SECT. I. ORGANOGRAPHY AND GLOSSOLOGY. 



61 



mains a long time attached to the stem before it falls off 

 and leaves the scar. Some petioles are termed " clasp- 

 ing," when they are attached for some extent around the 

 stem ; and they form " sheaths/' 

 when they wholly embrace it, as in 

 the grasses. In some, a membranous 

 limb-like expansion occurs on each 

 side of the petiole, which is then said 

 to be " winged." The limb in gene- 

 ral is similarly constructed on each 

 side of the midrib ; but to this there 

 are striking exceptions, as in the leaves 

 of Begonia (fig. 53.), Epimedium, 

 &c. 



(70.) Simple and compound Leaves. 

 The most obvious classification of 

 leaves, is in to "simple" and "compound." The limb of the 

 former consists of one piece 

 only (fig. 54.), which may 

 either be entire at the mar- 

 gin (a), or variously indent- 

 ed (6) ; and attached to the 

 stem with or without the in- 

 tervention of a petiole : in 

 the latter case it is said to 

 be " sessile." Compound 

 leaves (fig. 55.) are made up 

 of one or more pieces, called 

 " leaflets," each of which 

 is articulated to the petiole ; 

 and the degree to which it is 

 compounded, depends upon 

 the number of times in which 

 the main petiole branches, before the leaflets are attached 

 to its ramifications. Hence we have the simply (a), 

 doubly, triply (6), & c . compound leaf. 



(71.) Venation or Nervation of Leaves. The distri- 

 bution of the vascular tissue through the limb of the 



