PHOTOSYNTHESIS AND RESPIRATION. 97 



increased at the end of a sunny day, that it decreases after 

 some hours of exposure to darkness, increasing again on 

 exposure to light. These facts suggest that the starch formed 

 in light is digested and carried away during darkness, and we 

 find that the leaf-cells produce diastase just as do the cells of 

 the cotyledons in the Bean seedling. 



Keep a plant in darkness for two clays, then expose it to sunlight for 

 several hours. Remove some leaves, dry them in a slow oven (or on a 

 sand-bath), then powder them and let them soak in cold water for 

 about half an hour. Filter the extract thus obtained, and try its effect 

 on starch-paste, testing with iodine, and later with Fehling's solution. 

 An extract of leaves of Tropaeolum or of Broad Bean, made in this 

 way, quickly converts starch into sugar ; try these and other plants. 



132. Respiration. Every living being, plant or animal, 

 needs a continual supply of energy, without which, for 

 example, neither growth nor active movement is possible. 



Like animals, all plants respire that is, they absorb 

 oxygen and exhale carbon dioxide losing carbon during the 

 process. G-erminating seeds and fungi respire as actively as 

 do warm-blooded animals. The process of respiration is one 

 of slow combustion, and, although the carbon is oxidised at 

 a comparatively low temperature, enough heat is produced 

 to keep an animal warm, or to raise the temperature of a 

 plant by a few degrees. Plants have, however, so large a 

 surface relatively to their bulk that they lose heat very 

 rapidly, and are usually at almost the same temperature as 

 that of the surrounding medium. 



When a green plant respires it simply consumes organic 

 material, which it itself had previously constructed from 

 simple compounds by the aid of the energy contained in sun- 

 light. Thus starch is produced from water and carbon 

 dioxide, and a certain amount of energy fixed and oxygen 

 liberated. Then, at a later date, starch may be consumed in 

 respiration, oxygen being absorbed, carbon dioxide and water 

 liberated, and the " fixed " energy set free. Green plants are 

 unable to make direct use of the energy of sunlight they ab- 

 sorb, but, instead, adopt this apparently roundabout method. 

 Its utility is, however, sufficiently obvious, for if plants were 

 directly dependent upon the radiant energy of the sun for their 

 supplies of energy, they could only grow during the daytime, 

 s. B. 7 



