THE WORK OF LEAVES 



187 



gases over into lime-water, as 

 shown in the figure, it will soon become 

 milky. The starch, therefore, appears 

 to break up into water and carbon di- 

 oxide. What other substances may be 

 formed we cannot determine with the 

 means at our disposal, but these may 

 be neglected, since chemical analysis 

 shows that starch may be considered to 

 result from the union of these two sub- 

 stances. 



The question now arises, Is there any 

 evidence that the plant makes starch by 

 putting carbon dioxide and water to- 

 gether ? We know from previous experi- 

 ments that there is carbon dioxide in the 

 air, since lime-water exposed to air soon 

 forms a precipitate on the surface. We 

 also know that the leaf is well supplied 

 with water. Is it well supplied with air? 

 The best way to answer this question is 

 to place the leaf in an air-pump, which 

 we may construct as shown in Fig. 106. 

 A disk of rubber (leather or rawhide 

 may be used) is fitted to the inside of 

 a student - lamp chimney (the larger 

 size, No. 1, is preferable), and pushed 

 down to the neck of the chimney. The 



