PHOTOSYNTHESIS AND RESPIRATION. 



75 



used again and again ; starch once formed remains in leaves which 

 have been killed by boiling. In class-teaching, however, it is always 

 better to start experiments from the beginning. 



Fig. 34. 



11O. Is Air needed for Starch-Making? It is very 

 easy to prove that the green leaf contains 

 air and bears openings (stomates) com- 

 municating with the atmosphere; these 

 openings are usually most abundant on 

 the underside of the leaf. This at once 

 suggests a method of finding out whether 

 air is concerned in starch-making. 



* (a) Dip the leaves of various plants into very 

 hot (just boiled) water in a warmed tumbler, 

 and notice the expulsion of air- bubbles. In 

 many leaves the bubbles only appear on the 

 lower surface, e.g. Laurel, Rhododendron ; in 

 others they appear on both sides, but more 

 abundantly on the lower surface, e.g. Broad 

 Bean. Cut a large leaf (e.g. Laurel) across and 

 dip the cut edge under water, observing the 

 streams of bubbles given off. 



* (6) Try to blow through leaves dipping into 

 water, first with the stalk and then the blade 



under water. The simple apparatus shown in Fig. 34 may be used 

 to suck air through a leaf. Suitable leaves are Lesser Celandine, 

 Chinese Primrose, Marsh Marigold, Water Lily. 



* (c) Even when one cannot force air through a leaf with the lungs, 

 the presence of air-spaces and of openings on the surface can easily 

 be shown by using an air-pump or exhausting syringe to suck air 

 through, or a bicycle-pump to blow air through. The leaf-stalk should 

 be fixed in a bored cork, with another hole for a short glass tube to 

 connect with the syringe or pump by means of rubber-tubing. The 

 cork is fixed into a wide-necked flask or bottle, and the whole made 

 air-tight with plasticine and vaseline. 



What has the air, which can readily enter the leaf by the 

 stomates and circulate in the air-spaces within the leaf, got 

 to do with starch-making ? We shall study this question in 

 some detail presently. Meanwhile let us see what happens 

 if air is prevented from entering a leaf which is otherwise 

 placed under conditions favourable to the process of starch- 

 making. 



