8o THE BIOLOGY OF FLOWERING PLANTS 



give him bread," must be extended from the grass to the 

 plant in general, and from man to the animal kingdom. 

 Nor must a secondary effect almost as important be over- 

 looked — the elimination from the atmosphere of the carbon 

 dioxide constantly poured into it, naturally by the respira- 

 tion of living things, and, one might say, artificially, by 

 combustion ; and its replacement by oxygen. The warring 

 processes, as is well known, keep the amount of carbon 

 dioxide in the atmosphere roughly constant at about 3 parts 

 in 10,000. It was in searching for the process responsible 

 for keeping air " good " that the great chemist, Joseph 

 Priestley, made the first discovery of this activity of the 

 green plant in 1771. 



Essential for photosynthesis are the absorption of light 

 by the chlorophyll and a sufficient supply of carbon dioxide 

 and water ; the process goes on vigorously only within the 

 narrow range of temperature in which plant life is active. 

 The absorption of Hght is, of course, best carried out by an 

 expanded surface of chlorophyll. In the most primitive 

 form we see such a surface in the green dust of Protococcus 

 on the bark of a beech tree, or in the thin thallus of a sea 

 lettuce. In more highly organised plants, the evolution of 

 a more or less massive body has gone hand-in-hand with the 

 evolution of special light-absorbing and assimilating organs 

 — the leaves — having a greatly expanded surface capable of 

 forming organic substance in excess of the needs of the 

 actual assimilating cells, and so supplying those parts of the 

 plant which, with other structure and other functions, 

 possess no chlorophyll. 



The origin of the leaf is still obscure. It is regarded by 

 some as a specialised appendage or outgrowth of the stem ; 

 others look on it as the primary organ from which, in turn, 

 the stem has been derived ; or it may be that stem and leaf 

 have both been derived by diflferentiation of a priinitive 

 thalloid plant body, such as we see in the brown seaweeds. 

 It is possible that in different groups of plants the origin of 

 the leaf has been diverse. This question need not further 

 concern us here. The leaf is now a highly specialised, yet 



