1 62 



THE VASCULAR SYSTEM OF THE LEAF 



[CH. 



" Inversicatenales " for the whole series of ancient forms named by Seward 

 the Coenopterideae (Figs. 1 5 3, C, 1 54)- Further up in the leaf of many of these 

 ancient forms the vascular tract expands at the curved ends, giving a very 

 characteristic appearance in the case of the Zygopterideae. They thus afford 

 supply to the four rows of pinnae which their leaves bore (Fig. 73, p. 82). But 

 in the vast majority of primitive Ferns which had flattened leaves, and only 

 two lateral rows of pinnae, the undivided leaf-trace shows an adaxially 

 concave section, and often it may be sharply folded into a complicated 

 horse-shoe form. The steps leading to this state are illustrated, as seen at 

 successive levels from below upwards, 

 in the leaf-trace of Thamnopteris 

 Schlechtendalii, as it departs from the 

 stele of the axis (Fig. 155). Here the 

 xylem of the leaf-trace first appears 

 as a protuberance on the surface of 

 the xylem of the stem, and contains 

 a single xylem-group (i). It separates 

 as a mass elliptical in transverse sec- 

 tion (2). Further out an island of 

 parenchyma appears in front of the 

 protoxylem, and it gradually increases 

 until at last it opens on the adaxial 

 side (3, 4). The bay of parenchyma 

 thus formed widens until the xylem 

 resembles a crescent with curved ends, 

 while the protoxylem elements spread 

 over the margin of the bay (5), and 

 there divide into two, and later into 

 several groups (6, 7). At first the out- 

 line of the whole trace remains ellip- 

 tical: but further out in the cortex of 

 the stem a depression appears on the 

 adaxial side, which widens till the 

 trace becomes curved into a broad arch, while its ends are incurved. The 

 xylem has meanwhile thinned out, and numerous protoxylem-groups are 

 now seen along its concave margin (8, 9). It is believed that these stages 

 may be taken as indicating the changes undergone in the phylogeny of the 

 adaxially-curved leaf-trace so generally representative of the Filicales. But 

 in the more advanced types the cylindrical or oval region at the base is 

 frequently omitted, the trace being ribbon-shaped or curved from the first. 

 The whole progression may probably be related to the flattened form of the 

 leaf-blade, and to the progressive webbing of its segments laterally. 



Fig. 155. Diagrams illustrating the departure of 

 the leaf-trace in Thavinopte7-is Schlechtendalii, 

 Eichd. (After Gwynne- Vaughan. ) Compare 

 text. 



