138 Saunders . — The Leaf -skin Theory of the Stem: 
certain Gymnosperms several alternate ridges and furrows are visible 
in the hypocotyl, corresponding in number to the cotyledons, as e. g. 
in Pinus maritima (see Fig. 8) and Picea orientalis , Carr. We may 
place in another and probably much smaller category those species in 
which the width of the cotyledon insertion just falls short of half the 
circumference of the axis, so that the hypocotyl is not completely embraced. 
The result is a double contour line on each side extending down from the 
adjacent cotyledon edges and bounding a narrow longitudinal interspace 
which is seen as a slight furrow. This condition is well illustrated in 
species of Ipomoea (I. coccinea, /. sanguinea (see Figs. 5 and 6)) and 
Eucalyptus .(. E . alpina , E. diversicolor, E. gomphocephala ), and can be 
observed as soon as germination has taken place. The contour lines in 
the above-mentioned Ipomoea forms, which contain a considerable amount 
of anthocyanin, may often be still further accentuated by a sharp colour 
contrast. The pigment is here confined to the first sub-epidermal layer, 
and in seedlings grown in damp shade it may colour the whole exposed 
region of the hypocotyl except the two furrows, which, together with the 
plumule with which they are continuous, appear of a beautiful translucent- 
green when held to the light. Furthermore, the epidermal' papillae which 
develop on the rest of the hypocotyl, giving it a pile-like surface, are absent 
from the furrows. At a later stage or under other conditions of illumination 
and moisture, anthocyanin may overspread these intervening strips, obliterat- 
ing the colour boundaries but leaving the structural contour lines still 
plainly visible extending down the greater part of the hypocotyl, which, 
when fully grown, may measure from i-J to 2 inches in length. These 
shallow furrows, which are separated from the vascular ring by several 
layers of cortex, cannot be causally connected with any internal tissue 
distribution. Rather we are led by the facts adduced to regard ridge, 
furrow, hair line, and colour boundary as marking the limits in the 
superficial tissue of areas which are composed of downward extensions of 
the cotyledons between which, when the contours are double, appear 
similar prolongations of the next leaf-pair. In other words, this superficial 
tissue, i. e. the -epidermis and one layer at least of the hypoderm (and 
possibly deeper layers as well), must be considered, so long as it is desirable 
and practicable to maintain this distinction, as foliar and not axial in 
nature. We can then look upon Ipomoea , Lopezia , and Ulex as representing 
gradations in a series of stages passing from a condition in which the 
cotyledon-extension edges are actual ,* defined by anatomical and in certain 
circumstances by colour boundaries in addition : through that in which there 
is fusion of the contiguous margins, but a fusion of so superficial a character 
that the lines of junction exhibit features which we associate with potential 
1 In the sense that they are separate from each other although the whole extension is com- 
pletely confluent with the axis. 
