200 



BOTANY 



Sensitive leaves, like those of many Leguminosae, often suffer more 

 under such conditions than when the possibility of assimilation is pre- 

 cluded by their being deprived of light. 



The by-product arising from the assimilatory process is PURE 

 OXYGEN. The volume of oxygen thus set free is equal to the volume 

 of carbonic acid taken in. If plants assimilate in a known quantity 

 of air containing carbonic acid gas, its volume will therefore remain 

 the same. The chemical process of assimilation resulting in the de- 

 composition of the carbonic acid may be thus expressed : 



6C0 2 + 5H 2 = C 6 H 10 O 5 + 60 2 



(Starch) 



From this chemical equation it is evident that water is requisite 



FOR THE PROCESS OF ASSIMILA- 

 TION. The actual composition of 

 starch corresponds rather to a 

 multiple of the above symbol, or 



n(C 6 H 10 O 6 ), so that 

 should be 



the whole 

 multiplied 



Fio. 1S3. — Evolution of oxygen from assimilating 

 plants. In the glass cylinder C, filled with 

 water, are placed stems of Elodm canctdends; 

 the freshly-cut ends of the stems are intro- 

 duced into the test-tube R, which is also full 

 of water. The gas-bubbles B } rising from the 

 cut surfaces, collect at .S'. H, stand to sup- 

 port the test-tube. 



(at a temperature of 14° C 



equation 

 by n. 



The oxygen given off by green 

 plants, although not perceptible 

 when they are growing in the 

 open air, becomes apparent in the 

 case of water-plants. It was, in- 

 deed, through the evolution of 

 bubbles of oxygen from water- 

 plants that Ingenhouss first had 

 his attention called to the assimi- 

 latory activity of leaves. To see 

 this process, it is only necessary 

 to place a cut stem of a water- 

 plant in a vessel of water exposed 

 to the sunshine, when a continuous 

 series of small bubbles of gas will 

 at once be seen to escape from the 

 intercellular passages intersected 

 by the cut. The gas thus evolved 

 may be collected with little trouble 

 (Fig. 183), and will be found to 

 be chiefly oxygen, but containing 

 also traces of nitrogen and car- 

 bonic acid taken up from the 

 water. As water absorbs much 

 less oxygen than carbonic acid 

 100 vols, of water will dissolve only 3 



