7 
their structure may be seen in a piece of cane, and even the herbage of 
Grasses, Asparagus, Ruscus, &c. is sufficiently developed to exhibit the 
same appearance. | 
Our next cuts, figures 3 and 4, exhibit certain peculiaritiesin 
the “venation” or “nervation” of the leaves. It has been re- 
marked that the great majority of Exogene have the veins 
of their leaves distributed in a particular manner, whilst those gaygy 
of Endogene aré differently arranged. The probable cause E 
of this discrepancy is — by De Candolle to the more fre- rs 4 
or footstalk, commonly called the tail; and 2nd, the limb, or flat ex- 
panded portion. The veins in the petiole consist of bundles of vessels 
which run parallel to each other, and are more or less united into one 
mass; but when they reach the limb they diverge in various ways, sub- 
dividing as they proceed, and branching off at acute angles into smal- 
ler and smaller threads which give the limb a reticulated or net-like ap- 
pearance, as figure 3. In certain cases the limb is not developed, and 
then the petiole is frequently flattened out and becomes limb-like, ex- 
cepting that the nerves do not branch, but pee oe parallel when 
the form of the petiole is linear, or d converge 
towards the apex and base, when it is more or less oval. There are 
certain plants whose leaves are of two kinds, some of them being com- 
posed of a petiole and limb, and the rest consisting of a petiole only; 
and in these cases the difference in their venation is very distinctly 
marked. De Candolle therefore supposes that in other cases, where 
the venation of a leaf resembles that of these expanded petioles, we 
ought to consider them in the same light. There are some cases in 
which it would be difficult to allow such a conclusion, but in most others 
the inference is probably correct. But whether the reason assigned for 
these differences be the true one or not, we find the great mass of Ex- 
ogene have their leaves reticulated with yeins, as in figure 3, whilst 
those of the Endogene are seldom branched, but are disposed as in 
figure 4. 
Besides these characteristics derived from the stems and leaves, (two 
of the nutritive organs) there are others of equal or even greater gen- 
’ 
