WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 183 



supply, and thus causing an increase in assimilative acti* 

 vity ; or (3) supplying sugar from outside. Both in starchy 

 and starchless leaves, the formation of starch from sugar 

 can be effected in darkness ; and that this process is not 

 dependent upon chlorophyll, but can be carried on by leuco- 

 plasts, is shown by the fact that colourless parts can make 

 starch from sugar. 



242. Formation of Starch from Sugar by normally 

 Starchless Leaves. Cut leaves of Iris germanica into 

 pieces, say 10 cm. long. Test one or two pieces with 

 iodine; this plant does not form starch in its leaves. Float 

 some pieces in a dish containing 20 per cent, cane sugar 

 solution, others in a dish of water. Set both dishes in 

 darkness, covered with a sheet of glass raised slightly so 

 as to allow access of air. From day to day cut off 

 pieces and test them with iodine ; after about a week 

 the leaves supplied with sugar will be found to contain 

 starch, while those supplied only with water will remain 

 starchless. 



243. Formation of Starch from Sugar by normally 

 Starchy Leaves. Keep a plant of Tropaeolum or Tobacco 

 in darkness for two days, so that a piece cut from a leaf 

 shows no starch. Cut the rest of this leaf into two ; place 

 one half in sugar solution and the other in water, as in 

 the preceding experiment, and keep both in darkness. 

 After several days starch appears in the sugar- supplied 

 piece of leaf, but not in the other. 



Another method is to place water-plants (e.g. Elodea, 

 Callitriche, or Duckweed) in two jars of water, adding cane 

 sugar (about 5 per cent.) to one jar. Set the two jars in 

 darkness, and after about a week note that the starch- 

 supplied plants contain starch, while those in water are 

 starchless and unhealthy in appearance if not dead. 



G-lycerine (5 per cent, solution) may be used instead of 

 sugar in these experiments, but it is more difficult to keep 

 the glycerine culture free from moulds. In any case, it is 

 advisable to add to the culture a few drops of carbolic acid, 

 or thymol, or eucalyptus oil, as an antiseptic. 



