THE SEEDLING AND YOUNG PLANT 79 
met with on the same plant. ‘The young leaves are 
folded in the bud in such a manner that the two halves 
of the lamina lie one on the other, the upper surfaces 
being in contact (conduplicate vernation), the margins 
being therefore turned upwards. 
In order to understand the structure of the leaf, 
let us look at a section cut neatly across the midrib and 
lamina, and examined with the microscope. It is found 
to consist of three principal parts—an epidermis above 
and below, and all round the margins, and therefore 
over the whole of the leaf; this epidermis is, in fact, 
a continuation of that of the young shoot-axis, and 
envelops the whole of the remaining leaf-tissues. In- 
side this we have the main mass of the leaf substance 
—called the mesophyll—consisting of thin-walled cells 
arranged in a peculiar manner, and containing (in 
addition to less obvious structures) large numbers of 
green chlorophyll corpuscles; it is the predominance of 
these corpuscles which causes the leaves to appear 
uniformly green. Here and there we see vascular 
bundles, embedded, as it were, in the mesophyll, cut 
across in various directions, and when it is remembered 
that these vascular bundles constitute the venation of 
the leaf this phenomenon is easily explained. 
As we have already seen, the vascular bundles of 
the venation (fig. 20) are simply the much-branched and 
thinned-off upper ends of the vascular bundles from the 
shoot-axis, the lower ends of which join the vascular sys- 
tem of the latter lower down. Now the next point to be 
