LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 663 



structure suggests not a dichotomy but a local segregation of the vascular tissue 

 of the single strand in the middle region of the leaf, where the blade is broadest. 

 The analogy with what is seen in certain Conifers is obvious. Notwithstanding 

 this constant simplicity which the leaves of the Lycopodiales show, they do not 

 differ essentially in mode of origin from the leaves of other Vascular Plants. 

 Indeed, they may be matched very nearly in point of size and complexity by the 

 leaves of the Hydropterideae, while the primordial leaves of Pilularia and Marsilia 

 resemble in many of their features the submerged leaves of Isoetes. It may remain 

 an open question whether the simplicity of the Lycopod leaf is primitive, or the 

 result of reduction. A circumstance which favours the former alternative is the 

 great uniformity which they show, not only in the living forms, but also in the 

 fossils. The juvenile leaves in these plants do not show characters of comparative 

 interest, beyond the fact that they are often more succulent than the leaves of 

 the adult. 



I find myself unable to follow Lignier in the distinction which he draws between 

 " phylloides," by which term he designates the leaves of the " Prohepatiques " and 

 Lycopods, and the leaves of the Filicinese. As regards their relation to the axis, 

 their acropetal succession, and their general characters of form and structure, the 

 foliar appendages of the Lycopodiales fall into the category of "leaves" just as 

 much as those of other Vascular Plants. It is not, however, necessary to regard 

 them as all derived along a single ancestral line.* 



Eqtjisetales. 



The leaves of Equisetum are mere webbed teeth, each provided with a single 

 vascular strand, and united into a sheath. They are probably reduced representa- 

 tives of an ancestry with larger leaves, less fully webbed, or even free. Thus in 

 Annularia and Phyllotheca the leaves are slightly webbed : but in Asterophyllites , 

 which is traced back to the Devonian Period, the leaves appear quite separate in 

 widely divergent whorls. These leaves were simple, though of larger dimensions 

 than the leaf-teeth of Equisetum , and more effective than these as assimilating 

 organs. But among the earliest forms, such as Asterocalamites (Schimper) from 

 the Culm, the separate leaves were themselves branched in repeated dichotomies, 

 as shown in the well-known drawing of STUR.f 



These facts indicate that in the earlier Equisetales the leaves were separate 

 assimilating organs, and were sometimes susceptible of dichotomous branching. 

 That they were liable to cohesive webbing. That with this in some cases went a 

 reduction, so that they became mere protective sheaths to the assimilating axis, 

 as in Equisetum. In others they became enlarged, and the sheaths split into broad 



* Compare Lignier, "Equisetales et Sphenophyllales : leur origine Filicineenne commune," Bull, de la Soc. 

 Linn, de Normandie, 50 serie, 7 e vol., Caen, 1903. 



t See Land Flora, fig. 199 ; or Zeiller, Pal&obotanique, fig. 112. 



