184 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



244. Formation of Starch from Sugar by Colour- 

 less Leaves. That chlorophyll is not necessary for this 

 " chemosynthetic " form of starch -production is readily 

 shown by using, instead of green leaves, the white flowers 

 of various plants those of Phlox answer well, but others 

 should be tried. Test the leaves first with iodine, to ensure 

 that no starch is already present ; it is of course unneces- 

 sary to treat the petals with alcohol, but they should be 

 boiled in water before applying the iodine test. Float some 

 of the flowers in water, others in 5 per cent, sugar solution; 

 keep some in darkness, expose others to light. In both 

 cases note that in a few days the sugar- supplied flowers 

 contain abundant starch, while those in water are still 

 starchless. 



245. Which Light Rays are concerned in Photo- 

 synthesis ? We may investigate this by comparing the 

 effects of exposing plants to light of different colours i.e. 

 allowing only certain rays to fall upon the leaves. It is 

 usually found that the rays at the red end of the spectrum 

 are more active than any of the rest in promoting photo- 

 synthesis, and that for most plants the curve obtained when 

 the results of experiments are plotted on squared paper 

 shows two " humps " or maxima, a higher one in the orange 

 and a lower in the blue, with the lowest intermediate part 

 (minimum) in the green. 



It must be noted, however, that the experiments usually 

 made on this topic are open to various objections ; some of 

 these objections are mentioned in connection with the 

 following experiments, but the greatest are (1) the extreme 

 difficulty in obtaining spectroscopically pure colour screens ; 

 (2) the different heating effect of the different colours as 

 transmitted through screens. 



(a) A rough comparison may be made by setting a starch-free 

 plant, or a leaf with its stalk dipping into a bottle of water, in a 

 box, one of whose sides is replaced by a sheet of red glass, another 

 in a box with a side of yellow glass, another with green glass, another 

 with blue glass. After several hours' exposure to light, test each 

 for starch with iodine solution. However, coloured glass is pro- 

 bably never pure, in the sense of allowing only rays of one colour to 

 pass through it. This can easily be seen by testing coloured glass 



