64 



Classification of Leaves : Chlorophyll. 



growth arc obscurely foreshadowed iii these embryo forms, to be 

 displayed iii full maturity, when the conditions favor usually in 

 spring, but exceptionally in autumn. The latter cases are rare, 

 and occur only when a very dry summer is followed by a very mild, 

 humid autumn. 



Structure and Function of the Leaves. 



234. Leaves are the respiratory organs of plants. The juices are 

 there brought in contact with the air, when certain chemical changes 

 occur that fit them for the processes of assimilation that transform 

 them into every part of the growing tree or plant. 



235. Trees present an infinite variety of forms in their leaves, 



but may be divided into two great classes : the linear, 



or acicular form (sometimes shortened into imbricat- 



ing scales), that distinguish 



the coniferous order, and, with 



a few exceptions, remain green 



during the winter ; and. the 



broad-leaved form, supported 



by a network of ribs, and, in 



temperate and cold climates, 



generally falling from the 



trees upon the approach of 



winter. The latter are termed 



deciduous, when they fall from 



47 Leaves of the t ' ie * rees ^ n autumn, but this term also applies to a 

 pine - small number of the linear-leaved class, such as the 

 larch and the bald cypress. A leaf-stem is called a petiole, and 

 when there is no stem the leaf is said to be sessile. 



236. The framework of a leaf is filled in with a cellular structure, 

 and these cells are filled with a green matter, in the form of small 

 grains, called chlorophyll, and which, appearing through the trans- 

 parent walls of the cells, give the leaves their green color. In 

 most leaves, the upper side is more charged with chlorophyll than 

 the lower, and is therefore of a deeper green. The form of these 

 cells in the beech leaf are shown in the annexed engraving, 1 in 

 which oo is the upper side, and uu the under side ; o, the principal 



Leaf of the Mulberry. 



1 From liossmassler. 



