65 
Develop7nent in Equisetnm. 
Median longitudinal sections show that the central cavity 
extends throughout the internodes, but is interrupted at the 
nodes by a disk of tissue which is sometimes entire, some- 
times perforate. Williamson has shown that this canal 
existed during the life of the plant and was formed by 
absorption of the pith. 
Such sections also show that the cavities seen in 
transverse section at the inner angle of the woody wedges, 
represent canals which follow the woody Strands in their 
course through the internode. In the node these canals are 
sometimes interrupted, but sometimes bifurcate along with 
the woody strands and accompany them into the next 
internode. Thus the canals of adjacent internodes alternate, 
and may or may not be continuous. 
Radial sections further show a point of much importance 
to this paper. The thickness of wood in the node exceeds 
that of the internode, so that the wood -strands project at the 
node with convex contours, centrally into the pith, and peri- 
pherally into the rind. 
If the central cavity of such a specimen as has just been 
described were filled under pressure with material which 
would form a cast, the cast so produced would be cylindrical 
or somewhat conical, impressed at intervals with a circum- 
ferential groove corresponding with the nodal convex 
projections of the wood, and moulded into ridges and furrows, 
each furrow corresponding to the inner angle of. a strand of 
wood. Sometimes all that remains to represent one of the 
Calamitae is such a cast covered, perhaps, by a thin layer of 
coal derived from the actual tissue of the plant now crushed 
and altered beyond recognition. This is the teaching of 
Williamson enforced more recently and very convincingly by 
Stur. 
From this it may be inferred that there are three types of 
fossilization of Calamitae : — 
(1) Petrifactions showing actual plant-structure. 
(2) Mere casts invested by a thicker or thinner layer of 
coal. 
F 
