DESCRIPTION OF SPECIES— DICOTYLEDONOUS LEAVES. 125 



5. A hyphodrome nervation is that of thick, coriaceous leaves, whose mid- 

 rib only is strong and distinct, while the secondary nervation is indiscernible. 

 In this case, the nervation is about like the reticulate one, which, however, 

 has the second and tertiary veins about of equal thickness, distinct, united, 

 forming a prominent netting. 



6. The aerodrome nervation is that of the leaves with secondary nerves, 

 all at an acute angle of divergence, curving along the borders, the upper ones 

 passing up to the point of the leaves. Of this kind there are two divisions : — 



a. When all the secondary nerves are of the same order, with the upper 

 pair only ascending to the point of the leaves, as in the Dogwood ; 



h. When the two lower secondary nerves are stronger than those above 

 them, ascend to the point, and branch outside, as in Ceanothus Americanus, 

 the common Jersey Tea. 



This subdivision makes a transition to the triple-nerved leaves. 



2. Leaves palmate-nerved. — In this division, the primary nerves, three, 

 five, even seven, go out together from the base of the leaves, or from a short 

 distance above it, generally diverging fan-like, either all of the same thick- 

 ness or the lateral ones of different thickness, as in some Poplar leaves. With 

 palmate leaves, the areas are described as for the penninerved ones, the 

 primary areas being limited between the primary and secondary nerves on 

 both sides, and the name of cardinal areas being given to the large ones 

 inclosed between the primary nerves on two sides and the borders on the 

 other. The palmate-nerved leaves are subdivided also according to the char- 

 acters of their nervation in — 



1. Craspedodrome, with primary nerves going out to the borders : — 

 a. With secondary and tertiary nerves running also to the borders ; 

 h. With secondary nerves camptodrome ; 



c. With the same in part camptodrome, in part craspedodrome (mixed). 



2. Camptodrome, when the midrib only reaches the borders, and all the 

 others curve along them, as in the Poplar. 



3. Aerodrome, when the lateral primary nerves ascend to the point of the 

 leaves. 



3. Peltate-neuved leaves. — They have their primary nerves radiating 

 all around the top of the petiole, which may be either central to the leaves, 

 or on one side of them. The subdivisions Ijave the same characters as those 

 remarked above. 



