CHAPTER VIII 



THE VASCULAR SYSTEM OF THE AXIS {continued) 



(B) THE STELAR STRUCTURE OF THE LEPTOSPORANGIATE FERNS 



The Ferns quoted in the preceding Chapter, excepting only Cheirop/euria, 

 are either Eusporangiate, or near derivatives of that type. It will be shown 

 in Volume II that they are all relatively primitive Ferns, and that they 

 illustrate a number of distinct phyletic lines, most of which do not appear 

 themselves to have given rise .to any Leptosporangiate derivatives. This is 

 probably the case for the Ophioglossaceae, Marattiaceae, Osmundaceae, and 

 Hymenophyllaceae. But there is reason to believe that the Schizaeaceae do 

 represent with some degree of accuracy a source from which a considerable 

 section of the Leptosporangiate Ferns sprang. In these the sori are typically 

 marginal (Marginales), while others in which the sori are typically superficial 

 (Superficiales) may probably be traceable to some Gleicheniaceous origin. 

 The argument on these points will be developed later : but meanwhile the 

 statement of this general position in advance of the argument will have 

 cleared the ground for their anatomical study. Both the Gleicheniaceae and 

 the Schizaeaceae include permanently protostelic types, while in all Lepto- 

 sporangiate Ferns the axis of the sporeling is in the first instance protostelic. 

 Starting from this primitive stelar state it will appear from comparison that 

 transitions have occurred to variously elaborated, and finally to disintegrated 

 stelar structure. The types of vascular construction thus achieved in the 

 Leptosporangiate Ferns are known as Solenosfely, Dictyosiely, Polycycly, and 

 Perforation. Each of these will be described and illustrated. But underlying 

 them all are certain features that are constant in the Leptosporangiate Ferns. 

 However complicated the form of their vascular masses may be, they are 

 always strictly delimited from the snrrounding tissues by a contimwus sheet 

 of endodermis. This forms an unbroken barrier exercising a physiological 

 control over transit into or out of the conducting tracts. Another constant 

 feature is the absence of any ventilating system of intercellular spaces within 

 the conducting tracts. These structural points indicate a highly specialised 

 condition of the conducting system in the Leptosporangiate Ferns. 



SOLENOSTELY 

 The term solenostcle, or siphonostele as it is sometimes called, is applied 

 to that state in which the stele takes the form of a cylindrical tube, so that 

 in transverse section it appears as a ring of vascular tissue surrounding a 



