Seedling Structure of Gymno sperms. Ill . 437 
to one side and fuses with the adjacent bast of the epicotyledonary struc- 
tures, so there results a triarch root-structure. 
Thus there is exhibited a striking variation. The observations of 
Tansley and Miss Thomas , 1 and also our own, show that the normal occur- 
rence is for the vascular tissue derived from each cotyledon to form two 
poles of the root-structure, so that the number of poles in the root is double 
the number of cotyledons ; but, in the case under consideration, the seed- 
leaf-traces organize a root-structure having the same number of poles as 
there are cotyledons. This variation does not appear to be uncommon, 
for Sprecher 2 describes the root-structure as being hexarch in the higher 
regions which at lower levels becomes diarch, and Miss Thomas , 3 in a later 
publication, states that the tetrarch arrangement is fugitive, giving place to 
a diarch structure at a lower level. 
This tricotyledonous specimen also showed a few other, but minor, 
differences ; thus the mesarch structure of the cotyledonary bundles 
persisted relatively^for a longer time than in the case of dicotyledonous 
examples ; also, one seed-leaf joined the axis at a higher level than the 
other two. 
It remains to draw attention to a few outstanding features of general 
occurrence. 
The protoxylem of the cotyledonary bundles is well marked, it becomes 
lessen amount as the seed-leaf-traces pass towards the centre of the 
hypocotyl, and it becomes increasingly difficult to distinguish it from the 
metaxylem. Further, there is often seen an addition of xylem elements on 
the outer side of the protoxylem after the root-structure has been attained, 
a feature which has already been remarked upon as occurring in other 
plants, e. g. Jimiperus virginianus and Libocedrus decurrensl An endo- 
dermis is differentiated as soon as the root-structure is arrived at, and it 
occurs some distance out from the outermost vascular elements ; in other 
words the ‘pericycle’ is several cells in thickness. The upper region of 
the root is protected by a well-marked exodermis situated just beneath the 
superficial layer of cells. 
CYCADACEAE. 
The seedlings of these plants have received much attention from 
botanists during the last few years, and reference will be made to these 
researches as the occasion demands. We regret that we have been enabled 
to examine so few seedlings of these plants ; seeds of other genera and 
species were obtained but failed to germinate, and in other cases, even when 
germination was successful, many damped off before a stage suitable for 
our investigation was reached. 
1 loc. cit. 
3 loc. cit. 
2 loc. cit. 
4 Part I, Ann. Bot. 1908. 
