CHAPTER V 



THE LEAF 



IN the present lecture let us undertake the study in 

 general outline of the life of the leaf. This task will 

 be a little more difficult and complex than that under- 

 taken in our last lecture, since there appears to be more 

 confusion, among people not conversant with science, 

 about the leaf than about any other plant organ. No 

 other vegetable organ has suffered so much injustice. 

 For centuries, even down to the close of the eighteenth 

 century, people refused to see any direct utility in the 

 leaf. From time immemorial they had admitted as indis- 

 putable the utility of the root as an organ of nutrition, 

 and of the flower and seed as organs of reproduction ; 

 but the leaf continued to enjoy the superficial reputation 

 of a showy but useless ornament : the utmost that 

 people would consent to see in it was an organ for the 

 excretion of noxious vapours. Yet, as we soon shall 

 find, the leaf is as necessary as the root in the nutri- 

 tion of the plant ; moreover, it is the leaf that obtains 

 for the plant what is quantitatively and qualitatively 

 its principal food. It may, indeed, be said that the 

 leaf embodies the very essence of a plant's life. 



The erroneous ideas which prevailed for such a long 

 time with regard to the leaf and its significance are 

 fully accounted for by the peculiarity of the processes 

 of nutrition which go on in this organ. These are 

 totally different, both as regards the nature of the 

 food absorbed and the mode of its absorption, from 

 the nutritive processes of the animal organism, which 

 involuntarily recur to our mind whenever we speak 



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