504 Osborn. — Some Observations on the Tuber of Phylloglossum. 
The evenness of contour of the cell mass thus produced depends on 
how nearly equal the stimulus to division has been in neighbouring cells, and 
upon the absence of stomates from the proliferating area (Text- fig. 37 and 
PI. XXVIII, Photo 1). In Text-fig. 38 and Photo 2 it is obvious that growth 
has been less regular, the boundary between original groups of epidermal 
cells being clearly shown by anticlinal planes of division, extending for as 
many as eight or nine cell rows. There is thus developed a considerable 
cell mass due entirely to proliferation of the epidermis* Concurrently the 
sub-epidermal cells enlarge somewhat, and a few irregular walls are formed. 
These are best seen in a section tangential to the leaf-surface, when they 
resemble the secondary changes that may occur in the cortex of some 
herbaceous plants (Photo 4). In no case has the sub-epidermal tissue been 
observed to take any part in the structure of the main adventitious cell mass 
(cf. Text-fig. 39 and Photo 5), though in cases where the growth is close to the 
injured surface the activity of the sub-epidermal cells is greater than usual 
(Photo 4). The changes that occur within it are of such a nature as to 
reduce the amount of intercellular space below the adventitious mass, thus 
permitting of freer translocation between it and the leaf. 
The size of the cell mass produced appears to be directly related to 
conditions of growth. In contact with the soil or in very humid atmosphere, 
it may be small in amount ; in drier conditions growth is slower and a con- 
siderable and very irregular mass may be produced before further differen- 
tiation proceeds. A further factor, that of the age of the regenerating leaf, 
is discussed below. It has already been noticed that rhizoids were not of 
constant occurrence upon the cell mass, though they invariably occur in 
large numbers upon the tuber in the later stage of development. 
Once the cell mass is formed the next stage is the localization of 
growth at one or more centres by the differentiation of growing-points 
(Text-figs. 37-8 and Photo 3) consisting of a group of meristem cells. 
The growing-point is definitely geotropic, and quickly becomes buried in 
the soil. When, owing to its growth, the apical meristem has become 
buried, an invagination of the growing-point occurs as a result of unequal 
growth. This is well shown in Text-fig. 40 and Photo 6, which shows the 
growing apex already invaginated, and lying at the base of a short pit. 
In the case of this leaf a considerable irregular cell mass was formed owing 
to there being more than one centre of proliferation. However, only one 
growing-point developed. 
The subsequent growth of the stalk, after invagination of the apex, 
resembles that described for the normal new tuber. The apex is buried by 
intercalary growth of the stalk cells, which become considerably elongated 
in the vertical plane (Text-fig. 39 and Photos 5, 8, and 9). As in the 
normal tuber, the apex is in communication with the exterior along a 
channel (‘Canal de Braun’ of Bertrand) (Text-fig. 41). This, as usual, 
