252 Campbell. — On the development 
succession of the segments may be traced with little difficulty, 
whereas when no definite apical cell is present ( e.g . PL XIV, 
Fig. 1 7 a), no such arrangement of the cells is distinguish- 
able. The obliteration of the apical cell as such is brought 
about by a wall parallel to its outer face. The outer cell 
divides by vertical walls, soon becoming indistinguishable 
from the other epidermal cells, and the inner cell also dividing 
becomes part of the ground tissue of the leaf. About the 
end of the fourth day the leaf begins to grow much faster 
than the other members, and from this time onwards elongates 
with great rapidity. By the fifth day the differentiation of 
the future tissues is clearly indicated. A longitudinal section 
of the leaf at this stage (PI. XIV, Figs. 17 a, b) shows on the 
outside a single layer of nearly cubical cells, especially well- 
marked near the apex of the leaf, which is somewhat pointed. 
This layer of cells constitutes the primary epidermis. Beneath 
it are usually about two layers of cells arranged in nearly 
straight rows, which converge towards the apex of the leaf. 
These cells give rise to the mesophyll, and at an early stage 
large intercellular spaces are formed between them. Within 
these is a conical mass of cells, the outer ones of which differ 
but little from those lying outside them, but the innermost 
ones have undergone division by longitudinal walls forming 
the beginning of the procambium of the future vascular 
bundle. This longitudinal division ceases at some distance 
from the point of the leaf, and in consequence the vascular 
bundle does not extend into it. The cells of the leaf-tip 
above the point where the procambium ceases increase 
enormously in size, elongating to many times their original 
length, and forming thus a very loose large-celled parenchyma 
that ultimately dies away. From this time the growth of the 
leaf is due entirely to the activity of the basal part. If we 
examine somewhat older embryos (PI. XIV, Figs. 22, 23) the 
differentiation of the young tissues is still more evident, and 
the limit between the actively dividing basal cells and the tip 
of the leaf is very conspicuous. If the base of the leaf of such 
an embryo as that figured in Fig. 23 is examined, it is usually 
