156 WHAT IS A PLANT? 



be evolved from the leaves. That the gas is oxygen could be 

 proved in several ways. If we boiled the water before using it, 

 thus expelling the carbonic acid, no bubbles would come away, 

 showing that the oxygen was gained by decomposition of the acid 

 contained in the water. Suppose that our experiment had been 

 performed in the dark, no oxygen bubbles would have arisen, 

 showing light to be necessary for decomposition of carbonic acid. We 

 used green leaves ; our inference is that chlorophyll is the decompos- 

 ing agent. It is so ; it is the talisman with which the plant works 

 its will with the mineral world. Grow some mustard-seeds in 

 moist flannel in darkness ; the first leaves are almost colourless ; 

 expose the plants to the sunshine, they become green ; this shows 

 light to be necessary in oj'der to produce chlorophyll., which shall be 

 competent to decompose the carbonic acid of the atmosphere. 



Now, on the other hand, let an animal — say, one of ourselves 

 — breathe into a bottle containino; lime-water. Shake the bottle 

 well ; a milky fluid is seen, which, analysed, would prove to be 

 calcic-carbonate, obtained from the union of lime-water with the 

 carbonic acid from the animal's breath. In other words, the ani- 

 mal evolves^ not oxygen, but carbonic acid. We need not dwell 

 on the ascertained fact that animals absorb or take in oxygen, and 

 we have seen that a plant takes in and decomposes carbonic acid. 

 Now, Cuvier founded on such facts as hese this fourth distinction. 

 He held that animals gave off carbon and hydrogen in the form of 

 carbonic acid and water, absorbing oxygen from the air ; plants, 

 inversely, took in carbonic acid and water, retained the carbon 

 and hydrogen, giving off oxygen to the air. In short, his conclu- 

 sion was that evolution of oxygen^ under the agency of light, was 

 the special feature of plant-life — respiration — /.<?., evolution of car- 

 bonic acid., and absoiptio7i of oxygen., that of animal life. 



How does this distinction fail in its turn ? We saw just now 

 that our green leaves in darkness evolved no oxygen bubbles. We 

 did not say that no bubbles were evolved. As a matter of 

 fact, our green leaves in darkness would have given rise to bubbles 

 of carbonic acid, and had v/e led this gas into a botde of lime-water, 

 precisely the same result would have ensued as that produced by 

 the animal breath. 



Again. Either in sunshine or darkness, take some germinating 



