70 
Cor mack. — On a Cambial 
gamic forms though absent in their nearest modern repre- 
sentatives (present in Lepidodendron , absent in Selaginella). 
(7) Secondary thickening is not unknown in Vascular 
Cryptogams of the present day. It is recognized in Isoetes ; 
and it has also been described in Botrychium. Solms- 
Laubach 1 considers that in Botrychium the point requires to 
be cleared up by further investigation ; for the present we 
defer such examination. The objection that such secondary 
thickening is slight, is invalid, for mere questions of size give 
no sure basis for scientific classification. 
Thus we conclude that secondary thickening must some- 
times be homoplastic rather than homogenetic ; and conse- 
quently that, even in the absence of a cambial activity in 
modern Equisetaceae, it would be unwarrantable to deny the 
possibility of affinity between them and Calamitae with 
secondary thickening. 
While this is true, it is well that the Equisetaceae should be 
re-examined with a view to discovering whether cambial 
activity is really quite absent. This paper originated in a 
suggestion made by Professor Bower, with full appreciation 
of its palaeobotanical bearing, that in Equisetum some slight 
cambial activity might be found at the nodes, where, as is 
well known, the wood is more extensive than in the inter- 
nodes ; sections cut there and then gave indications which 
provoked more deliberate investigation. 
Williamson 2 , as the result of his examination of Equisetum 
maximum , brings forward certain differences of structure 
which render it undesirable to unite Catamites , with Equiseta- 
ceae £ though there are some points of resemblance between 
the two plants that sorely tempt a botanist to do so: We 
seek to discover by a re-examination of this species and of 
one of the Calamitae, how far such differences exist, and to 
estimate roughly what may be their systematic value. 
Fig. 1 is a diagram from a camera lucida outline of a radial 
section of Equisetum maximum . The section passed through 
1 Fossil Botany, Eng. Ed., p. 223. 
2 loc. cit. p. 503. 
