96 PHOTOSYNTHESIS AND RESPIRATION. 



When a plant is burnt, oxygen is consumed and carbon 

 dioxide and water produced, while the stored energy is 

 liberated again in the form of heat. This energy was 

 stored up in latent form during the assimilation of carbon 

 dioxide, and it really represents that portion of the sunlight 

 absorbed by the plant, which was utilised in the process, 

 and which provided the energy necessary to produce a 

 chemical change of this kind. Coal consists of the remains 

 of plants of past ages, and hence, when a piece of coal burns, 

 the heat and light which are liberated simply represent so 

 much sunlight which has lain dormant for millions of years. 



130. What becomes of the Starch formed in the 

 Foliage-Leaves? Is it absorbed by the plant, like the 

 starch stored in the cotyledons of the Bean ? If so, we ought 

 to find less starch in the leaves in the morning than in the 

 evening, since the leaf cannot make more starch until it is 

 again exposed to light. 



* (a) Remove some leaves from a plant in the early evening (an hour or 

 two before sunset) and place them in alcohol. Early next morning remove 

 some more leaves from the plant, arid also cut out differently shaped 

 pieces from some of the leaves and place them in alcohol overnight ; 

 then compare their starch-test with that given by the rest of the leaf 

 the next morning. The starch formed in the leaf during the day is 

 found to have disappeared, more or less completely, in darkness. 



* (b) Would the starch have disappeared had the leaves not been left 

 on the plant ? Repeat the experiment, but cut some of the leaves off, 

 set them in a tumbler with their stalks under water ; cut pieces out, 

 test them for starch, then test the rest of each leaf in the morning. 



131. How does Starch disappear in Darkness? We 



saw that the starch stored in the Bean cotyledons' is on ger- 

 mination converted by the action of a ferment (diastase) into 

 sugar, which is soluble in water and therefore able to travel 

 by diffusion to the growing parts of the young plant and 

 supply them with food. We have seen that the leaf when 

 exposed to light produces starch, which disappears during the 

 night, and if we cut out small pieces from the same leaf every 

 day and tested them, we should find that the alternate pro- 

 duction and disappearance of starch go on day after day. By 

 cutting out pieces of leaf of a certain area, drying them and 

 weighing, we should find that the dry weight of the leaf is 



