Anatomico-physio logical Relations in the Spermophyte Shoot. 137 
look for some other explanation. Now it is to be observed that in some 
species the margins of the cotyledons become confluent at the level of 
insertion, completely enveloping the axis and forming a sort of cup or 
socket out of which the plumule emerges. This condition was found to 
exist in a very large number of the Dicotyledons examined and in several 
of the Gymnosperms. Where the cotyledons are of considerable thickness, 
as e. g. in Gorse (Ulex europaed) and Stock ( Matthiola incana ), the socket 
rim may be quite substantial (see Fig. 1) ; where they are less solid in 
structure it is correspondingly thinner. In the former case the hypocotyl 
has the same appearance all 
round and is usually featureless, 
but in certain species coming 
under the latter head a distinct 
ridge or welt may be traced down 
either side from the point of 
coalescence of the cotyledons 
towards the root, as in Rivina 
humilis} Plum. (Phytolaccaceae) 
(see Fig. 9). Or in place of a 
ridge there may be a well-defined 
line of hairs such as is seen under 
certain conditions in Veronica 
hederae folia and Lopezia coronata 
(see P'igs. 3 and 4), or, again, 
the two characters may coexist 
Thus in Ruellia amoena the ridge 
is ordinarily surmounted by a 
conspicuous line of character- 
istically shaped hairs, the rest 
of the surface being glabrous 
(see Fig. 7). When the seedling 
is growing under conditions which 
are still more favourable to hair 
formation the hairiness may, however, become general, and in the same 
way Rivina , in which the ridge is more often glabrous, will some- 
times develop a hair line under altered conditions. In the Polygonaceae 
the coalescence is emphasized by the production of a tubular sheath 
extending upwards for a short distance above the node level. In 
1 The presence in Rivina of a shallow furrow on either side of the ridge, indicated in the trans- 
verse section but more clearly seen in a surface view of the seedling, might suggest at first sight that 
this type should be classed with Eucalyptus and Ipomoea (see later). But the furrows in this 
plant have a different origin. A close examination shows that they are not intercotyledonary, but 
arise through a foreshadowing in the cotyledon stalk at its insertion of that four-angled shape which 
is assumed by the petiole of the leaves. 
Figs. 7-10. 7, Ruellia amoena , hypocotyl in 
transverse section ; the potential edges of the cotyledon 
extensions are thrown up into a ridge surmounted 
by hairs. 8, Pinus maritima , hypocotyl (after 
Chauveaud). e., epidermis; x., xylem; p., phloem; 
r.c . , resin canal. 9, Rivina humilis , hypocotyl show- 
ing the same feature as Ruellia but without the hairs ; 
the shallow groove on either side of the ridge results 
from the angled form of the cotyledon petioles. 
10, Stellaria media, internode in transverse section. 
M 2 
