160 THE LEAVES. 



obese or misshapen, like those of the Ice-plant and other species 

 of Mesembryanthemum, &c. : sometimes they are reduced to tri- 

 angular projections or points, or are perfectly confounded with the 

 unusually developed green bark of the stem, which fulfils their 

 office, as in the Stapelia and most Cacti. 



274. The Development Of Leaves proceeds from the apex (which 

 first appears, in the form of a little tumor or papilla) towards the 

 base, which is later eliminated from the axis. The apex is pushed 

 forward by the formation and growth of the parts beneath : after 

 the blade has shaped itself, the rudiment of the petiole, if there is 

 to be any, begins to be visible, and this grows in like manner from 

 the apex downwards, the lower part of it being the last formed : 

 its growth subsequent to its first formation is greater in proportion 

 to its original size than that of any other part of the leaf. The 

 sheath at the base (as in most Monocotyledons), or the stipules 

 (304, which principally belong to Dicotyledons), are at first con- 

 tinuous with the blade, or divided from it by a mere constriction : 

 the formation and elongation of the petiole soon separate them. 

 The stipules, remaining next the axis or source of nourishment, un- 

 dergo a rapid development early in the bud, so that, at a certain 

 stage, they are often larger than the body of the leaf, and they ac- 

 cordingly form in such cases the teguments of the bud.. Divided 

 or lobed and compound leaves are simple at the commencement, 

 but the lobes are very early developed ; they grow in respect to 

 the axis of the leaf nearly as that grew from the axis of the plant, 

 and in the compound leaf at length isolate themselves, and are 

 often raised on footstalks of their own. Commonly the upper 

 lobes or leaflets are first formed, and then the lower : but in those 

 of the Walnut and Ailanthus, and other large compound leaves, 

 new leaflets continue to be produced from the apex, even after the 

 lowermost are nearly full grown. In the earliest stage leaves con- 

 sist of parenchyma alone : the fibro-vascular tissue which makes 

 the ribs, veins, or framework appears later. No good researches 

 have yet been made upon the mode and order of its production. 



275. The Forms Of Leaves are almost infinitely various. These 

 afford some of the readiest, if not the most certain, marks for char- 

 acterizing species. Their principal modifications are therefore 

 classified, minutely defined, and embodied in a system of nomen- 

 clature which is equally applicable to other parts of the plant, and 

 which as an instrument is indispensable to the systematic botanist. 



