7 8 
Cor mack. — On a Cambial 
fascicular cambium (Fig. 1 1). He remarks that ‘ the cells of the 
pith become more regular in disposition 5 as we pass into the 
primary medullary ray ; and again notes the suddenness of 
transition from pith-cells to ‘primary medullary ray’ cells 
seen in longitudinal section. As cambial activity in Equise- 
tum has not extended to the formation of an interfascicular 
cambium, we can sufficiently account for the absence of such 
muriform tissue. However, as a matter of fact, cells which 
might be described as ‘ muriform 5 are found in the primary 
medullary rays of Equisetum maximum. 
Certain others of the distinctions drawn are not involved in 
secondary thickening. 
Williamson writes: ‘We further discover in the mode of 
Equisetum that, in addition to the cellular diaphragm or 
extension of the pith that stretches across the fistular cavity, 
a still more dense layer exists, not only within the diaphragm, 
but which, as shown in Fig. 41, is continued in a direct line 
across both the vascular and cortical zones .... this dense 
layer truncates the vascular masses.’ 
Fig. 1 is a diagrammatic representation of the tissues in the 
node of Equisetum maximum — the species examined by 
Williamson. As might be expected, the cells of the node are 
little elongated except in the vascular system ; but there is 
a gradual transition to greater elongation in the internode. 
In Fig. 1 there is an attempt to indicate relative shortness of 
cells in the parenchymatous portion by means of greater depth 
of shading. The shorter cells must, we conclude, be the 
‘ denser layer ’ referred to by Williamson ; but the layer, such 
as it is, cannot be described as ‘ intersecting the course of the 
vessels.’ The vascular system passes, so to speak, without 
interruption through the layer. 
No great importance can be attached to the distinction 
between Calamitae and Equisetaceae, which is based on more 
complete absorption of the pith and consequent continuity of 
the central cavities of certain Calamitae. Diaphragms are 
not always absent from the nodes ; they are sometimes thick 
throughout, and sometimes thin down towards the centre. 
