THE GREEN LEAF 63 
‘* veins ’? which are left, and constitute the 
delicate network of “skeleton leaves” on 
macerating leaves in water. The vascular 
bundles are rather complicated in structure, 
and they represent the most highly specialised 
tissues of the plant body. <A_ vascular 
bundle consists of wood (xylem) and bast 
(phloém), and a thin band of tissue known 
as cambium often lies between them. They 
anastomose freely in the oak leaf, and in 
the stalk they are collected into a few large 
vascular strands which join the vascular 
tissues of the stem. Similarly the root pos- 
sesses vascular strands, and these are like- 
wise joined with those of the stem or trunk, 
and thus there is a general continuity of the 
vascular tissue throughout the plant. The 
water enters the root from the soil, passes up 
the trunk, and flows thence into the leaves, 
travelling through certain specialised cell 
elements of the wood. In the leaf it is dis- 
tributed to various kinds of cells, and especially 
to those containing the bulk of the chloro- 
phyll (P, in Fig. 9) in which photosynthesis 
is especially active. The greater part of it 
evaporates from the cells into the large air 
spaces which are present in the leaf substance, 
and the contained air is thus saturated with 
aqueous vapour. 
The cuticle, which forms the outermost 
membrane of the epidermis, would prevent 
the exit of any water, either as liquid or 
vapour, if it were perfectly continuous. Inthe 
