IX Leaves 171 



green plants the absorption of oxygen and the liberation 

 of carbonic acid gas is counteracted by the reverse nutri- 

 tive process. 



(3) Leaves are the principal organs by means of which 

 the plants absorb carbonic acid gas from the air, and by 

 aid of the radiant energy, which passes through the screen 

 of green colouring matter, split it up, liberating the oxygen, 

 and using the carbon as the foundation for the upbuilding 

 of organic products, in which water from the roots, and in 

 some cases salts as well, are also ultilised. 



The student may indeed feel that these results might 

 have been reached much more rapidly by a priori reason- 

 ing. As plants live they must breathe, and the leaves are 

 obviously the organs fitted for this. As plants build up 

 carbon-compounds, and yet get no carbon from the soil, it 

 is evident that they must get it, as carbonic acid gas, from 

 the air, and the leaves are obviously the organs fitted for 

 this ; and so on. But this a priori method is never con- 

 clusive, and though we have done little more than indicate 

 the lines of experiment, we have shown how the functions of 

 the leaf may be experimentally demonstrated without taking 

 very much for granted. To do so even imperfectly is better 

 than to follow the short cut of dogmatic assertion ; and with 

 Professor Detmer's convenient and well-illustrated manual 

 of experimental physiology, now EngHshed, the student, 

 even without a teacher, may fairly set to work, and 

 profitably read also, as his experimental course demands, 

 the appropriate passages in the manuals of Sachs and 

 Vines. 1 



The Structure of the Leaf. — We are now in a 

 position to advance to an examination — at first a very 

 general one — of the internal structure of the leaf. 



^ Sachs, Lectures on Vegetable Physiology, Oxford, 1887. Vines, 

 Vegetable Physiology, Cambridge, 1886. 



