504 
PROEESSOR W. 0. WILLIAMSON ON THE ORGANIZATION 
graph leaves unnoticed some points of importance to my present inquiry. In Plate 
XXIX. fig. 41 I have represented a longitudinal section of a node in a cylinder of 
Equisetum maximum. Fig. 42 represents a segment of a transverse section of an 
internode of the same plant, whilst fig. 43 exhibits one of the canals (42, e ) with its 
surrounding parts yet more highly magnified. 
The internal organization of these plants presents some striking resemblances to that 
of the Calamites. Their stems are jointed and their internodes fistular, whilst the me- 
dulla stretches permanently across the stem at each node (41, i), forming a cellular dia- 
phragm. (41, n). There is a thick persistent medullary layer (41, b) separated from an 
outer or cortical layer (41, h) by a series of canals (42, e, 43, e), like those common in 
the Calamites, and which reach from node to node without penetrating either. At each 
of the outermost angles of these canals we find a few longitudinal spiral vessels (43, g), 
and in the mass of dense cellular tissue which lies externally to each canal, we have 
two other small clusters (43, g') of similar vessels. Primarily cellular, these vessels be- 
come variously modified into annular, reticulated, barred, and scalariform types. The 
first glance at these sections tempts us to regard the canals (e) as the homologues of the 
longitudinal canals of the Calamite, and the vessels (g, g') as the degraded representa- 
tives of the woody wedges, the copious vascular laminae of which seem thus to have 
dwindled down to a few detached vessels. This resemblance is rendered yet more re- 
markable if we examine the corresponding vessels in some other species of Equisetum. 
Dr J. Milde has shown that in E. variegatum , robustum, brachyodon , hyemale , and ramo- 
sissimum, these vessels are arranged in two laminae, which radiate from the outer angles 
of the canal towards the cortex in two parallel lines. Let the intervening space be- 
tween these lines be filled up with similar ones, and we should have the woody wedge 
of the Calamite ; at the same time we have no evidence that this arrangement in the 
recent forms is the result of an exogenous growth*. When we turn to the longitudinal 
section (fig. 41), we again discover points of resemblance. The canals (42, e) are not 
very visible ; but we can readily mark their course by tracing that of the vessels ( g ) which 
accompany them, whilst the outer series ( g') is also seen imbedded in cellular paren- 
chyma. But the most striking features of this section appear immediately beneath the 
node. At this point we have a dense mass, convex externally, and consisting of very 
large reticulated vessels (41, y), in which the vessels (g, g') coming up from below sud- 
denly disappear. This cluster of enlarged reticulated vessels terminates very abruptly 
at the node, as if truncated, having previously given off a divergent cluster (41, z) to the 
base of an adjoining branch (41, to), and from which a few spiral vessels alone are prolonged 
into the branch, but between which and the cluster (z) whence they spring is placed 
an extension (i 1 ) of the nodal septum which bounds the branch cluster (z) in the same 
way that the main node (i) does the cluster y. These dense additions to the vascular 
tissues in the neighbourhood of the node appear at first sight to represent the arched 
* Sachs figures the superficial arrangement of these vessels at the nodes of Equisetum maximum ; it exactly 
corresponds with that of the woodj* wedges in my fig. 2, a fact favouring the idea of close Equisetacean affinities. 
See Sachs’s ‘ Lelirbuch der Rotanik,’ p. 349 D. 
