. Owing to the longer days 

 during the summer of temperate regions, however, plants 

 actually receive more light per summer day than they do in 

 the twelve hours' day of the tropics. 



Even in shade-loving plants photosynthesis stops when 

 the light-intensity is reduced to *001 that of sunlight ; in 

 sun-loving plants the minimum is reached long before this. 



(a) Cover half of a leaf with thin paper, or a piece of ground glass, 

 and expose it to light ; test for starch and note result. Compare the 

 colour, after testing with iodine, with that of an uncovered leaf on the 

 same plant. 



(6) Place some healthy cut branches of Elodea or other water-plant 

 under water, and select one which gives a good stream of oxygen- 

 bubbles (fairly rapid and constant) from its cut end. Count the time 

 required for, say, 10 bubbles to be given off, and repeat the counting 

 several times till you get a fairly constant result. Then remove the 

 jar into the shade, or cover it with a sheet of thin white paper to 

 weaken the light, and take times as before, noting the change in the 

 rate of bubbling. 



(c) Cover the jar containing the water-plant with a black cloth except 

 at one side, and throw light on the plant by placing an incandescent 

 gas lamp, acetylene lamp, or electric lamp at different distances from 

 it, noting the distances and the rates of bubbling. Bring the lamp 

 into such a position that bubbles begin to come off, and count the rate ; 

 when it becomes fairly constant, bring the lamp to half this distance 

 from the plant and count again. Part of the effect, however, is due to 

 the heat given out by the lamp ; a flat-sided bottle, or other vessel, 

 containing water (kept cold by constant renewal) should be used as a 

 screen to absorb the heat. 



