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 Equisetuni 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 Equisetuni 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 Equisctum, 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 Equisetum 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. 1 But it has been 

 seen that early segmentation is not a constant index of morphological 

 character, and, accordingly, the stelar condition of Equisetuni 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 (I.e., p. 131). He himself suggests a different origin of the sporangio- 

 phore for Eqtiisetum 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 lias done with 

 some ingenuity for the individual case ; but the more elastic view of the sporangiophore as 

 a part siti 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 iii.). 



1 Campbell, Mosses and Ferns, p. 460. 



2 P. 



