A natomico-physiologicai Relations in the Spermophyte Shoot. 139 
edges : to one in which the union is sufficiently deep or so exactly adjusted 
that no anatomical demarcation is traceable. But though in this last case 
the boundaries are uncharted and signposts are wanting, we may safely' 
presume that the development of the cotyledons follows the same course 
as in the two other types. When the potential edges are not only fused 
but become thrown out into ridges the explanation may be that growth in 
circumference in the cotyledons is more rapid than in the axial core, and 
that accommodation takes place in this way, while the condition in which the 
edges become actual may arise through the initial increase being greater in 
the core than in the cotyledons. 
Now it is a familiar fact that anthocyanin production and hair formation 
are both characters which vary considerably in accord with environmental 
conditions. We may find, for example, that an Ipomoea grown exposed 
to full sunlight in dry soil has the entire hypocotyl surface uniformly 
coloured ; or again, that some Veronica individuals will show a completely 
glabrous hypocotyl, while others have hairs distributed over the whole 
surface. But the important point which . concerns us here is not that tender 
certain conditions hairs and anthocyanm can overspread the entire area , or on 
the other hand that their formation can be entirely suppressed , but that , under 
the appropriate conditions , the disposition of hairs > colour , or other anatomical 
features always follows a certain definite arrangement in relation to the 
morphological ground-plan. 
But if this view of the composite nature of the hypocotyl is well 
founded, are we to conceive of the epicotyl and the shoot 'thereafter 
produced from it as likewise consisting of a foliar skin enclosing an axial 
core? A careful examination of the stem surface shows that precisely 
similar and equally plain evidence is obtainable from the later developed 
as from the seedling shoot, the foliage-leaf extension being clearly outlined 
in many species by a furrow or ridge, or by a hair or colour line. And 
further, that where these topographical features are present they stand in 
a direct relation to (1) the general orientation of the individual, (2) the 
mode of phyllotaxis, (3) the proportion of the circumference of the axis 
occupied by the leaf-insertion. For example, if, in a species exhibiting one 
or other of these features, the leaf arrangement is decussate with a leaf- 
insertion width of half the stem circumference, then the congenitally fused 
but potential edges of the foliar extensions will show two longitudinal 
boundary or fusion lines in each internode, extending from the points 
where the free portions of each leaf-pair come into contact down 
to the mid-axils of the leaf-pair below, as e. g. in Hypericum calycinum 
and Epilobium parviflorum (see Fig. 21). If the insertion width is less 
than a half but more than a quarter of the circumference, there will 
obviously be, not two, but four such contour lines . traceable in each 
internode (one extending down from either margin of each leaf-base), which 
