THEIR DEVELOPMENT, ETC. 



153 



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

 angular projections or points, or are perfectly confounded with the 

 green bark of the stem, which fulfils their office, as in the Stapelia 

 and most Cacti. 



273. The Development of Leaves. At their first appearance, each 

 leaf is a minute papilla or projection of parenchyma on the nascent 

 axis : as it grows, this shapes itself into the blade, and is eliminated 

 from the axis. The petiole, if any, is later formed, and by its 

 growth raises the blade from the stem. Commonly the apex of the 

 blade first appears, and the formation proceeds from above down- 

 wards. The sheath at the base (as in most Monocotyledons), or 

 the stipules (259, which principally belong to Dicotyledons), are at 

 first continuous with the blade, or divided from it by a mere con- 

 striction : the formation and elongation of the petiole soon separate 

 them. The stipules, remaining next the axis or source of nourish- 

 ment, undergo 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 accordingly form in such cases the teguments of the bud. 

 Divided or lobed and compound leaves are simple at the commence- 

 ment, 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, the 

 formation proceeds from below upwards, and new leaflets continue 

 to be produced from the apex, even after the lowermost are nearly 

 full grown. In the earliest stage leaves consist of parenchyma 

 alone: the fibro-vascular tissue which makes the ribs, veins, or 

 framework appears later. 



274. At the points on the surface of the developing leaf where 

 stomata are about to be formed, one of the epidermal cells early 

 ceases to enlarge and thicken with the rest, but divides into two (in 

 the manner formerly described, 33), forming the two guardian-cells 

 of the stomate : as they grow, the two constituent portions of their 

 common partition separate, leaving an interspace or orifice between. 

 In some cases, each new cell divides again, when the stomate is 

 formed of four cells in place of two. 



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

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



