LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 699 



and that, as far as observation extends, the architecture of all leaves of Ferns may 

 be interpreted in terms of dichotomy, with or without sympodial, and ultimately 

 monopodial, developments. 



It has been seen above that in other phyla of Pteridophytes the leaf is either 

 unbranched, or that it bifurcates. The only case of difficult interpretation is the 

 three-lobed sporophyll of Cheirostrobus, and even there the distal end of each lobe 

 forks equally. It is possible that it shows either trichotomy or a very condensed 

 pair of dichotomies. Thus it may be stated generally for the Pteridophyta that in 

 them the leaf is either unbranched or dichotomous ; or it shotvs branching and webbed 

 venation that may be recognised as a sympodial derivative of dichotomy. 



Comparative Conclusions. 



In order to help in obtaining a scientific foundation for views of " leaf- 

 architecture," the leading facts have been summarised in the above pages as to the 

 branching and venation of the leaf in those Vascular Plants which are held to be 

 primitive. More especially attention has been given to their juvenile leaves, as 

 elucidating the construction of the adult. But in many characters of form, of 

 venation, of webbing, of basal growths, and even of apical arrest, the leaves of 

 Pteridophyta prefigure the leaf-construction of Flowering Plants. We may now 

 consider how far the conclusions stated above can be extended so as to include all 

 leaves of Vascular Plants. And it will be found possible to base upon the data 

 before us, together with other facts already well known, certain general conclusions 

 relating to leaf-architecture at large, which will now be briefly discussed and 

 formulated. 



I. The conception of the shoot, and of its constituent parts, axis and leaf, as 

 laid down by Sachs,* is adopted for the purpose of this discussion, but it is the 

 leaf of the sporophyte only that is treated here. The leaf is normally a bifacial 

 member, with the adaxial and abaxial relations indicated structurally and develop- 

 mentally. Its form is as a rule flattened, with convex margins. But even where 

 the form is cylindrical, as in Pihdaria, the segmentation clearly indicates the 

 bifacial character. Or, in the case of the leaf-stalks of certain Ferns that appear 

 cylindrical, the margins are still indicated by lines. Putting aside exceptional 

 cases of high specialisation, it may be stated that the leaf is a bifacial member with 

 margins defined structurally, though these may be obscured. 



II. In the simplest cases the leaf has not a marked apical growth : it is un- 

 branched, and may contain only a simple unbranched vein. This is the constant 

 state in the Lycopodiales, fossil and modern. It is also generally seen in the 

 Equisetales, though in Asterocalamites and Pseudobornia dichotomy existed. 

 Dichotomy is, however, the rule in the Sphenophyllales and Psilotaceee, though 

 in the Psilotacese the vegetative leaves are simple. This is also seen occasionally in 



* Textbook, 2nd Engl, ed., p. 153 ; and Lectures on the Physiology of Plants, Lecture I. 



