LEAVES. 



671 



Leaves which are developed at the end of a chief axis are termed blossoms. 

 Of course it must be understood that all the 

 different kinds of leaves do not occur upon 

 the same plant. The leaf may be accepted 

 to mean the stem-leaf. 



Leaves are folded up in various ways, 

 and the manner in which this is accomplished 

 is termed the vernation of the plant. The 

 leaves of endogens and exogens differ in their 

 veining. The former veins do not touch ; 

 there is none of that beautiful interlacing 

 which we find in the exogenous leaves, 

 rise from base to apex, curving as they 

 known lily of the valley. This " nervous 



Fig. 762. Net-veined leaf. 



In the former the veins 

 advance, as in the well- 

 system ". of the leaf is 



its "venation," and the veins distributed in the blade or lamina of the 



leaf are twofold, as remarked, 

 ascending in curves, or diverging 

 from a central nerve called the 

 " mid-rib." These lateral nerves 

 are either parallel or " reticulate " 

 that is, net-like. 



We will now examine the 

 forms of leaves which are regu- 

 lated by the divergence and exten- 

 sion of the divisions of the mid- 

 rib. Thus we get an orbicular, or 

 peltate leaf; palmate, digitate, and 

 pedate forms also occur, as may 

 be seen in the illustrations, pages 

 672 and 673, where all the varied 

 shapes can be studied. The leaf 

 consists of a petiole or stalk, and 



the lamina or blade. The petiole is composed of bundles of vascular tissue ; 



the lamina is formed by their extension, the interstices 



being filled with cellular tissue. So we perceive that the 



leaves and stems are composed of similar materials. To 



defend the tissue a skin, or epidermis, is placed upon the 



surface of the leaf, and this epidermis is full of breathing 



holes, or pores, called stomates (compare page 664). 



There are also cells filled with chlorophyl, which gives 



the leaf its green tint. 



The petiole may be absent in a leaf, and when it is the 



leaves are termed sessile, or sitting leaves. These leaves 



sometimes coil round the stem, and are called " amplexi- 



caul," or stalk embracers. These are simple leaves. Compound leaves are 



Fig. 763. Linden tree. 



Fig. 764. Stomates, highly 

 magnified. 



