ANATOMY 385 
ANATOMY. 
For the purposes of the present discussion the ,chief points of 
importance in the anatomy of the Equisetales relate to the structure of the 
axis: the leaves and roots carry only a minor interest. It will suffice to 
say of the former that their structure in EZguésetum points to a probability 
of reduction from a condition more effective in assimilation, which was 
their state in some at least of the Calamites. The roots of Eyudsetum are 
essentially of the Fern-type, though with some peculiarities of detail of 
their own: the roots of the Calamites show in their primary structure 
striking similarity to those of Zguisetwm, including the peculiar double 
endodermis; but they show in addition a cambial thickening, which is 
quite in keeping with the secondary growth of the axis which they 
support. 
In discussing the structure of the axis the same order may be observed 
as in the external morphology, and the living genus Lguisetwm will be 
taken first. Transverse sections of the internode show the well-known 
disposition of the chief tissue-tracts, though with varying proportion and 
.structure of the several tissues according to the species and the grade of 
the axis cut: viz. a peripheral epidermis, a broad cortex, and a central 
stelar region. The chief interest naturally centres in the tissues of the 
stele, and indeed it is unnecessary to discuss here the special characters 
of the superficial tracts. It may be noted first that the outer limit of the 
stele is not defined by the first apical segmentations: the inner cell cut 
off by the first periclinal wall in each segment of the apical cell forms 
only the pith, while the vascular tissues originate together with the 
cortex from the outer products of each segment. But it has been 
seen that early segmentation is not a constant index of morphological 
character, and, accordingly, the stelar condition of Lguésetum may 
properly be compared with that of other Vascular Plants, irrespective 
of its origin in the primary segmentation. The stele consists of a 
which the whole theory is based. Thirdly, the same difficulty will arise from the alternation 
of the whorls of bracts, and the superposition of the sporangiophores as opposed to Jeffrey’s 
suggestion. Fourthly, the theory is quite inapplicable to the Equisetales at large, as is 
admitted by Lignier (/.c., p. 131). He himself suggests a different origin of the sporangio- 
phore for Hyguisetum and Archaeocalamites, where they are held to represent whole 
leaves. These two hypotheses of origin of the sporangiophore put forward by Lignier seem 
too divergent to explain satisfactorily the nature of substantially the same part within 
the same natural phylum. Such difficulties are sure to arise where the attempt is made 
to reduce variable forms to a strict morphological scheme. This Lignier has done with 
some ingenuity for the individual case ; but the more elastic view of the sporangiophore as 
a part sud generis appears to accord better with the natural facts. The sporangiophore 
may have a more or less definite relation to the ‘sterile bracts, and it often has; but the 
facts for the Equisetal phylum do not indicate this as an obligatory relation. The 
nature of that relation will be best considered when corresponding facts from other 
sporangiophoric types are available (see part ili.). 
1Campbell, Mosses and Ferns, p. 460. 
2B 
