362 Clark. — Secondary Thickening in 
side, and secondary phloem to its outer side. The xylem is composed 
mainly of short, square-ended tracheids, with but few vessels. Many 
of these xylem-cells present rather a curious appearance, due to the 
shrinking and curling up in the cell-cavity of the separated inner layer 
of the thickened cell-wall. This layer is presumably cellulose, from the 
fact that it stains blue with Schulze’s Solution. The xylem of 
Medinilla^ another member of the Melastomaceae, also shows this same 
feature. At this stage neither primary nor secondary medullary rays, 
nor even isolated groups of parenchyma cells, are to be seen. 
In slightly older stems, small patches of thin-walled parenchyma cells, 
with large intercellular spaces, are found situated at irregular intervals 
throughout the xylem-ring. These patches are, in all probability, 
equivalent to medullary spots. 1 They are developed from the inner side 
of the cambium ring, and consist of cells, identical in size and form with 
the xylem-cells, from which they differ solely by the absence of thick 
walls (Figs. 2, 3, and 4). The large intercellular spaces might be 
considered as another point of difference, but this is not so, because 
patches of thick-walled xylem-cells with similar spaces are also met with 
at irregular intervals in the xylem-ring (Fig. 5). In addition to these 
patches there are a number of large areas of parenchymatous tissue, 
forming wedges which project radially inwards ; they are opposite the 
slight indentations of the stem surface, and are seen to have developed 
from the inner side of the cambium-ring. These wedges are never, in 
stems of this age, continuous with the parenchyma of the pith, nor do they 
communicate with the parenchymatous patches. The cambium opposite 
each wedge is slightly deflected towards the centre, in correspondence 
with the surface indentation, but it is always external to the area of 
xylem-parenchyma forming the wedge. These wedges are not merely 
radial spokes in the xylem, but are continuous as broad plates or 
wedges throughout a considerable length of the stem. The fact that 
there is no equivalent development on the phloem side of the cambium 
seems to differentiate these structures from ordinary, secondary medullary 
rays. The parenchymatous cells of the wedge are identical in appearance 
with those of the patches (Figs. 2, 3, and 4). In transverse section, in many 
instances, the cells have rounded-off edges with intercellular spaces so 
exceedingly well marked that the whole tissue presents somewhat the 
appearance of being composed of marbles, arranged in a series of regular 
rows (Fig. 7). At somewhat rare intervals, one or perhaps a small group 
of three to four thick-walled xylem-cells may be seen enclosed in the 
parenchymatous wedge. This in no wise disturbs the regularity of the 
tissue-arrangement, as the xylem-cell is identical in size with its paren- 
chymatous neighbours. This regularity of arrangement, which is also 
1 De Bary, Comp. Anat., Engl, ed., p. 492. 
