STRUCTURE.] DEVELOPMENT OF SIMPLE LEAVES. 285 



or partly (half way only when the leaves are opposite), so 

 that the internal sides of the two surfaces cover each other 

 above the top of the axis ; as for example in Bryophyllum 

 calycinum, the Gentians, and many other species. 



The most usual plan of development in Dicotyledonous 

 plants is that in which the halves of the leaves are applied 

 the one against the other ; this plan is, however, subject to 

 many modifications, as each half can be rolled or folded on 

 itself, and may have transverse or longitudinal folds. That 

 part of the leaf which resembles a rim is the first to lengthen 

 and become thin at its edges, which turn in, and at length 

 generally meet on the medial line of the blade. The result 

 of this is at first a slight furrow, commencing below the apex 

 of the leaf, and extending to its base. It is easy, at a certain 

 period, to see that this line is formed by the approach of rims 

 manifested at the edges of the blade, and that it is lost near 

 the base. After a time, when the furrow is deeper, and it 

 extends as far as the base of the blade, the two halves of the 

 latter are sufficiently developed to touch, or every where to 

 cover, one another. 



In this plan of development, the young laminse cover one 

 another, either by turning their edges to the axis, ( Vernatio 

 duplicativa], as in Liriodendron, Leguminous plants, &c., or 

 by turning their edges from the side opposite the axis ( Ver- 

 natio replicativa], as in some Labiates. 



The progress of the line formed by the contact of the edges 

 of the lamina is not always equally rapid in all parts of the 

 leaf. 



The formation of a peltate leaf is owing to a very strong 

 development of the lower part of the blade. A rudimentary 

 leaf of Tropseolum majus resembles afive-lobed or quinquefid 

 leaf, furnished with a short broad petiole ; it is about a mil- 

 limetre long. At the point where the petiole joins the 

 lamina, i. e. at the edge of the latter, are two small bosses 

 which belong to it. The bosses are the most recently 

 developed of the whole blade ; they afterwards form two small 

 lobes; the leaf has then seven distinct lobes. All these 

 lobes approach each other, in consequence of the develop- 

 ment which takes place in all directions, but especially at 



