164 Saunders . — The Leaf -skin Theory of the Stem: 
they obviously appear to be so, but generally and probably universally 
throughout the group. This appears to be the case also in some at least 
of the Pteridophyta. 
2. In those species in which a hypocotyl is developed, its superficial 
tissues are similarly derived from the cotyledons, and it is therefore to the 
cotyledons that we must look for an explanation of its external features. 
3. This ‘ leaf-skin ’ is formed by a downward growth and extension of 
the leaf primordium, which keeps pace with the extension of the central 
axis with which it is fused, after the manner conceived by Hofmeister, who, 
however, in his Ber indung theory goes so far (in the case of Equisetum 
specifically, and it would appear in the higher plants too) as to refer all the 
tissue external to the pith to the leaf. 
4. The contact edges of these extensions may be so adjusted or so 
deeply fused as to show no outward trace. Or they may exhibit the 
characteristics of potential edges and be demarcated in various ways (by 
ridges, furrows, lines of hairs or of colour). 
5. When each leaf-insertion completely encircles the stem (as in most 
Monocotyledons and a few Dicotyledons), or when the leaves, being opposite 
or whorled, come into lateral contact at the level of insertion (as in many 
Dicotyledons and some Conifers), the downward extension of each leaf is 
limited to a single internode. 
6. Where the leaves are opposite, but where the insertion width occupies 
less than \ the circumference of the stem (especially well seen in some 
Dicotyledons with four-angled stems), the leaf-extension stretches through 
two internodes. 
7. Where the leaves are spirally arranged with an insertion width less 
than the circumference of the stem, the pattern traced by the fused edges 
of the extensions and the number of internodes through which the extension 
of any one leaf descends will depend on the leaf-divergence and the leaf- 
insertion width. 
8. If the leaf-divergence and the leaf-insertion width are expressed by 
the same fraction (e. g. f ), the contact line descending from one edge of 
each insertion will c pick up and the one from the other will c be picked 
up ’ by a corresponding line descending from the neighbouring leaf below 
and above respectively, and each fusion line thus formed will extend down- 
wards until it terminates in the axil of a particular leaf in the cycle. 
9. When the leaf-insertion width occupies some lesser fraction of the 
circumference no ‘ pick-up ’ occurs ; the boundary lines run a separate 
course, and hence two terminate in each axil, one on either side the 
mid-point. 
10. In the circumstances described under (8) and (9) the boundary 
line from one edge of the leaf-insertion extends through a greater number 
of internodes than the one from the other edge of the same insertion. 
