886 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
fact is, that as we trace the ray outwards we often meet with elongated cells of 
prosenchymatous form, but apparently without pits (see fig. 16). Such cells are most 
often found at the margins of the ray, but also occur in its interior. We must regard 
these prosenchymatous elements as intermediate forms between ray-cells and 
tracheides,* 
The study of transverse sections shows us that the intruding tracheides, whether 
appearing at the sides or in the middle of the ray, are not isolated, but form, almost 
from their first origin, continuous radial series.* 
The question now arises how this interfascicular wood which we have described is 
developed. The principal medullary rays consist of relatively short cells, which, 
judging from the analogy of recent plants, must have arisen from cambial cells of like 
form. Now the difficulty is, that wherever interfascicular tracheze appear, we find 
these short elements replaced by extremely long ones. A numerical estimate of this 
difference has little value, in view of the great variations in length of both ray-cells 
and tracheides, but in many cases we may safely assume that the interfascicular 
tracheides are twenty times as long as the parenchymatous cells of which they take 
the place. Yet the radial seriation of the elements is scarcely disturbed by this 
enormous change in their dimensions. “Sliding growth” to such an extent would 
inevitably bring with it a complete obliteration of the original radial arrangement of 
the secondary tissues.t 
The hypothesis that the interfascicular trachee arose by cell-fusion is at first 
tempting, but there is no sufficient evidence to support it. We have already dis- 
cussed the rare cases in which there are traces of transverse walls; in the vast 
majority of trachez nothing of the kind is visible. There is no analogy among recent 
plants for the existence of xylem-vessels without any trace of the limits of the cells 
from which they are formed.§ 
The sculpturing of the cell-wall is so often perfectly preserved in the fossil Cala- 
mates, that remains of transverse walls can scarcely have escaped observation, 
considering how obvious they are wherever they exist in recent plants. 
Besides this negative evidence, the existence of elements intermediate in form and 
length between the parenchymatous cells of the ray and the trachee is a stroug 
argument for the origin of the latter by growth rather than by cell-fusion, especially 
as these transitional elements appear just where they are wanted, namely, where the 
interfascicular wood is beginning to form. 
The solution which we suggest is that the interfascicular tracheze arose by the 
* They are well shown in C.N. 20z, 83, 90, and 130*. 
+ See Witramson’s figure above cited, Part I, Plate 25, tig. 17. 
¢ Cf. Du Bary, loc. cit, p. 470; Kraspe, ‘Das Gleitende Wachsthum,’ 1886, 
§ Cf. De Bary, loc. cit., p. 165. The case of Dracena and its allies which might once have been 
supposed to afford such an analogy, is now known to be one of sliding growth (see Scorr and Bresyek, 
“Secondary Tissues in Monocotyledons,” ‘ Annals of Botany,’ vol. 7, 1893). 
