514 Osborn.— Some Observations on the Tuber of Phy lloglossu m . 
response to the stimulus of gravity. It has been pointed out that the new 
tuber is definitely geotropic. The adventitious tuber behaves similarly, but 
the cell mass or protocorm, which is developed first by the regenerating leaf, 
does not. It may grow from any side of the leaf in relation to the ground, 
and, even when developed in contact with it, shows no penetration into the 
soil beyond such amount as is due to expansion of the group of cells. The 
cell mass is a transitory structure and rarely attains any size, but where, as 
in Text-fig. 12, an unusually large one has formed, its growth is plagio- 
tropic rather than geotropic. 
It is not intended to discuss the morphology of the tuber at any length, 
but the following remarks maybe deemed pertinent. In 1886 it was shown 
by Bower that the new tuber in sterile plants is formed directly from the 
apex of the plant, while in fertile plants, the apex of which develops the 
peduncle and cone, the new tuber is an adventitious growth bearing no 
relation to the leaf arrangement. This is substantially our position to-day, 
though recently Miss Sampson 1 has on anatomical grounds attempted to 
show that in fertile plants the apex bifurcates and that the new tuber is of 
the nature of a branch. The vascular anatomy of a minute plant such as 
Phyiloglossiim is subject to considerable variation, as was shown by Bower. 2 
The writer has hardly examined sufficient plants in serial section to be able 
to affirm or refute Miss Sampson’s contention, but it seems significant that 
in two fertile plants recently studied no connexion existed between the 
vascular supply to the new tubers and the stele of the plant producing it. 
In one case, a four-leaved plant with two roots, the vascular strand from the 
new tuber completely died out in the stem before the xylem masses of the 
two roots coalesced, and far below the point to which any leaf trace pene- 
trated. It is therefore hardly legitimate to assume that in all cases the 
tuber of fertile plants of Phylloglossum is a modified branch, or, indeed, that 
it represents 4 a highly specialized leafy axis’. The ‘ vascular supply ’ to the 
adventitious tuber has been shown to be a late development and in no case 
connected with the tracheides in the regenerating leaf. The wide tracheidal 
cells of the 4 vascular supply ’, with their loose spiral or reticulate lignifica- 
tion, resemble rather 4 storage tracheides’ than conducting elements. Such 
a function affords a reasonable explanation of the cup of tracheides formed 
at the distal end of the stalk round the new tuber apex. The reproduction 
of tubers by detached leaves agreeing, even to details of vascular supply, 
with those formed normally by the plant in each growing season, suggests 
that caution is necessary before adopting a generalization as to the 
morphology of the tuber which is based entirely on anatomical evidence. 
1 Sampson, K. ( 1916 ): Ann. Bot., xxx, p. 331. 
2 Bower, F. O. ( 1885 ) : loc. cit., p. 674, and PI. 70, Figs. 42, 43. 
