116 DEVELOPMENT OF COMPOUND 



Therefore the truly simple leaf is the one with the entire 

 edge, and all other leaves with any irregularities of margin, 

 such as lobes, teeth, crenatures, etc., are transitional forms 

 between simple and compound leaves. For such irregularities 

 are in reality so many incipient advances to the compound 

 condition of the lamina or blade of the leaf. 



It has been shown (page 112) that a leaf can develop other 

 leaflets from its margin, and thus become a compound leaf. 

 Now, when this process ceases in its first stages, and the 

 apices of the new leaflets are only just visible on the margin 

 of the parent leaf, then that margin is rendered serrulate, 

 crenulate, denticulate ; or if the formative process is carried a 

 little further, so as to render the summits of the new leaflets 

 rather more apparent, then the leaf-margin is said to be ser- 

 rate, crenate, dentate. 



In the lobed variety of leaf, the upper portion of the lamina 

 of each leaflet is fully formed, and the number of forming 

 leaflets, as well as the extent to which their formation has been 

 carried, is expressed botanically by the terms bilobed, tri- 

 lobed, quinque-lobed, etc.; the words bipartite, tripartite, 

 and quinque-partite are expressive of a still higher degree 

 of development, and indicative of the formation of the greater 

 portion of the lamina of the leaflets, and that their detach- 

 ment from each other has been nearly carried down to their 

 main petiole or support. 



If the reader compares the leaf of the False sycamore (Acer 

 pseudo'plalanus), Plate IY., Fig. 3, with that of the Castor-oil 

 plant (Ricinus communis), Fig. 4, he will see that the transition 

 from the one form of leaf to the other has been brought about 

 by the partial development of the lamina of an additional leaf- 

 let, and that the extra lobe thus formed fills up that sinus or 

 opening which is left between the two lower lobes of the 

 Sycamore leaf, and thus produces in the Castor-oil leaf the 

 palmately lobed variety. The terms palmately- cleft and 

 palmately-parted, point out a still further formation of the 

 lamina of the leaflets, whose several bases nevertheless still 

 remain in a state of coalescence. 



But in the digitate leaf of the Horse Chestnut (CEsculus 



