ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 687 
all other specimens the pith is either solid, or, if hollow, there is every appearance of 
the cavity being due to decay or to the intrusion of Stigmarian rootlets.* 
The pith varies enormously in size in the different specimens, and may only consist 
of three or four cells, as seen in transverse sections, while in the smallest rootlets it 
disappears altogether. Except in these extreme cases the structure is fairly uniform 
in all. The pith consists of rather large-celled parenchyma, and presents no pecu- 
liarities (see Plate 16, figs. 1, 2,and 3). It is surrounded by a ring of primary xylem- 
groups, varying in number from 25 downwards. When the pith is large the xylem- 
bundles are usually separated from one another by broad primary rays (see Plate 15, 
photographs 1 and 3); when it is small the bundles may be nearly in contact with one 
another, and in the latter case their limits may be somewhat difficult to trace. The 
presence, however, of these primary groups of xylem is absolutely constant, and their 
structure is always essentially the same. Hach group, as seen in transverse section, 
has an approximately triangular form, with one angle directed outwards (see Plate 16, 
figs. 1 and 2).t It consists of a varying number of tracheides, of which there are often 
about 20. The elements become smaller towards the peripheral angle, and it is con- 
stantly at the angle itself that the most minute tracheide is situated (figs. 1 and 2, pa.). 
In accurately transverse sections, if the cells of the pith have rather thick walls, it 
may not be easy to distinguish them from the primary tracheze. With care, however, 
this can always be done, in good preparations. Wherever the section is at all 
oblique, the primary trachez can be recognized with ease by the pits on their walls. 
The arrangement of the trachee at once suggests, even from the inspection of 
transverse sections alone, that the development of each group of primary xylem 
was centripetal, the small trachee at the external angle representing the protoxylem, 
or first-formed elements. This conclusion is rendered certain by the examination of 
oblique and longitudinal sections, in which we find that the smillest and most 
external elements of the primary xylem are spirally thickened. ‘This, for example, is 
beautifuliy shown in the radial section (C.N. 131 7) a part of which is represented in 
fig. 8. The spiral protoxylem-trachee are quite unmistakable, both at the place 
figured, and at several other points in the section. Similar observations have been 
made by Renavtt, who has shown clearly that the development of the primary 
wood was centripetal in his specimens, which he refers to different species from ours.t 
We find that the spiral tracheides are few, and their spirals densely coiled, a 
peculiarity characteristic of the root in recent plants, and due to the shortness of its 
growing region. (See pp Bary, “‘ Comp. Anat.,” p. 352.) 
* See Wintsauson, “ Organization,” Part IX., Plate 19, figs. 1-7; Part XII., Plates 27 and 28, 
fies. 2, 3, and 6; also Photographs 1, 3, 4, 6, and 7 in the present paper. 
+ See also WiLIamsoy, “ Organization,” Part L., Plate 25, fig. 16; Part XII., Plate 27. fig. 3. 
+ Rewauut, “Recherches sur les Végétanx fossiles du Genre Astromyelon,” ‘ Annales des Sciences 
, Ps ? a 
Géologiques.’ vol. 17, 1885; see p. 12, Plate 8, fig. 8, &c.; ‘Flore fossile d’Autun, Part IT., Plate 57, 
fig. 7, we. 
