ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 869 
(2.) The arrangement of the vascular bundles in a single regular ring, enclosing a 
large pith-area, agrees well with Hguisetum, but equally well with any normal 
Gymnosperm in Dicotyledon. We shall show in detail below that the canal at the 
inner edge of each bundle represents the disorganized protoxylem, so that in this 
respect the agreement with Hquisetwm is exact. We shall further show that in 
favourable cases there are distinct remains of the phloém to the outside of the wood, 
so that there is no doubt as to the bundles being normally collateral, with centrifugal 
xylem, just as in the recent genus. 
(3.) For the comparison of the minute histological structure, we are practically 
limited to the wood, and as only the primary region can be compared, the material is 
somewhat meagre. The primary xylem-elements of Calamites, the spiral, reticulated 
and scalariform trachez, are on the whole such as we find in Equisetum, and certainly 
agree less closely with those which we find in the corresponding position in other 
groups of vascular plants. The distinctive structure of the nodal, as compared with 
the internodal xylem, also shows a close agreement in the two genera. 
In a few cases the structure of the cortex shows the distinct peripheral strands of 
sclerenchyma, which are frequent in the sub-aérial stems of Hquiseta.* 
To sum up our preliminary anatomical comparison. Although there is no secondary 
thickening, so far as we know, in any recent Hquzsetum (if we except the slight 
indications investigated by Mr. Cormack)t, yet the primary structure, on which in 
the Calamutes the secondary zone is superposed, is almost identical in the two genera. 
PRIMARY STRUCTURE OF THE STEM. 
a. The Internodes. 
The first subject to be considered in detail is the primary structure of the stem. 
It is comparatively rare to find specimens in which this primary structure is unaltered. 
Where this is the case the specimens in question are usually, though not always, small 
twigs, with few vascular bundles. It must not be supposed that these twigs repre- 
sent the earliest stages of the principal branches. They are probably ramuli of a 
high order, which in many cases seem never to have undergone any great degree of 
secondary thickening. In such small branches the pith is often solid, just as is the 
case in the ultimate ramuli of some Equiseta, as, for example, HZ. pratense. In other 
* See Wriuiamson, “ Organization,” Part XIL, ‘Phil. Trans.,’ 1883, IL, Plate 33, fig. 19 (C.N. 62). 
Nore.—Reference numbers preceded by the letters C.N. (= Cabinet Number) always indicate the 
number of the slide or specimen in the WiLLrAmson collection. 
+ “On a Cambial Development in Hquisetum,” ‘ Annals of Botany,’ vol. 7, 1893. 
+ See Wintramsoy, “ Organization,” Part IX., Plate 19, figs. 8 and 9 (C.N. land 2). See also our 
own fig. 1, in Plate 77, though here the pith is not quite complete. 
