484 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
spaces are wholly occupied by cellular tissue, but when made in the centre of the woody 
zone, we have the structure representedfin Plate XXIV. figs. 13 & Plate XXVIII. fig. 38. 
In fig. 13 the central mass (m) is a combination of vascular and cellular tissue. The 
distinction between the two elements is not seen in this tangential section, which is 
practically a transverse section of the structure. But in longitudinal sections, like 
fig. 10, we obtain evidence that a considerable number of vessels, derived from the 
woody wedges, are suddenly deflected and proceed outwards towards the bark in a compact 
parallel series. The point specially to be noted in these vascular communications with 
external branches is their small size. The aggregate of the cells and vessels composing- 
each one is always less than that of the woody wedge which it penetrates. These arrange- 
ments will be referred to again when describing fig. 38, which represents another type. 
The next structures demanding attention are the primary medullary rays, or radiating 
cellular masses (fig. 1, c ), which separate the woody ridges, and which like the latter 
extend as continuous planes from node to node (fig. 2, c ), where, as already indicated, 
they become merged with the woody tissue in the shape of enlarged secondary medullary 
rays. As the irregular parenchymatous cells of the pith pass outwards between each 
two contiguous woody wedges, they gradually assume a more regular disposition. Their 
parallel faces become also parallel with the surface of the pith, throwing them into linear 
series, which, in the transverse section, radiate from the medulla to the bark. At first 
these cells are of large size, but they rapidly become less as they pass outwards. This 
circumstance, combined with the simultaneously increased regularity in the arrangement 
of the cells and with their more uniform size, causes their appearance, as they approach 
the bark, to differ very little in the transverse section from the vessels of the woody 
wedges on each side of them. Indeed in many old stems the line of demarcation is only 
to be traced with exactness in the tangential sections. These cells seem to vary consider- 
ably in size. Sometimes they are comparatively narrow, as in fig. 9 ; but in others the 
central row stretches across the ray as a series of long narrow parallelograms, the long 
axis of each cell extending nearly from one woody wedge to the next. So far as I can 
ascertain, these variations are merely the result of differences in the rate and conditions 
of growth, since I often find them exhibiting considerable difference on opposite sides of 
the same stem. In the tangential section (fig. 2, c) these cells do not appear to be 
arranged in any regular order, merely presenting the aspect of an irregular parenchyma. 
But the case is different with the radial sections, one of which is represented in fig. 8. 
At b we have the medullary cells assuming the vertically elongated aspect frequently seen 
in all longitudinal sections of the pith, whether radial or tangential ; but all the re- 
mainder of the section is composed of more or less regular lines of muriform cells, iden- 
tical in every point of form, size, and general aspect with those of the secondary medul- 
lary rays (fig. 11). 
In determining the physiological character of these cellular masses we labour under 
some difficulties, because of the general aspects of the plants to which they belong. The 
plants are, as abundant evidence proves, cryptogamic in their fructification and affinities; 
