Osborn. — Some Observations on the Tuber of Phylloglossum. 507 
proceeding the tuber becomes further differentiated. The outer layer or 
coat consists of rather large cells with stout cell walls, and numerous rhizoids 
are developed (Photos 8, 9, 10, and xi). The coat rarely is more than one 
cell thick, and in irregular tubers may be absent. In this respect it differs 
from the new tuber coat described by Bower, 1 which consists of two or 
three cell layers besides the epidermis, the latter having characteristically 
thickened walls and rhizoid-like hairs. The tissue comprising the main 
body of the tuber consists entirely of parenchyma, with few intercellular 
spaces. This tissue is packed with minute starch grains, and forms an 
almost spherical body of cells, near the upper pole of which the growing 
apex is found. The cells of the apex are clearly differentiated from the 
rest of the tuber by their large nuclei, abundant protoplasm, and the absence 
of cell inclusions. The appearance of a fully-developed adventitious tuber 
is well shown in Text-figs. 41, 42, and Photos 8, 9, 11, especially Text-fig. 
41, which shows the ‘ tracheide ’ strand from its expansion in the adventitious 
cell mass, throughout its whole length, to the cup around the tuber apex. 
The same section also shows the channel throughout its whole length, 
including the rather prominent opening to the exterior (see also Photo 10). 
The position of this shows that invagination of the apex did not occur till 
after an appreciable amount of geotropic growth had occurred. Only two 
adventitious tubers showing 4 leaflets ’ developed from the cell mass have 
been examined in serial section. The 4 leaflet ’ structure is exceedingly 
delicate, and consists of an epidermis, a spongy mesophyll, with large inter- 
cellular spaces and a central strand. In one 4 leaflet 5 this central strand 
consists of elongate parenchyma only, and, as the main leaf had already 
begun to rot when fixed, little further development could have occurred. 
In the other case there was a well-developed vascular strand seen in trans- 
verse section to consist of 4-6 larger 4 tracheides * around two or three 
smaller ones. This vascular strand passed down the leaflet and connected 
with the tracheidal cells in the cell mass. It was noted that certain 
epidermal cells of the leaflet near to its apex developed fine rhizoid-like 
processes. 
General Results and Conclusions. 
The observations on the behaviour of the tuber of Phylloglossum 
recorded in the opening sections of this paper serve to emphasize the 
importance of the structure as an organ of perennation. This view, referred 
to by Professor Bower in his Presidential Address to Section K of the 
British Association, 19 14, 2 expresses more nearly the true value of the 
tuber than one which regards it as an annually produced protocorm 
developed by a permanently embryonic Lycopod. 
1 Bower, F. O. (1885) : loc. cit., p. 666. 
2 Bower, F. O. (1914) : Brit. Ass. Report, p. 565. 
