CHAP. II. 



LEAVES. 



107 



the veins of leaves are at present described ; whence it hap- 

 pens that no precise idea can be attached to the different terms 

 that have been contrived to designate particular forms of 

 venation. 



A third objection is this, — that, while slight modifications 

 in the arrangement of the veins have received distinctive 

 names, others of much greater importance, and of a more de- 

 cided character, have received no distinctive appellation what- 

 ever. For these reasons, the practical weight of which I have 

 long experienced, it has occurred to me that the following- 

 changes in the language used in speaking of venation will be 

 found better, at least, than that for which they are substituted, 

 if they are not entirely what could be desired. 



It has been usual to call that bundle of vessels only which 

 passes directly from the base to the apex of a leaf the rib or costa, 

 or midrib. This term I would extend to all main veins which 

 proceed directly from the base to the apex, or to the points of 

 the lobes. There is no difference in size in these ribs; and 

 in lobed leaves, which may be understood as simple leaves, 

 approaching composition, each rib has its own particular set 

 of veins. 



The midrib {Jig. 53. 7) sends forth, alternately right and left 

 along its whole length, ramifications of less dimensions than 

 itself, but more nearly approaching it than any other veins : 

 these I would call primary veins {Jig- 53. 3). They diverge 

 from the midrib at various angles, and pass to the margin of 

 the leaf, curving towards the apex in their course, and finally, 

 at some distance within the margin, forming what is called an 



