STRUCTURE.] LEAVES OF CERATOPHYLLUM. 291 



a small scale, the perfectly developed leaf, the length of which 

 is from 150 to 200 millimetres. The leaflets then acquire, in 

 the way above described, their leafy halves. The short broad 

 petiole lengthens rapidly, and at the same time there appears 

 immediately below, but only on one side of each leaflet, a 

 thickening which is continuous with the edge of the leaflet, 

 forming at first an unequal rim, and afterwards a small den- 

 tate leafy blade, continuous with the lamina of the leaflet. It 

 is this that constitutes the winged petiole. 



The formation of the lamina of Papilionacese is tolerably 

 alike in all the species that I have examined. The leaflets are 

 folded so as to form a duplicate vernation. Th leaflet, at first 

 doubly convex, is hollowed or flattened on the side that after- 

 wards forms the upper or under surface ; its edges are raised 

 in the shape of a rim, which gradually spreads on the concave 

 part in which the midrib is afterwards formed. When they 

 touch each other on the medial line, they form a deep opaque 

 furrow, situated between two projecting ridges, which become 

 gradually thinner, and ultimately form the leafy lamellse. 



" History of the Development of the Leaves of Ceratophyllum 

 demersum. The axis of the leaf-bud is a rounded cone, 

 having below its summit a great quantity of nascent whorls, 

 of which the upper are completely covered by the lower. 

 There is no whorl immediately below the top of the axis; not 

 far below it we, however, find two or three indistinct rings; 

 a little lower still, we see several whorls of contiguous little 

 tumours ; and below these again, several crowns formed by 

 little straight points are observed. The whorls are separated 

 from each other by small portions of the axis, which are 

 either naked or covered with the rudiments of leaves. The 

 interstices are smaller near the top than near the bottom of 

 the axis. 



The number of leaves of which a complete whorl is formed 

 varies from ten to sixteen. It is alinost impossible to count 

 the exact number of little projections in the nascent whorls; 

 there are often from thirty to forty of them. These nascent 

 whorls bear a great resemblance to the integument of an 

 ovule, when being formed. 



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