XVIIl] 



COMPARISON 



35 



monodesmic, which is a primitive state. But the meristele may be 

 elaborately moulded: this is seen in less degree in the small plants of the 

 Botryopterideae, but in sections of the larger petioles of the Zygopterideae 

 the meristele shows very complex outlines, which however are all referable 

 to amplifications of the primitive type. It will be shown that this also has 

 its relation to size (see Vol. I, Chapter x). Such structure provides the 

 leading anatomical features by which the Zygopterideae are characterised. 

 The Anachoropterideae show a somewhat similar amplification, but along 

 divergent lines. All these vegetative features accord with the early appear- 

 ance of these Plants as fossils, and indicate that in them we are dealing 

 with primitive and probably synthetic examples of the Filicales. Their 

 most peculiar feature is the high degree of elaboration of the vascular 

 system seen in some of them, though this is always closely related to the 

 protostele and the monodesmic trace: and it demands some elucidation 

 when found in families so primitive in type and early in occurrence. 



If the illustrations of the Coenopteridaceae published in standard works be 

 measured, and the results divided by the magnifying power, it is possible to 

 obtain the approximate actual diameters of the parts together with the 

 diameters of their conducting tracts. If these be tabulated in order, from 

 those which are smallest to those which are largest, it is found that parallel 

 with the increasing measurements there is an increase in complexity of the 

 vascular system. The following table demonstrates the result for the stems 

 of the Coenopteridaceae named, and in the last column the approximate 

 ratio of the diameter of the axis to that of the stele is also given for some 

 of them. The figures can only be approximate, since in certain examples 

 the phloem and sheaths are imperfectly preserved, and the measurements 

 have then been taken from the xylem alone: 



Table I. Stcjus of Coenopteridaceae 



