Vascular System of the Genus Equisetum. 2 1 1 
vascular supplies from the aerial shoots (Text-fig. 7). There are no spiral 
and annular elements present, and this may be accounted for by the absence 
of elongation in this region of the axis once the conducting tissue has 
developed. This underground region thus presents features which only 
appear in a modified form at the bases of the ordinary aerial axes. As has 
been shown above, these structural peculiarities can be directly attributed to 
the early and repeated branching by which it is built up. 
Origin of the Metaxylem. 
The structure of the bundles in the internode of the mature stem is too 
well known to need detailed description. Of the three groups of xylem 
normally present, the two lateral groups, commonly known as the meta- 
xylem, develop later than the elements of the protoxylem which are 
associated with the carinal canal. 
There have been two different interpretations of the nature of the 
metaxylem of the internodal bundle. The earlier investigators concluded 
that it comprised the laterally developed elements of a collateral bundle of 
which the first-formed elements were represented by the protoxylem 
adhering to the carinal canals. The similarity in the general arrangement 
of the tissues as seen in transverse sections of the internodes of any 
Equisetum compared with that of a young herbaceous dicotyledon naturally 
gave rise to the opinion that the structures were analogous. Such a type 
of structure is, however, unusual in Vascular Cryptogams apart from the 
well-known case of the Osmundaceae. In all the groups of Vascular 
Cryptogams other than the Equisetaceae the mature structure, even in the 
most elaborate types, can be shown to have been derived, in the course of 
ontogeny, from a simple protostele. 
In 1901 Prof. Gwynne-Vaughan ( 9 ) suggested another interpretation 
for the metaxylem. He pointed out its independent nature and showed 
that the protoxylem of the carinal canals was alone associated with the 
leaf-trace system, and he concluded, at least for E. hiemale and E. maximum , 
that the metaxylem strands were continuous over the nodes, merely altering 
their position laterally and becoming associated with neighbouring leaf- 
traces. He went farther and suggested tentatively that this xylem might 
represent the last remnants of a mass of centripetal wood of a primitive 
protostele. This would involve the centripetal development of this portion 
of the wood, and he supported his theory by reference to E. giganteum , 
which showed what he interpreted as indications of such a development, 
but owing to the mature condition of all the available material he could not 
demonstrate this conclusively. 
The question has thus definitely centred upon the order of differentia- 
tion of this metaxylem. Eames (10) and Queva ( 7 ) have subsequently 
stated that the whole of the lateral metaxylem is centrifugal, although the 
