LEAVES. ; 59 
branch ; opposite, when there are two, which are at the same 
height, but on the sides opposite to each other ; verticil- 
_ late, or in the form of a whorl, when there are several sur- 
_ rounding the stem or axis at the same level. The alternate 
_ is considered the normal or regular mode of distribution— 
the others arising from the non-development of the interme- 
diate parts called internodes—the places at which the leaves 
are developed being termed nodes. 
196. Leaves are formed by an expansion of the fibres of 
_ the stem at particular parts. These fibres are bundles of 
vessels, chiefly spiral vessels, along with woody tissue, which 
by their ramification form a network, or skeleton, which i8 
filled up by cellular tissue, continuous with the tissue of the 
bark. The whole is covered by the cuticle, except in leaves 
which grow under water, which appear to be destitute of 
cuticle. The spiral vessels are derived from the medullary 
sheath. Many of the vessels must be continuous with ae 
alburnum. 
197. When the bundle of fibres ee ee out Seatnisdiately 
on leaving the stem, the leaf is called sessile, as in the White 
Poppy (Papaver somniferum). See Fig. 8, the leaf of Horned 
Poppy (Glaucium luteum). 
Fig. 8 
Sessile Leaf. 
