362 KIDSTON : THE FLORA OF THE CARBONIFEROUS PERIOD. 
this is added a zone of secondary wood, formed from a cambium layer, 
and as this secondary zone increases in width, the star-like form of 
the primary wood is lost and the bundle becomes circular in form 
through the external addition of the secondary wood. The whole 
is enclosed by a firm bark. 
Giving to the dimorphic condition of the leaves, it was formerly 
supposed by some that SphenopJiyllum was an aquatic plant, but 
this is not the case. The whole structure of the plant makes it clear 
that S phenophyllum was terrestrial, though from their long delicate 
stems they must have had some support to keep them in an upright 
position, and this support they probably found by scrambling amongst 
the surrounding vegetation. 
S phenophyllum, not only in the structure of its stem, but also in 
that of its cones, exhibits so many peculiarities that it is impossible to 
include it with any other group of plants. With the casts of Calamites 
it has a certain superficial resemblance 
in its noded stems and whorled leaves, but 
^^^^ the solid axis and non-alternating ribs of 
\JkV S phenophyllum, along with its dichoto- 
\ ( mously-divided leaves as well as the 
structure of its cone, differ so much from 
Fig. 15. - SphenophyUum ^j^^ Calamites that any systematic rela- 
trichomatosum Stur. Ar- . , . . . , i i i tt- i 
rangement of sporangia ^lonship IS entirely precluded. With 
(enlarged). Asterocalamites it has a greater resem- 
blance in the ribs of both not alternating 
at the nodes and in the leaves being dichotomously divided, but 
SphenophyUum differs here also in its solid axis and in the structure 
of the cone, presuming that Pothocites is the cone of Asterocalamites. 
Nor with the Lycopodiacese does SphenophyUum seem to have 
any close connection. The jointed stem, dichotomously-divided 
leaves, and structure of the fructification are very different from 
anything found amongst the Lycopods. 
The genus SphenophyUum must therefore stand alone as a peculiar 
and interesting type of plant which appears to have become extinct 
in early geological times. 
