194 NUTRITION 



in darkness, nor if the plant be killed, nor if the water con- 

 tains no carbonic acid, nor if the roots which are without 

 green colouring-matter be used instead 

 l|IU=^== ^ ^ ,iJ of the green shoot. Consequently we 



jpiU 11 conclude that the oxygen is given off 



ImLgg^-^. ,^ -_^^J I only from the parts which are receiving 

 ||L^^^^i^Srl73 I carbon from the carbonic acid. In 

 Ill^^^^^toSl other words, the living plant exposed 

 llE^^^^^^^ 11 ^° ^^^ ^€!A takes in carbonic acid by 

 || ^^^^^^^^3 IP means of its green parts, it retains tbe 

 ^BlBms^simimssair ^^-^^^^-^ ^s food but gives the oxygen 

 Fig. 238. - Evolution of ijack to the air. 



bubbles of oxygen from the _ _ «. ■« j. ^x. 



cut end of a stem under water. Influence of Temperature on the 

 (BaseduponA.Mayer'sfigure.) absorption of carbonic add and on 

 the evolution of oxygen by green parts. — If the experi- 

 ments just described are to be successful, the green parts 

 must be exposed to a proper temperature. If the bottle or 

 the water be kept too cold, or too hot, there will be no absorp- 

 tion of carbonic acid, and no evolution of oxygen. There are 

 three important temperatures — (i.) the lowest temperature, or 

 minimum, at which the plant can still absorb carbonic acid and 

 exhale oxygen ; (ii.) the highest temperature, or maximum, at 

 which these processes still go on ; (iii.) the best or most suit- 

 able temperature, or optimum, at which the_ plant is perform- 

 ing these processes most rapidly. Of course, the optimum 

 temperature lies between the minimum and maximum. 



Influence of the intensity of Light on the absorption of 

 carbonic acid and evolution of oxygen by green parts. — Light 

 is essential to the process. Commencing with darkness, as we 

 increase the intensity of the light to which the plant is exposed 

 the two processes become more and more active. This may 

 be illustrated by a simple experiment made on the cut shoot of 

 the water-plants. We note that near the window, exposed to 

 the sun, the bubbles of oxygen come off rapidly, but when the 

 plant (in the water) is transferred to a gloomier part of the 

 room the bubbles appear more slowly, till in absolute darkness 

 they cease entirely. 



Chlorophyll, or the green colouring-matter. — The substance 

 which causes the green colour of leaves and stems may be 

 termed chlorophyll. Placing leaves in water, the chloro- 

 phyll is not removed from them, for it is insoluble in water. 



