Development in Equisetum . 69 
stem. In (c) and (d) the woody cylinder is thick and massive 
and the stems are often large and nodose. 5 
Williamson regards (c) in the foregoing table as the 
Calamodendron of Brongniart : Dawson (b). That such 
difference of opinion is possible, proves how very closely 
related are the two types. The four types form a series 
in which (d) stands in the same relation to (c) as (b) 
does to (a). 
Having regard to all these facts we conclude with William- 
son that we are dealing with a group of plants which are 
closely allied to one another. 
We are thus brought to the consideration of the problem 
regarding the systematic position of this group. 
The removal by Brongniart of the forms in question to a 
place among the Gymnosperms was the outcome of his 
assumption that a plant which showed secondary thickening 
could not be a Cryptogam. 
The following considerations lead me to regard this 
assumption as unjustifiable : — 
(1) The vascular and reproductive systems are not so 
intimately connected that we can infer that because one plant 
has open bundles, and another plant closed bundles, therefore 
their reproductive organs will be essentially different : nor can 
we infer that because in two given plants the bundles are open, 
therefore their reproductive organs will be similar. 
(2) Cambial activity is found in plants whose affinities must 
be very remote, for example, in Quercus and in Laminaria. 
(3) While normally absent from a class (Monocotyledons) 
secondary thickening may be present in some members of it 
(certain Liliaceae). 
(4) In a class which normally possesses secondary thick- 
ening, it may be present in one species, and practically absent 
in another of the same genus ( Ranunculus Jiuitans ). 
(5) Among extinct plants also secondary thickening is 
known to have been absent from a species of a genus possess- 
ing it ( Lepidodendron Harcourtii). 
(6) Further, secondary thickening existed in ancient crypto- 
