112 DEVELOPMENT OF COMPOUND 



Now, it is plain, from simple inspection of the fibrous part 

 of these different varieties of leaf, that in proportion as that 

 is developed, the leaf itself becomes more highly organized. 

 These leaves were detached in regular order from the side 

 of the shoot, commencing with those situated in the neighbor- 

 hood of the flower and proceeding downward into the vege- 

 tative region. At a we have the lamina of the leaf with only 

 a biserrate margin, and no petiole. This specimen was nearest 

 the flower. At b we have a trilobed condition of the lamina, 

 or a partial formation of one terminal and two lateral leaflets. 

 At c one of the leaflets is formed ; and the other two are still 

 in a state of anastomosis, d, The three leaflets detached from 

 each other and fully formed, e, The same more developed. 

 /, A bilobed condition of one of the leaflets, and an increased 

 development of the petiole, g, Both of the leaflets bilobed. 

 A, One of the new leaflets fully formed, i, Complete forma- 

 tion of both of the new leaflets. 



Here, then, we have plainly visible the intermediate stages 

 in the formation of the lamina or blade of a leaf with only a 

 biserrate margin, into five distinct leaflets. Surely, after 

 such a lesson from Nature, we may safely regard the lobed 

 variety of leaf as the condition of a lamina whose leaflets are 

 in a state of partial formation, and remain still organically 

 united with each other. 



The leaves of the swamp maple (Acer rubrwri), the sugar 

 maple (Acer saccharinum), the Plane-tree maple (Acer pseudo- 

 platanus), and the Castor-oil (Ricinus commnnis), Plate IV., 

 Figs. 1, 2, 3, 4, may therefore be regarded as transition forms 

 between simple and compound leaves. In all these cases the 

 lamina of the leaf consists of several partially-formed anasto- 

 mosing leaflets, whose number is indicated by the number of 

 lobes, and the extent to which their formation has been 

 carried, by the depth of the divisions between the lobes, 

 precisely as the number of petals and sepals united and the 

 extent of their union is shown in the gamopetalous and gamo- 

 sepalous envelopes of the flower, by the number of lobes or 

 teeth on their superior margin. 



When the fibrous ramifications of these lobed leaves are 



