in the ‘ Seedlings ’ of Certain Leptosporangiate Ferns. 405 
as being a solid rod of xylem surrounded by phloem, and later an amphi- 
phloic tube with an inner endodermis surrounding non-stelar tissue, but 
as a solid strand of vascular tissue which may be either a solid rod of 
xylem ensheathed by phloem (protostele), or a ring of xylem surrounded 
by, and surrounding, phloem (amphiphloic protostele). 
It may be objected that at the very base of the young stem a few 
parenchyma cells are differentiated at the centre of the xylem immediately 
before the appearance of internal sieve-tubes, and that such parenchyma 
cannot be regarded as vascular tissue. Definite internal phloem is so 
quickly differentiated, however, that there can be little doubt that the 
parenchyma cells, which are in organic continuity with the internal phloem, 
are best regarded as potential phloem. The wholly vascular nature of the 
cauline strand at this level is further emphasized by the occasional differ- 
entiation of tracheides in the internal phloem, an example occurring in 
Nephrodium setigerum being illustrated in Fig. 203. 
The insertion of the leaf-traces upon this primitive strand has resulted 
in the latter becoming moulded and modified along certain definite lines, 
the culminating structure being the dictyostelic complex. In all probability 
this arrangement of the vascular tissues is the most satisfactory for allowing 
an adequate supply of water and mineral food to reach the relatively large 
leaves. The portion of the cauline system actually in connexion with the 
leaf-strands is probably quite insufficient to afford this supply, and the 
latter is only effected as a result of the ready transference of water from 
one part of the rhizome to another, which is rendered possible by the 
complete network of the vascular strands. 
It has been stated above that the ontogenetic elaboration of the 
vascular system proceeds along certain constant and well-defined lines. 
As has been shown by Boodle and others, not the least interesting point 
in connexion with this fact is that many of the admittedly more primitive 
ferns have proceeded for only a relatively short distance along these onto- 
genetic and, presumably, phylogenetic lines. The elaboration of their 
vascular system stops short, as it were, at some intermediate stage or 
condition which is adopted by the plant as the most suitable for its 
mature habit. Take, for example, a typical dictyostelic fern such as 
Blechnum brasiliense. The first stage is the solid protostele, a structure 
adopted as the mature organization by Lygodium. The appearance of 
central parenchyma follows, a condition which is exactly matched in 
Schizaea. Later, well-defined sieve-tubes appear in the central parenchyma, 
and this structure, although rare in mature plants, is found in Lindsay a 1 . 
The region at which we have the differentiation of the ground-tissue pocket 
is the most interesting in the whole series of elaborative processes. If we 
imagine this conical pocket continued through successive internodes (cases 
1 Tansley and Lulham, loc, cit. 
