44 PROCEEDINGS OF THE 



In the first place, the variability of the position marked by the 

 first tangential wall in the different great groups of Pteridophyta 

 was pointed out ; and Van Tieghem's statement that it coincided, 

 in the Filicinese, with the outer limit of the monostele or (in the 

 polystelic forms) of the ring of steles, was held to rest on far too 

 narrow a basis of evidence. This outer limit is said to correspond 

 in most cases with the external limit of the protophloem, the 

 sheath-layers (pericyele and endodermis) of the monostele being 

 supposed to be derived from " cortex." However, the histogenetic 

 origin of these sheath-layers is extremely variable ; they are 

 derived from " cortex," or from a separate layer (coleogen), or 

 sometimes partly from the young stele itself. The most striking 

 deviation at present known in Ferns is found in Schizcea malaccana^ 

 where each primary segment of the apical cell divides into an 

 anticlinal series of three cells, of which the middle one is an initial 

 of the vascular ring and the sheath-layers. If these histogenetic 

 differences were to be seriously taken as a clue to morphological 

 (phylogenetic) distinctions, the most patent absurdities would 

 result, e. g. the outer endodermis would not be homologous in 

 different Ferns, a practically impossible conclusion. The con- 

 sideration of the homologies of tissues must be based primarily 

 on a common-sense comparative consideration of adult structures. 



The question of " pith" in Ferns was then touched upon, and 

 it was concluded that while the pith of Scliizcm is intrastelar, 

 histologically part of the amylom, and developed in place of the 

 central tracheids of a primitive protostele, remains of which are 

 found in some species of the genus, the large-celled pith enclosed 

 by an internal endodermis, which begins to make its appearance 

 in ScJiizcea and is normal in solenostelic Ferns, is a netv tissue, 

 phylogenetically the descendant of the intrastelar pith, or in other 

 cases of the central phloem, though in connection with, and often 

 histologically identical with the cortex. In considering this 

 question, a contemplation of the actual histogenesis of the central 

 tissue prevents our using the phrases " internal cortex " and 

 " intrusion of cortex." 



Mr. L. A. Boodle, F.L.S., followed with a paper entitled 

 " Stelar Structure of ScMzcea and other Ferns," illustrated by 

 lantern-slides. In the rhizome of Schizcea dichotoma the stele has 

 normally a ring of xylem enclosing a centi'al pith, which is usually 

 largely sclerotic. A group of tracheids sometimes occurs in the 

 pith, and is either isolated or connected with the normal ring of 

 xylem. Endodermal pockets are present in connection with some 

 of the leaf-traces, and may pass obliquely inwards to near the 

 centre of the pith. Besides these an isolated inner endodermis 

 is occasionally found in the pith. The central tracheids and the 

 isolated inner endodermis appear to be vestigial ; the former are 

 probably remnants of the central part of the xylem of a proto- 

 stelic form, such as is seen in Lygodium, though it is possible that 

 they may represent a centrally placed protoxylem embedded in 



