vii] STELAR THEORY 139 



The term stele was applied to it by Van Tieghem {Ann. Set. Nat. Bot. S^r. 7, 1886, p. 276). 

 There can be no doubt of the existence of such a tract in ordinary adult stems. The question 

 remains how is it to be interpreted? Is the stele a real entity, forming an essential con- 

 stituent of all axes : or is it a composite structure built up merely of decurrent tracts from 

 the leaves? Or does the truth lie between these extremes? Thisquestion has been raised 

 afresh by Campbell {Amer. Journ. of Bot. 192 1, p. 303). 



A provisional answer is suggested by examination of the apical structure of the adult 

 shoot in some Fern with elongated protostelic rhizome and isolated leaves, such as Cheiro- 

 pleuria (Fig. 115). Here the disproportion of size between stele and leaf-trace is great. The 

 former may be followed up to the apical cone as a column of procambium maintaining its 

 identity, while giving ofif only small fractions of its tissue as a leaf-trace to each isolated 

 leaf. The residual vascular tissue appears to be truly cauline and not referable in origin 

 to leaf-trace tissue. In elaborated vascular systems the compensation-strands and medul- 

 lary strands, such as are seen in many Leptosporangiate Ferns (Chapter vili), are clearly 

 cauline. Thus it appears that in adult shoots a part of the vascular system of the axis may 

 be, and often is, not common but cauline. Here then the stele is not merely a sympodium 

 of leaf-traces. But Campbell has pointed out in relation to the sporeling in certain Eu- 

 sporangiate Ferns that there is "an absence of a cauline stele in the young sporophyte," 

 and he contemplates with Delpino the sporeling plant as being composed of leaves as 

 primary organs, and the so-called stem as being formed by the coalescence of leaf-bases. 

 This may appear as though it were the fact in extreme cases such as certain Ophiogloss- 

 aceae and Marattiaceae selected by Campbell. But that does not demonstrate that it is 

 the correct view for general application, nor even that it is the true fact for the phyletic 

 history of the selected examples. 



The vascular system follows the Hnes of evolutionary progress, it does not dictate them. 

 If the progression be towards a preponderance of leaf over stem as is the rule in Ferns, the 

 vascular development of the latter will recede, and the former become dominant. Conversely, 

 whereas in Lycopods the stem preponderates, it is quite obvious that the greater part of the 

 axial stele is cauline. It may be held that the shoot in the Filicales fluctuates between a pre- 

 ponderance of leaf over axis, and a reasonable balance between the two. Where, as in the 

 juvenilestateof the Ophioglossaceae, the former is seen in its extreme state, there may appear 

 to be no cauline stele : nevertheless in a type such as the adult Cheiropleuria the cauline 

 factor may preponderate with converse result. Other Ferns may take intermediate positions. 

 Accordingly it would appear that extreme monophyllous examples, such as Ophioglossum 

 (the actually primitive character of which genus may be held in doubt), are not those best 

 suited to serve as a foundation for a general stelar theory. A more illuminating line would 

 appear to be to start from a really ancient fossil type, such as Botryopteris cylitidnca. It 

 may, however, be submitted that in the present state of uncertainty as to the early evolu- 

 tionary relations of the leaf and axis it is impossible to speak with confidence, and in general 

 terms, as to the origin of the stele. This much however seems clear : that at an early stage 

 in the ontogeny of Ferns a coherent body of tissue, "partly made up of elements which 

 have a truly cauline origin, serves to connect up adjacent leaf-traces" (West, A7tn. of Bot. 

 iQi?) P- 369)- It may be added that the greater the preponderance of the leaf over the 

 axis in the young shoot, the later will be the stage when this composite nature of the stele 

 can be recognised. 



Note. To avoid needless repetition the bibliography of stelar tissues will be given collectively 

 for Chapters vii, viii, and X at the close of Chapter x. 



