THE LEAVES OF PLANTS. 



103 



Stipules are leaf-like organs occurring in pairs at th 

 petiole with the stem (Fig. 184*). 

 They are formed at the very ear- 

 liest appearance of the leaf, and 

 then are seen as two small tu- 

 mours, continuous with the leaf- 

 like expansion ; and since they 

 grow more rapidly than the loaf 

 itself, they at length become one 

 of its protective coverings. They 



point of connexion of the 



n. ie^< ;f Q f;,i M Fi S- 185*. Exhibiting pitchers of various plants produced 



Fig.184*. A pairof stipules, f modified petioles, a, pitcher of sarracenia. b, pitcker 



b, attached to a stem a, at of nepentes . pitcher O f cephalotus. 



the base of the petiole d. 



usually assume all the external characters and internal anatomy of leaves (except in size 

 and position) , and, no doubt, perform the functions of those organs. In certain pod-bearing 

 plants, as the sweet pea (lathyrus), they cannot be distinguished from leaves ; and, although 

 they appear as distinct organs in certain roses, they sometimes subsequently become true 

 leaves. la the polygonums and rhubarbs they do not assume this leaf-like character, but 

 appear simply as a membraneous, almost colourless, sheath, which surrounds the base of 

 the petiole and the stem, and is known as 

 an ochrea (Fig. 186). When they are 

 found at the base of the petiole of a pin- 

 nate leaflet, they are distinguished from 

 the stipules of the whole leaf by the 

 term stipels. The stipel differs from the 

 stipule in being developed after its leaf, 

 and in proceeding in its growth very 

 slowly. It is occasionally difficult to 

 distinguish the stipule from certain mem- 

 braneous parts formed at the base of the Fi &- 186.-Showing the ochrea, a, or sheath, sur- 

 ,....., rounding the stem in the polygonum, and which 



petiole of the common crowfoot and um- is a modified stipule. 



bellifers ; and in most monocotyledonous or endogenous plants, they are not met with. 



We have now completed our account of the fully developed leaf, with its lamina, 

 petiole, and stipules, without having as yet discussed the constitution of the embryo 

 leaf or leaf-bud, because, although the leaf is developed from the bud, and the bud is the 

 first to be formed, yet in the earliest development of a plant the first leaf is produced 

 without a bud, and passes through its course of development before a leaf-bud appears. 



The leaf-bud is an imbricated or scaly coniform organ, placed in the axis of a leaf, 

 and is a rudimentary leaf or branch formed as the growing season is about to close. 



