60 THE LEAF. 



influence of heat and light. In consequence of this expo- 

 sure, and the action of certain organs with which this part is 

 furnished, the qualities oi the sap are changed, and it as- 

 sumes the odor or fragrance peculiar to the leaves of some 

 plants. After the sap has undergone this change, it is sent 

 back to the bark of the tree, where it suffers another change, 

 and is converted into wood. Meantime the watery and su- 

 perfluous parts of the sap are thrown off from the under sui- 

 face of the leaf. 



Fig. 26. 



FORMS OF THE LEAVES. 



With respect to form, leaves are divided into simple and 

 compound. 



Simple Leaves, Leaves are simple when only one grows 

 on the same petiole, as the Oak, Chestnut, Rhubarb. 



Compound Leaves. Leaves are said to be compound, when 

 several leaves or leaflets grow on the same petiole, as in the 

 Rose, Elder, and Senna. 



The outlines, or shapes, most common to the simple leaves, 

 are the following. 



Round, Fig. 26, (orbiculata,) having the lon- 

 gitudinal and transverse diameters equal. Ex. 

 Pimpernel, (Anagallis tenella.) It is very rare 

 that precise examples of this leaf occur in na- 

 ture. The specific name, rouivl ea.ved,(rotun- 

 difolia,) though often applied, is therefore very 

 seldom strictly applicable. 



Ovate, Fig. 27, (ovata,) egg shaped, having the 

 form of an egg divided lengthwise into two halves. 

 The length is greater than ihe breadth, and the 

 base broader than the apex. Ex. Chequer-berry, 

 (Michella repens,) Pear. Obovate is this form re- 

 versed. 



Roundish, Fig. 28, (subrotund,) nearly round, 

 having little difference between the longitudinal 

 and transverse diameters. Ex. Round-leaved 

 Wintergreen, (Pyrola rotundifolia) Green-briar, 

 (Smilax.) 



What effect do the leaves produce on the sap ? How are leaves divided 

 with respect to form ? When are leaves said to be simple ? When are 

 leaves compound ? Give an example of a round leaf. Of an ovate leaf 

 Of a roundish leaf. 



