64 Classification of Leaves : Chlorophyll. 



growth are obscurely foreshadowed in these embryo forms, to be 

 displayed in 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 liuBar, 

 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 J\^\ -^ , >^ y- y, 

 by a network of ribs, and, in fiv^\^^'^^Xr'^2>-^ 

 temperate and cold climates, f\c^ ] 

 generally falling from the v;^-V^;^t^I^^-<I^^ 

 trees upon the approach of '^^^/^ >---■ 



winter. Thelatter are termed "'^ 



7 .J \. i.\. c n I 48. Leaf of the Mulberry. 



deciduous, when they tall from 



47 Leaves of the *^^® trees in 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 cMwophyU, 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,' in 

 which 00 is the upper side, and uu the under side ; o, the principal 



' From Rossmassler. 



