196 ANATOMICAL EVIDENCE 
We see among the former group plants in the very act, so to speak, of 
developing a conducting system in response to vital needs, and others in 
the most various stages of its evolution in complexity. The conditions 
under which this evolutionary development occurred must have been 
practically identical with those to which the primitive Pteridophytic 
sporophyte was subjected,—gradually increasing adaptation of a simple leafy 
form to terrestrial life. And the final result, as seen in the highest Poly- 
trichaceae, is so strikingly like the state of things obtaining in the true 
vascular plant as to furnish probably one of the completest and most 
interesting cases of homoplastic development in the plant-kingdom. It can 
hardly, therefore, be denied that the study of the conducting system in 
Mosses is calculated to throw most valuable side-lights on the question 
of the evolution of the vascular systems of the higher plants. 
As the result of his careful analysis of the tissues, Tansley} concluded 
that the highly developed Polytrichaceous stele is in the aerial stem 
essentially double in nature and. phylogenetic origin, consisting (1) of a 
central primitive hydrom-cylinder originally developed, and still serving to 
supply the apical bud, sexual organs, and sporogonium with water; and 
(2) of a double peripheral mantle of hydrom and leptom separated by a 
starchy hydrom-sheath, and all three layers composed of the joined bases 
of leaf-traces, and designed between them to conduct water to and formed 
material from the leaves. 
The bearing of these considerations on the problem of the nature and 
origin of the primitive stele among the Pteridophytes, as we find it, for 
instance, among the Sphenophyllales and Lycopodiales, is a very interesting 
question. Two alternative explanations of such a stele are possible. 
According to a strobiloid theory, we may suppose the primitive Pterido 
phyte descended from a form bearing a terminal fruit-body; this contained 
a primitive hydrom-stele comparable with that of the Mosses, but supplying 
the fruit-body directly, since it is developed in the sporophyte, instead of 
merely leading up to the base of the sporogonium. The lineal descendant 
of such a primitive hydrom-stele would then perhaps be seen in the central 
metaxylem of, for instance, Sphenophyllum, Chetrostrobus, the Lepidodendra 
with solid steles, the monostelic Se/agimel/as, and (modified in various ways) 
in Pstlotum, Lycopodium, etc. (Fig. 99). Added to this would be the bases 
of the leaf-traces represented by the peripheral protoxylem-strands, and only 
evolved after the primitive sporophyte had thrown out leaves requiring a 
vascular supply connected with the main channel of the stem. The fact 
that they appear before the central xylem in the development of the 
individual stem would be merely in relation to the need for the early 
establishment of conducting channels to the leaves—a need which is 
universal in leafy vascular plants. 
On the other hand, under some phytonic theory we might suppose that 
the formation of leaf-structures requiring a vascular supply preceded the 
1Z.c., pe 35. 
