CHAP. I. THE LEAF. 53 



organ ; the midrib of the expansion being merely a 

 direct prolongation of the central fibres of the 

 petiole, and the lateral nerves being merely ramifi- 

 cations of the midrib, or secondary, or divergent 

 prolongations of the petiole ; so that no separation 

 of them can be effected without absolutely cutting 

 or tearing them asunder. The point from which 

 the fibres of the petiole begin to diverge is generally 

 in the margin of the expansion ; but in some leaves 

 this point is placed in the centre, and the leaf said 

 to be peltate or target-shaped (PI. II. Fig. 3.), as 

 that of the common Nasturtium or Indian Cress. 



If the petiole supports several distinct expansions Support- 

 it is not always simple, and not always to be re- 



Slons * 



garded as constituting an integral part of the leaf. 

 The petiole of the leaf of the Horse Chesnut, which 

 supports several distinct expansions, is indeed sim- 

 ple, but does not constitute an integral part of the 

 leaf, at least in the rigorous sense in which the 

 petiole supporting only a single expansion may be 

 said to do ; because the several expansions, which 

 are indeed the real leaves, are not direct and imme- 

 diate prolongations of the petiole, but distinct or- 

 gans attached to it by a natural and conspicuous 

 joint, at which, in the season of the leaf's fall, they 

 spontaneously detach therqselves from the petiole, in 

 the same manner in which the petiole detaches 

 itself from the tree. 



But when the petiole supports several distinct Com- 

 expansions it is itself generally divided, or, in the \\^f pe " 



