138 THE VASCULAR SYSTEM OF THE AXIS [CH. 



The interest of these examples Hes not so much in the fact of secondary- 

 thickening having been established, as that none of the later Filicales ever 

 appear to have developed so important an innovation to the best advantage. 

 It is seen only in Ferns of primitive type. For some reason which is not 

 obvious Ferns worked out the evolution of all their more modern forms 

 upon the basis of the primary vascular tissues alone. In the great majority 

 of them these were composed of strictly circumscribed tracts, without inter- 

 cellular spaces, enclosed within continuous and well-defined endodermal 

 sheaths. These will be described in the next Chapter, as they are seen in 

 the Leptosporangiate Ferns. 



All the Ferns whose stelar state has been described in this Chapter are 

 relatively primitive. Most of them are Eusporangiate, or are intermediate 

 between this and the Leptosporangiate state. Some are actually fossils, 

 others belong to affinities which have an early fossil record. None but Cheiro- 

 pleiiria show the full characters of the Leptosporangiate Ferns. It is from 

 such sources that suggestions of advance rather than of simplification of 

 vascular structure may be expected. Though structural simplification may 

 naturally have occurred in some of them, it cannot reasonably be assumed 

 that it has affected all of these early and fossil types. The rational view 

 would rather be to expect the reverse, and to assume that the apparent 

 anomalies which have been described embody tentative advances upon the 

 simple protostelic structure. An impressive feature underlying the whole 

 series of facts relating to the vascular system of these relatively primitive 

 Ferns is that they all start Trom a protostele. In some, as Campbell has 

 pointed out, its structure is ill-defined, giving the appearance of a mere 

 sympodium of leaf-traces. This may be held as a consequence of a mega- 

 phyllous disproportion of axis and leaf, with the axial constituent of the 

 stele reduced in extreme cases to vanishing point. But that does not alter 

 though it may obscure its stelar nature ; and an interesting feature of the 

 facts disclosed is in what different ways the ontogenetic amplification may 

 be carried out. It will be shown in Chapter x that they are natural conse- 

 quences of increase in size of the conducting system, such as is seen in the 

 individual life in the course of its progress from the sporeling to the adult. On 

 the other hand, fi'-om the phyletic point of view, the nearer the adult structure 

 remains to that of the original protostele the more primitive the plant which 

 shows it is to be held in respect of that itnportant structural feature. 



Stelar Theory 



If an adult stem of a Dicotyledon, a Gymnosperm, a Lycopod, a Horsetail, or a 

 simple type of Fern be cut transversely, a more or less compact column of conducting 

 tissues is found lying centrally and connected outwards with the leaves by strands of the 

 jeaf-trace. Often it is seen to be delimited by an endodermis or "phloeoterma " which gives 

 precision to it, and appears to justify the recognition of the whole column as an entity. 



