670 
PROFESSOR F. 0. BOWER ON THE DEVELOPMENT AND 
an approximately central position, notwithstanding differences in number and arrange¬ 
ment of the leaves. Following still the changes of external conformation, it is found 
that the young tuber begins to project, since the growth of tissue is continued 
actively round the apical depression (fig. 16, a) ; it gradually becomes inverted and 
elongated, as the result of a special localisation of intercalary growth (figs. 17-20). 
Meanwhile the swelling of the tuber begins by increase of the tissues lying below the 
depressed apex (fig. 20). Further details of the development of the tuber, as illus¬ 
trated by sections, will be given below. 
In some cases a leaf smaller than the rest and of later development is to be seen 
inserted above the tuber (fig. 18) ; its position is, as a rule, not directly above the 
tuber (fig. 17). A leaf holding this position has been observed by other writers, but 
it is by no means of constant occurrence, and its occasional appearance illustrates once 
more how great is the irregularity of number and position of the leaves in Phyllo- 
glossum. The description above given of the development of those simpler plants of 
Phylloglossum which do not bear sporangia coincides with the structure of those 
plants when mature, and it may especially be noted that in the mature plant there is 
no trace of an apical cone or bud to be found in a central position at the base of the 
leaves, an observation which completely bears out the view that in these plants it is 
actually the apex of the plant which gives rise to the new tuber. 
While the development of the tuber is progressing, an outgrowth makes its 
appearance on the opposite side of the plant to that on which the tuber is formed; it 
is situated at a point below the insertion of the oldest leaf. It arises as a smooth 
rounded cone ; there is no sign of any rupture of the tissues, indicative of endogenous 
origin, its outer surface being unbroken and glossy; as it grows older a slight 
excrescence appears at its apex, the convexity of its surface being there greater; this 
outgrowth ultimately develops as the first root (r, in figs. 9, 166, 18, 19, 20). In one 
plant this outgrowth did not appear as a simple cone, but, as shown in fig. 21, two 
cones appeared, joined laterally, a mode of development which may well be compared 
with that exceptional development of the leaf shown in fig. 20. As far as external 
observation is concerned, it may be concluded from the above description that the root 
is of exogenous origin. This is borne out by the results derived from the study ol 
sections, which will be detailed below. 
The second alternative mode of development remains to be described, viz., that in 
which the plant forms sporangia. As above stated, this appears to occur only on the 
germination of large and well-developed tubers. The formation of leaves and their 
order of succession appear to proceed in the same way as above described, but it 
is to be noted that the number of ordinary vegetative leaves is usually smaller m 
sporangium-bearing plants than is the case in plants of purely vegetative development. 
When sporangia are to be formed, the apex of the plant is convex, and grows on in an 
erect position ; on it are formed successive leaves, which originate like the ordinary 
foliage leaves, but remain of relatively small size, while the axis which bears them 
