a 
3 
: 
P 
4 
3 
q 
ON LEAVES. 81 
the leaf, as in the Bauhinia, Fig. 103, the leaf of which gives a 
very good idea of the cleft leaf. The leaf of Castor-oil tree 
(Ricinis communis), Fig. 104, is cleft in eight sections. Finally, 
leaves are partite when the separation penetrates thearly to the 
petiole, or reaches the centre of the limb, as in Fig. 105, a bipartite 
leaf, and again in Fig. 106, which represents the leaf of Cannabis 
sativa, the Hemp; as Fig. 107 does Echinops spherocephalus, and 
as Fig. 108 does the leaf of Scholymus hispanicus, in which the 
divisions of the leaf are more numerous still. 
The leaf is, as we have said, a flattened organ having two 
surfaces, and border, the whole of which constitutes the /amina, or 
blade. The blade of every leaf is traversed by prominent lines or 
ridges called veins, which are, however, more saillant on the lower 
face than on the upper. These are formed of woody tissues, spiral 
vessels and cellular tissues. They form the nervure of the leaf, and 
are retained in their positions, and the intervening space is filled up, 
by cellular tissue. The tissues of the veins are brought into closer 
proximity in the petiole, which is a small stem. Having passed 
into the stem, one part ef the food of the plant enters the bark, while 
the other traverses the wood and vay 
penetrates to the medullary sheath 
at the centre of the stem. Every 
leaf is thus in communication with {‘ 
the stem, and not this only: itis,in {* 
fact, aprolongation of the pith, spiral \ 
vessels, and wood of the system. 
The disposition of the nervure, or 
veins, differs very little in the three 
principal types. In the Chestnut, 
whose leaf is represented in Fig. 
100, the nervure runs from the base ny 
to the summit of the blade, sending _Fig- 109—Leaf of the Marsh Mallow. 
out to the right and left a secondary set of veins parallel to each 
other, disposed like spray of a feather. In the Mallow (Malva 
sylvestris), Fig. 109, five principal nerves run from the base of the 
leaf to the apex, and radiate in the blade, like the foot of a web- 
footed bird, In the Iris, of which leayes are represented in 
Fig. 30, p. 30, a great number of delicate veins run from the base 
G 
