144 



THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



system. The result of this will be physiological delimitation of the stelar 

 pith below it from the pith above, while histological continuity is established 

 between the cortex and the pith above that barrier. jfj 



There is no sign of intrusive flow of the outer tissue 

 inwards, nor of displacement of cells as an initial step 

 from the medullated protostele to the solenostelic state. 



This description is written as a simple objective state- 

 ment of what is seen in successive transverse sections of a 

 young plant of Gleichenia pectinata, without any precon- 

 ceived theoretical interpretation. The species is one with 

 prolonged internodes in the adult: it is thus a peculiarly 

 favourable example for testing the validity of such 

 theoretical interpretations as have been suggested in ex- 

 plaining the origin of solenostely. The structural changes 

 are correspondingly extended, a fact which makes their 

 succession clearer than is usual in Ferns. They support a 

 view of change of procambial destination of cells /« situ 

 in the formation of pith, phloem, and internal endodermis. 

 They do not give ground for a belief in a flow or intrusion 

 of cortex into the stele. Consequently the pith, whether 

 in the medullated region below the cross-barrier of in- 

 ternal endodermis or above it, is to be regarded still as 

 the product of procambial tissue, however nearly the 

 character which it may assume when mature may re- 

 semble that of the external cortex. This conclusion 

 harmonises readily with the facts of the opportunist 

 appearance of internal endodermis in the primitive Ferns 

 described in the previous Chapter, and especially with 

 that within the stele of the root of HelmintJwstachys. 

 Such facts lead to the general position that endodermis 

 is a tissue that can be formed when and where it is re- 

 quired, and its presence in imperfect sheets or groups of 

 cells does not demand explanation in terms of reduction, 

 i.e. of origin from some pre-existent and complete endo- 

 dermal layers imperfectly developed in the individual 

 under examination. 



The typically constructed solenostele of Gleichenia 

 pectinata finds its correlative in many others of the 

 Superficiales. It must suffice here to mention Metaxya, 

 Lophosoria, Matonia, and Dipteris, in all of which the 

 internodes are long, a feature which conduces to the typical presentment of 

 the solenostele. 



Fig. 134. Plan of stelar 

 construction of a 

 juvenile plant of 

 Gleichenia pectinata 

 as seen in median 

 section. For detailed 

 description see text. 

 (After Dr M'^Lean 

 Thompson.) 



