172 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



228. Disappearance of Fhotosynthetic Starch in 



Darkness. On a bright day expose to light a healthy pot- 

 plaut, e.g. Tropaeolum or Fuchsia, or a young Sunflower 

 or Phaseolus raised in a pot, and in the afternoon remove 

 from two or three of the leaves a piece about 1 cm. square ; 

 if the plant has plenty of leaves, remove larger pieces or 

 even half of the leaf. Place these pieces in boiling water 

 for a minute or two, then into a tube of alcohol labelled A. 

 Now set the plant in darkness, after watering it. Next 

 day remove similar pieces from the leaves, and place them 

 in a second tube of alcohol labelled B. Repeat this on 

 the third day (0). Then test with iodine the pieces in 

 A, B, and C, placing each lot separately in a saucer. Note 

 that as the result of keeping the plant in darkness the 

 starch present in the leaves diminishes until in two or three 

 days it has disappeared altogether. 



Now expose the plant to the light again, and after a few 

 hours remove pieces of leaf; note that they contain starch. 



This experiment is made more striking if each time we 

 cut out differently shaped pieces from each leaf, and then 

 place the different pieces together in the saucer of iodine. 

 Note that the leaves do not change in colour, or suffer any 

 other change (excepting the disappearance of the starch) 

 when kept in darkness for two or three days. 



In investigating the conditions required for photosyn- 

 thesis, we must begin with starch-free leaves, and in order 

 to do this all we simply set in darkness for a few days a 

 plant whose leaves have been found to contain starch under 

 normal conditions. 



229. Isolated Leaves (Appearance and Disappearance of 

 Starch). Repeat the preceding experiment with isolated leaves 

 instead of the entire plant : Tropaeolum is especially suitable. Cut 

 off the long-stalked leaves and set them with the stalk dipping into 

 a bottle of water. For comparison, set a pot-plant along with the 

 isolated leaves. The results will show that (l)the starch which 

 appears in the leaves of plants exposed to light is actually made in 

 the leaves, and is not derived from sugar or other substances carried 

 to the leaves from other parts of the plant ; (2) starch disappears 

 more slowly from isolated leaves than from those remaining on the 

 plant ; (3) evidently starch accumulates in the leaves because it is 

 formed more rapidly than it can be removed by translocation. 



