Anatomico-physiological Relations in the Spermophyte Shoot. 161 
The circumference of the axis here is often far too great to be encircled in 
this way by the leaf-bases. But we find contour lines starting from the 
insertion which spread out sideways far enough to meet those descending 
from neighbouring insertions, so that it becomes evident that the width of 
insertion cannot be taken as an invariable guide to the width of the 
descending extension since a sharp contraction may occur at the node. 
These contour lines in Senecio as well as the line from the midrib are free 
from stomata which are scattered thickly over the intervening areas. On 
the other hand, in the succulent stem of Euphorbia pendula , where the 
leaves are reduced to non-vascular scales, there is no trace of a fusion line, 
and the stomata are scattered round the whole circumference, midrib line and 
line of fusion included. From these facts it would then be possible to argue 
either way in regard to the appearance presented by Psilotum , but the clear 
evidence afforded by Tmesipteris inclines one to the view that an enveloping 
leaf-skin exists also in the related genus . 1 
6 . The Bearing of the ‘ Leaf-skin’ Theory on certain 
Genetical Problems. 
It will be clear from what has been said in the course of this account 
that it may be possible in a plant exhibiting a varying degree of hairiness 
to resolve this variable character into two components : one primary (i. e. 
exhibited in the earliest stages of development), constant (up to a point), 
fixed in position, inherited (subject to permissive conditions), because it is 
the expression of certain physiological conditions definitely associated with 
a particular anatomical configuration. Since the anatomical contours are 
constant, the manifestation of the character especially associated with them 
is constant also under a range of conditions, wider or narrower for different 
types and not at present capable of precise formulation. The other, 
adventitious, appreciably inconstant in degree, variable in position, not 
inherited, representing a secondary effect superposed upon the primary, 
a response in areas still plastic enough to respond to conditions set up at 
a later stage in development, and causing a flooding effect which may lead 
to blurring or obliteration of the inherited feature. This secondary effect 
is of common occurrence in the flowering region of many plants which are 
more or less free from hairs in the vegetative region. The production of 
flowers and small bracts in place of much larger foliage leaves must involve 
a considerable physiological readjustment, and may well be a prime cause of 
the general condition of hairiness which often sets in at this point. 
We are in the habit of classifying the characters of organisms under 
the head of those which are inherited and those which are not, although 
1 The early divisions of the daughter segments of the apical cell in Psilotum have been followed 
for some distance by Solms-Laubach, but the origin of the leaf rudiment is not shown (see Ann. du 
Jardin botanique de Buitenzorg, vol. iv, 1884). 
