outer walls of its cells unlike
the inner walls.[52] Especially significant is the structure of such
intermediate types as the Liverworts. Beyond the differentiation of the
covering cells from the contained cells, and the contrast between upper
surface and under surface, the frond of _Marchantia polymorpha_ clearly
shows us the direct effect of incident forces; and shows us, too, how it
is involved with the effect of inherited proclivities. The frond grows
from a flat disc-shaped gemma, the two sides of which are alike. Either
side may fall uppermost; and then of the developing shoot, the side
exposed to the light "is under all circumstances the upper side which
forms stomata, the dark side becomes the under side which produces
root-hairs and leafy processes."[53] So that while we have undeniable
proof that the contrasted influences of the medium on the two sides,
initiate the differentiation, we have also proof that the completion of
it is determined by the transmitted structure of the type; since it is
impossible to ascribe the development of stomata to the direct action of
air and light. On turning from foliar expansions, to stems and roots,
facts of like meaning meet us. Speaking generally of epidermal tissue
and inner tissue, Sachs remarks that "the contrast of the two is the
plainer the more the part of the plant concerned is exposed to air and
light."[54] Elsewhere, in correspondence with this, it is said that in
roots the cells of the epidermis, though distinguished by bearing hairs,
"are otherwise similar to those of the fundamental tissue" which they
clothe,[55] while the cuticular covering is relatively thin; whereas in
stems the epidermis (often further differentiated) is composed of layers
of cells which are smaller and thicker-walled: a stronger contrast of
structure corresponding to a stronger contrast of conditions. By way of
meeting the suggestion that these respective differences are wholly due
to the natural selection of favourable variations, it will suffice if I
draw attention to the unlikeness between imbedded roots and exposed
roots. While in darkness, and surrounded by moist earth, the outermost
protective coats, even of large roots, are comparatively thin; but when
the accidents of growth entail permanent exposure to light and air,
roots acquire coverings allied in character to the coverings of
branches. That the action of the medium causes these and converse
changes, cannot be doubted when we fi