Ill] THE SEARCH FOR NEW CRITERIA 6i 



may also be great diversity in outline of the segments, and in their relation 

 to one another, and to the main rachis. Such features offer an easily observed 

 criterion of comparison. They are closely connected with the venation. 

 Sometimes, as in Dryopteris (Fig. 2), the veins end blindly near to the 

 margin, the venation being then described as open. But in other Ferns they 

 are connected so as to form a coarse network of large meshes (Fig. 16): or 

 others again may show a finer network of smaller meshes (Figs. 17, 18). It 

 is thus apparent that the architecture and venation of the leaf shonXd provide 

 another valuable criterion of comparison, and one easy of observation. 



IV. If the veins of the leaf of Dryopteris be followed inwards from the 

 margin of the pinnule, they connect downwards into the midrib of the pinna, 

 and pass on through the rachis to the leaf-base. There they enter the stem 

 as a number of isolated strands, which insert themselves on the margin of 

 one of the meshes of the cylindrical network of vascular tissue which traverses 

 the massive stem (Fig. i, D, E, F). These together constitute the stele 

 which is here broken up into parts (meristeles). Thus the shoot is traversed 

 by a connected system of conducting tissue. It happens that this tissue, so 

 important physiologically, retains with great persistence its characters of form 

 and distribution in individuals of the same affinity. But a comparison of 

 different types of Ferns discloses features, both in the stele of the stem and 

 in the leaf-traces which spring from it, that are different and distinctive for 

 the several groups. Moreover, since these resistant tissues are preserved 

 commonly in the fossil state, they provide a wide basis for comparison. 

 Accordingly the vascular tissue of stem and leaf affords perhaps the most 

 reliable structural criterion of comparison of all those which lead towards a 

 phyletic grouping of the Ferns. 



V. The leaf-stalk of the mature leaf of Dryopteris, the whole leaf while 

 young, and the apical region of the stem are closely invested by a dense 

 covering of broad protective scales, or ramenta. These are pale coloured 

 while young, but are rusty brown when mature, after which time many of 

 them dry up and fall away. These broad scales are of the nature of trichomes 

 originating from superficial cells. But there is a good deal of variety in this 

 investiture in Ferns. Many possess no flattened ramenta, but only hairs 

 composed of a single row of cells. Frequently the hairs may bear glands on 

 their ends, or be variously branched. They may even be borne on large out- 

 growths which sometimes mature as hard woody emergences covering the 

 older parts. Such dermal appendages, simple or complex, will also be brought 

 into the comparative treatment. 



VI. The preceding characters all relate to the vegetative system, which 

 in plants at large is found to be so directly adaptable to circumstances that 

 its features are commonly accorded only a minor place as a basis for classi- 



