144 Saunders . — The Leaf- skin Theory of the Stem: 
dons. In many Conifers, in fact, the leaf is so obviously decurrent that it is 
invariably described as such. 
Enough has now been said to show the general nature of the evidence 
upon which the view here advanced is based. It has been made clear that 
as regards certain definite surface patterns which are visible on the shoot 
axis, the form of the quasi-geometrical areas which go to their make-up is 
the outcome of the particular leaf-divergence and leaf-insertion width. Also 
that the ‘ direction of fit ’ of these areas corresponds with the orientation of 
the leaf-spiral — that it is, in fact, merely another outward expression of the 
same internal relation. These facts are patent to the eye, and their signifi- 
cance can scarcely be disputed. They lead directly to the conclusion that 
in Spermophytes the shoot axis consists throughout of an axial core 
enveloped in a covering of tissue of foliar origin — the leaf-skin. I have 
used the word ‘ skin ’ with intent, for the reason that it does not connote any 
precise number of cell layers. For although the evidence adduced relates 
only to the first hypodermal layer in addition to the epidermis, it may be 
premature at this point to assume that in no case does any deeper layer give 
indication of these boundaries. 
By the downward extension of its tissue the leaf is braced against the 
stem, and in this way we may suppose obtains a better point d appui, 
an arrangement which may have its value where the petiole has to carry a 
large outspread lamina. As is well known, the subcylindrical petiole of 
many Dicotyledons is frequently channelled on the upper side. Where this 
character is well marked, there may be a distinct appearance of a wrap-over 
of the edges, as in Lopezia coronata , where hairs border either rim of the 
channel, thus continuing in this middle region the demarcation by hairs of 
the actual (here arched-over) edges which has been shown to exist for the 
potential edges of the downward extensions, and which in the expanded 
lamina takes the form of a fine ciliation. 
The differentiation of the vascular tissue of the leaf in the higher plants 
begins, as we know, in the periblem, about at the level at which the leaf is 
conceived as arising from the axis, and develops in both directions, upwards, 
keeping pace with the increasing length of the free portion, and downwards, 
usually for a definite distance which varies with the particular type, until it 
makes connexion with an older leaf-trace at a lower level. Now, although 
there are good grounds for maintaining that the region of transition from the 
arrangement typical of one member to that characteristic of another is not 
necessarily identical for all the tissues involved, nevertheless, the very fact 
that the leaf-trace pursues a downward as well as an upward course of 
development renders it at least not inconceivable that the more superficial 
tissues may do so too. 
A decurrent mode of development of the Spermophyte leaf seems 
to have been envisaged by Sachs, if we may judge from the diagrammatic 
