2 20 
A. G. Tansley. 
wall of the stele. At the same time the dorsal wall of the stele 
breaks in the middle line, so that the whole vascular system 
acquires the general form of a leaf-trace with deep inward folds 
A' and B') on the two flanks. The two branch steles of the 
’/ V hum,n 
Fig. 65. Pteris incisn var. integrifolia. Vascular system of node seen from 
dorsal side. The stele below the node is seen in sections, the parts between 
cut away. A, 13, lateral folds of solenostclc, A', 13', of leaf-trace. From 
Tansley & Lulham. 
dichotomy are now separated off to the right and to the left from 
the dorsal half of the trace, hut they arc contributed to by hollow 
cylindrical strands arising from the ventral half ( e , etc., Fig. 66). 
The result is that the extended flanks of the trace are consi¬ 
derably abbreviated, though the whole remains of the same general 
form, which is that of the Hypolepis-typc but considerably more 
folded. The two conspicuously deep lateral folds (A' — B') are con¬ 
tinuous below with the internal folds (A, B) of the solenostele 
already described. Since these folds gradually die out in the inter¬ 
node below there can be little doubt that they represent a decurrent 
development of the lateral folds of the leaf-trace. Part of the wall 
of each branch-stele (black in Fig. 66) is also continuous with the 
corresponding fold of the main solenostele, and this is connected 
with the absence of gaps, such as are found in Hypolepis, at their 
bases (see Gwynne-Vaughan, ’03, Plate 33, Figs. 5, 6). 
Pteris aquilina (Fig. 67) has, strictly speaking, a dissected 
dorsiventral solenostele on the definition adopted in these lectures, 
since the leaf-gaps are comparatively remote, and do not overlap, 
though there are numerous other gaps which do. The petiolar 
system, as is well-known, consists of a number of separate 
strands, the arrangement of which eventually becomes very con¬ 
fused. At the base, however, the outline of the leaf-trace as a 
whole is of exactly the same type as in the plant just described. 
The deep lateral folds of the trace are here also exclusively supplied 
by the internal vascular strands of the stem, which take the well 
