590 
Holden and Bexon.—On the Seedling 
o 
march of events in the vascular development of many seedlings, there are 
a number which present difficulties requiring further elucidation and explana¬ 
tion. The chief of these are two in number, and reference has already 
been made to them by previous investigators. The first is the difficulty 
presented by the behaviour both of the protoxylem and metaxylem which 
constitute the intercotyledonary poles in tetrarch roots, and here, as 
Dr. E. N. Thomas ( 16 ) points out, Chauveaud’s theory breaks down to 
some extent, the metaxylem of these poles being ‘ continuous with the 
“elements superposes” of the lateral strands of the cotyledon ’. Although 
the collateral segments may be almost wholly concerned in augmenting the 
cotyledonary vascular supply, they frequently combine this function, as 
Compton (6) has shown, with that of ensuring a closer contact with the 
epicotyledonary strands. Occasionally, as in Juglans nigra (8) and Caesal- 
pinia separia (6) for example, this function may be the only one, and the 
intercotyledonary root poles are then directly continuous with the epicoty¬ 
ledonary traces and contribute no elements to the cotyledons. This 
extreme condition is one which is frequently associated with hypogeal 
germination, and the fact of its occurrence raises the question as to what 
extent the intercotyledonary root protoxylems generally, in tetrarch types, 
are referable to epicotyledonary sources. Miss Davey, in a paper dealing 
with the seedling anatomy of the Amentiferae (8), suggests that this may 
be the case in certain species (e. g. Myrica Gale , Alims, Carpinus) even 
though direct connexion between the epicotyledonary protoxylem on the 
one hand, and that of the root on the other, cannot be established. This 
lack of continuity is held to be due to the delay in plumular development. 
As a result of this delay 1 differentiation is not complete at the upper end of 
the seedling, so that the protoxylem centre “dies out” in ascending the 
hypocotyl, and actual connexion cannot be established ’. The same author 
(8, p. 598) cites Compton as describing a similar distribution of the inter¬ 
cotyledonary vascular units between cotyledons and plumular leaf in the 
Leguminosae ‘ as a phenomenon of replacement in which the cotyledons are 
being supplanted by plumular leaves \ It would appear, however, that 
Miss Davey had misinterpreted Compton’s views, since this investigator 
(6, pp. 102-4), after stating the following alternatives: £ (i) The plumular 
traces may tend to replace the intercotyledonary root poles, or vice versa 
their relation to one another being complementary and one of supplantation. 
(ii) The intercotyledonary xylem may combine with the leaf-trace to form 
a joint conducting channel, their relation to one another being supplementaryl 
and after discussing the evidence for each, pronounces in favour of the 
latter. 
The evidence in the case of the sycamore seems definitely against the 
view that the intercotyledonary protoxylems are plumular in origin. In 
the majority of cases the midrib bundles fork at a relatively high level in the 
