96 MR. J. H. BURRAGE ON THE 
support that attain these dimensions, although mere proximity 
encourages the formation and partial development of the dises. 
On branches which project freely into the open they are very 
small or altogether absent. The size of the organs is con- 
siderably modified by surrounding conditions—a damp position 
favouring their growth to a considerable extent, even when quite 
removed from proximity to a support. A warm sheltered position 
also influences their development. Thus, in a specimen growing 
on the outside of a wall of a forcing-pit in the Royal Botanic 
Garden, Edinburgh, the dises were largely developed, occurring 
in various sizes just above the insertion of most of the leaves; 
while in specimens at Kew and Oxford, in more exposed situa- 
tions, they were absent except where in absolute contact with, 
or quite near to, the support. 
When the dise is in contact with a support the leaf cannot 
occupy a normal position, but the lamina is brought out into the 
open by means of the ‘ twisting’ of the petiole. 
Development.—The primary divisions in the development of the 
dise are initiated at an early stage of the bud. The organ is 
formed by division and subsequent growth of the cortical cells 
of the stem, and possesses no definite growing-point, as in tbe 
ease of a lateral root. Tangential divisions first appear in the 
layers immediately below the epidermis, just above the axil of 
theleaf. Similar divisions spread gradually in the subepidermal 
layers, towards the apex of the stem, and laterally. At the 
lowest point irregular divisions occur in the deeper cells of the 
cortex, which become divided up to form a strand of merismatic 
cells connecting the outer layers with the procambial strand. 
As the result of these tangential divisions there is formed, 
immediately below the epidermis, a layer of columnar cells 
(Pl. 5. figs. 2 & 3). The latter are uniform in size, large 
and much more elongated than those below. They possess? 
large nucleus, and appear, in all respects, very similar to the 
cells of the epithelial layer of many secreting glands. Towards 
the centre the cells divide, forming a double layer. The second 
layer is not continuous over the whole surface, being always 
absent at the margin of the protuberance, and only irregularly 
formed in central positions (Pl. 5. fig. 2a). In the median 
line of that part of the protuberance nearest the leaf a series of 
elongated cells are formed, passing in a radial direction from the 
procambial strand to within three or four layers of the columna 
