56 



LEAVES. 



No matter how much the lamina may be divided, if the divi- 

 sions are not articulated to a common petiole, the leaf is simple ; 

 and if the lamina is not divided at all, but articulated to the 

 petiole, the leaf is compound, as in the Orange. The principle 

 of formation of this class of leaves will be readily understood by 

 the above explanation of simple leaves. 



94. All compound leaves may be reduced to two varieties, 

 corresponding to the feather- veined and radiated forms of retic- 

 ulated leaves. If we recur to the feather-veined leaf given above, 

 and conceive each of the primary veins with the secondary 

 veins belonging to it, to form a distinct lamina, we should have 

 a true type of the Pinnate leaf, as exhibited in the Vicia, Pea, 

 &c, and by the continuation of the petiole we have the ten- 

 dril (Fig. 85) ; and by continuing our supposed dissection of the 

 Chestnut, and conceiving not only each primary vein as being 



Fig. 85. 



Fig. 86. 



Bipinnate leaf. 



independent of the others, but each secondary one forming a 

 lamina, and the primary vein becoming a common petiole for 

 them, we then shall have the type of the bipinnate leaf, as ex- 

 hibited in Fig. 86. We may conceive this division to go on 



Fig. 87. 



Tripinnate leaf. 



Unequally pinnate leaf. 



94. To what varieties mav all compound leaves be reduced? What, is a 

 pinnate leaf.' When bipinnate! 



