STRUCTURE.] THE ENTIRE LEAF. 275 



principally occurs in Monocotyledons. The lamina, ligula, 

 petiole, and sheath, can be distinguished in it. The formation 

 of this leaf has been already described in a general manner in 

 what precedes. When the leaf offers the four above-mentioned 

 parts, the apex and the lamina are formed first, then the 

 sheath, and lastly the petiole. 



The process of development is precisely the same in Dico- 

 tyledons, except for the lamina. In Monocotyledons the 

 lamina appears from the very first, in the form of a flat 

 expansion which afterwards rolls up : but in Dicotyledons 

 it generally appears as a fleshy petiole, continuous with 

 the axis. 



When the vernation of the leaf-bud is duplicate, the leaves 

 have at their first appearance, hardly any resemblance to 

 the forms they afterwards acquire. 



The plan of vernation is seen from almost the earliest 

 period of the development of the leaves, but it is modified in 

 many ways by subsequent developments. 



In Liriodendron tulipifera for example, the youngest leaves 

 form a semicircle on the axis ; afterwards when the petiole is 

 formed, their vernation becomes inclinative ; and lastly, but 

 before the expansion of the bud, the leaves are almost erect. 



" Formation of the Lobed or Divided Leaf. The apex of the 

 leaf rises on the axis in the shape of a little tumour ; other 

 small tumours joined by a fleshy rim, are emitted right and 

 left at the same time that the original one is being developed ; 

 new tumours join themselves to the first, so that the axis at 

 length becomes a large lobe with five, seven, nine, or more 

 notches ; this lobe rests on a base which is more or less 

 enlarged, or on a short thick stalk, which, if there are any 

 stipules, is not clearly defined until after their formation. 



In leaves which, when completely formed, have small lobes 

 alternating with the large ones, the small ones are never 

 visible at the birth of the larger, but they appear at a later 

 period, and are developed in the same way. 



On stripping a bud of Acer campestre below the outer 

 scales, from three to five pairs of leaves almost completely 



T 2 



