84 



LECTURE v. 



the tube d which contains pumice-stone saturated with solution of potash : the 

 ' p /plant passes through a tubulature in the dish b. 



"The left-hand plant is the control-experiment ; the dish b' contains only water, 

 and the air has free access to the interior of the jar. 



The air in c can contain no carbon dioxide, whereas it is present in the air 

 contained in c'. Moll found that the chlorophyll-corpuscles in the leaves of the 

 plant in c contained no starch grains, after exposure to light, whereas those of the 

 plant in c contained them abundantly. 





FIG. 1 8. The glass bell-jar i stands in a vessel containing water ; in this is placed 

 a leaf be ; carbon dioxide to the extent of 5 per cent, is introduced into the 

 air of the bell-jar by means of the tube k : the apical end of the leaf b is intro- 

 duced into another glass vessel containing a quantity of solution of potash d. 



Thus one part of the leaf c is in an atmosphere which contains a large quantity 

 of carbon dioxide, the other part b is in an atmosphere which contains none. 

 Moll found that when the whole apparatus was exposed to light, starch was 

 formed in the chlorophyll-corpuscles of c, but none in those of b. 



The amount of carbon dioxide absorbed by a plant is so 

 considerable that there can be no doubt that it is chemically 

 transformed, after its absorption, in the metabolism of the 

 plant. The nature of the changes which it undergoes will be 

 discussed when we are considering the constructive metabolism 

 of the plant. 



Absorption of Nitrogen. 



The question as to whether or not plants can take up any 

 appreciable quantity of free nitrogen from the air has long 

 engaged the attention of chemists and physiologists. Priestley 

 and Ingenhousz concluded from their experiments that plants 



