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 l concluded 

 that the highly developed Polytrichaceous stele is in the aerial stem 

 essentially double in nature and phylogenetic origin, consisting (i) 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, Cheirostrobus, the Lepidodendra 

 with solid steles, the monostelic Selagindlas, and (modified in various ways) 

 in Psilotum, 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 



1 /..<., P . 35. 



