On Secondary Thickening in Pteridophyta. 
2 I 1 
EQUISETACEvE. 
Literature. 
Cormack.—On a Cambial Development in Equisetum. Ann. Bot,, 
Vol. VII., 1893. 
Equisetum maximum. 
Stem. The examination of a transverse section through a 
mature node shows that the xylem of the bundles is relatively very 
extensive, and that the elements of the wood and of the phloem, in 
the central region of the strand, are arranged in radial rows. This 
regular seriation is not obvious in the tracheides nearer the centre 
of the axis nor in the more peripheral phloem-elements, in fact 
“ the whole arrangement suggests that a plate of tissue has been 
intercalated between two older growths by the activity of a cambium¬ 
like meristem.” The opinion here expressed is strengthened by the 
examination of tangential and radial longitudinal sections, from 
which it is seen that the characters of the elements interposed 
between the xylem and phloem are those usually associated with a 
normal cambium ; and further, the formation of new cells is similar 
to that obtaining in ordinary secondary thickening. 
Corroboration is to be obtained by the comparison of sections 
of stems of different ages. By this means it is found that the 
number of elements in the radial thickness of a bundle is about 
equal both in a young and an old internode. Thus in this region 
the tangential division is arrested very early. This, however, is not 
the case at the nodes; the immature node has fewer elements in 
the radial thickness of a bundle than has the mature node. 
Cormack’s conclusion is “ that after the bundle has attained in 
the internode its full number of cells in radial thickness, and after 
tangential division in the corresponding tissues of the node has 
ceased, a plate of tissue has been intercalated between the proto- 
xylem and protophloem of the node; that the xylem thus formed 
is mostly reticulately thickened, whereas the thickening of the 
protoxylem is annular ; and that the intercalation has been accom¬ 
plished by the activity of a meristem whose cells are cambiform.” 
PSILOTACE/E. 
Literature. 
Boodle.—Secondary Tracheides in Psilotum. New Phytologist, 
Vol. III., 1904. 
Boodle.—On the Occurrence of Secondary Xylem in Psilotum, 
Ann. Bot., Vol. XVIII., 1904. 
Ford.—The Anatomy of Psilotum triquetrum. Ann. Bot., Vol. 
XVIII., 1904. 
