146 Saunders . — The Leaf -skin Theory of the Stem: 
extension of the component cells in the direction of their length. A some- 
what similar view was held by Nageli, who considered it probable that the 
tissue of the stem surface immediately surrounding the visible leaf-base 
belongs to the leaf. 1 Various modifications of this conception of the dual 
nature of the stem have been brought forward from time to time, the most 
recent being the 4 pericaulom ’ theory of Potonie, briefly outlined in 1902, 2 
and more fully elaborated in his 4 Grundlinien ’ in 1912, to which work the 
reader is referred for a more detailed discussion of the views of other writers 
on the subject. In the course of this survey the writer insists that his theory 
must be distinguished from any form of Berindung theory hitherto 
advanced, since he includes under the term 4 pericaulom 5 not only the cortex 
but the deeper-lying tissue, at least in that part of the stem occupied by leaf- 
traces. Somewhat earlier, in 4 Die Gliederung der Kaulome ’, 3 Celakovsky 
based his fundamental thesis that the Stengelglied (internode with 
succeeding leaf or leaves), and not the single cell nor the whole plant, con- 
stitutes the 4 individual 5 — a view again not new (see Asa Gray, ‘ Structural 
Botany’, 1881) — upon the conception that the leaves furnish a complete 
Berindung to the axis. In illustration of this argument several schematic 
drawings are given which, phylogenetic implications apart, the evidence 
here adduced fully supports (see PI. IV, Figs. 7 a and 19). For the moment, 
however, we are concerned less with the points of difference between the 
several theories than with the fact that they have this feature in common, 
that they rest for the most part on metaphysical conceptions, palaeonto- 
logical argument and evolutionary theory. There is little or no direct 
evidence adduced which can be said to afford any very convincing proof of 
the position which it is sought to establish. In the present attempt to 
elucidate further the morphological nature of the shoot in the higher plants, 
the aim has been, by setting forth certain demonstrable facts and making 
clear their relation to the general anatomical scheme, to furnish evidence 
tantamount to proof of the existence of a 4 leaf-skin ’ in the different Spermo- 
phyte groups, and by establishing this conception on a firm basis of fact to 
settle a question which ever and anon gives rise to renewed discussion, and 
upon which there still exists some division of opinion. 
In this conception of the foliar nature of (at least) the superficial layers 
of the whole shoot axis, of the formation of a skin by downward extensions 
of the leaf-rudiments which are fused along their contiguous margins and 
which keep pace in their growth with the stem core, we find at least a par- 
tial solution of our original problem (see p. 135). We learn that it is to 
the surface character of the cotyledons that we must look for a guide to the 
surface character of the hypocotyl, since these two regions are morphologi- 
cally one as regards their outer tissue. If the cotyledons are wholly or for 
1 Abstammungslehre, 1884. 2 Ber. d. deut. bot. Gesell., xx, p. 502, 1902. 
8 Bot. Zeit., 1901. 
