Vv 
ramifying in every direction, by which the sap is brought into 
contact with the air. These vessels are arranged in two sets; the 
upper one, m exogenous plants, communicating with the vessels of 
the wood, and the lower with those of the bark. The nutriment 
taken up by the root passes through the upper series to the upper 
surface of the leaves, and after undergoing the necessary chemical 
changes, returns by the lower through the bark to the root, 
depositing various secretions by the way. The spaces between the 
vessels are filled up with cellular matter, in which numerous minute 
pores establish communication between the fluid in the vessels and 
the air; they are chiefly on the under surface in most plants. 
In exogenous plants the principal fibres or veins of the leaf branch 
off from a central vem or midrib, distributing themselves in 
numerous net-like ramifications over the whole leaf, as in Fig. 3. 
In plants with endogenous stems, on the other hand, they are 
usually arranged in a parallel series, diverging from the stalk and 
uniting again at the apex of the leaf, bemg connected during their 
course only by minute cross-veins at right angles to the others, as 
in Fig. 4, The leaves are generally supported on a stalk called a 
petiole, but are sometimes stalkless, or, to speak technically, sessile. 
At or near the base of the petiole two or more leaf-like appendages 
are often found, to which the name of stipules is given. The angle 
between the leaf and the stem is called the aai/, and is the place 
where the bud of the ensuing growth is developed. In some few 
plants the buds drop off, and falling on the earth become bulbs, 
and thus propagate the plant, as in the Tiger Lily; a bulb is, in 
fact, only an underground bud. 
Leaves are either simple, or composed of a number of leaflets 
arranged upon the same stalk, when they are styled compound. 
Simple leaves are either endire, or variously cut and divided; but 
their forms are far too numerous to admit of mention here, and 
for the explanation of the technical terms by which botanists have 
been compelled to distinguish the diversified figures of these im- 
