ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 887 
elongation of single cells, but that this elongation took place in the cambium-cells before 
the tracheze were cut off from them. We may suppose that an ordinary short cambial- 
cell, belonging to a principal ray, before beginning to produce trachez instead of ray- 
cells, itself became elongated by sliding growth. The existence of intermediate forms 
renders it probable that this elongation did not take place all at once. It is probable 
that a cambial-cell, after growing to some extent in length, divided, and cut off a 
short tracheide or transitional element; before the next division the cambial-cell 
may have grown to a greater length, and then have cut off a longer tracheide, and 
we may suppose this process "continued until the interfascicular cambial-cells attained 
a nearly constant length, after which the development would go on uniformly, as in 
the fascicular wood. 
This account of the process is, of course, only a hypothesis, for the direct observation 
of the successive stages of histological development is impossible, even in the best- 
preserved fossil. We think, however, that our hypothesis is one which explains the 
facts, and we do not see how the great elongation of the tracheides can be reconciled 
with the preservation of radial seriation in any other way. 
A certain small additional amount of sliding growth may also have taken place in 
the young tracheides themselves, after their separation from the cambium, just as is 
known to happen in Pinus.* The existence of occasional intruding ends of elements, 
presumably tracheides, seen in transverse section among the cells of the ray, renders 
this probable. 
We have not had an opportunity of making a similar study of the type (our sub- 
type B1) in which the interfascicular wood at once bridges over the entire width of 
the principal rays. Probably a like explanation would apply here also ; we need only 
suppose that a larger proportion of the cambial cells forming the ray undergo 
simultaneous elongation. It is evident that in both cases the process must involve 
thg interruption of a considerable number of the parenchymatous cell-series of the 
ray, and we have already seen that this actually occurs. 
Before Jeaving the subject of the principal medullary rays, something must be said 
of their modified structure in the infranodal region. Such a ray almost always 
broadens out just below the node, forming, as seen in tangential section, the “lenti- 
cular organ” of previous memoirs.t In this region the cells of the ray are very 
numerous, and generally isodiametric throughout, often contrasting sharply with the 
more elongated elements in the rest of the ray (see Plate 72, photographs 5 and 6). 
The cells toward the middle of the infranodal tissue of the ray are generally smaller, 
and frequently have thicker walls than their neighbours. The most careful investi- 
gation, however, has failed to reveal any traces of vascular tissue in this position. 
The preservation of the specimens is often so perfect, that such tissue, if it had been 
* See Ky, ‘‘ Zur Kenntniss der Tracheiden,” ‘ Ber. d. Deutsch. Bot. Gesellschaft,’ vol. 4, 1886. 
+ Wivtiamson, * Organization,” Part TX., p. 326. 
MDCCOXCIV.—B. 5X 
