CHAPTER XXVII. 



SPORANGIOPHORIC PTERIDOPHYTES. 



I. EQUISETALES. 



UNDER the common designation of the " Sporangiophoric Pteridophytes " 

 may be grouped together those forms whose sporangia are disposed, either 

 singly or in larger numbers, upon more or less elongated vascular stalks, 

 which are enlarged as a rule at their distal ends. The existence of the 

 sporangiophore clearly distinguishes these plants from the Lycopodiales, 

 though it may for the present remain an open question whether any 

 genetic connection existed between the latter and the sporangiophoric types. 

 Under this designation are included the Equisetales and the Sphenophyllales 

 (incl. Psilotaceae), while, according to the view which will be developed 

 below, the Ophioglossales will also appear as an outlying group sharing 

 the same character, though in a more elaborated form. It will be a 

 matter for later discussion how far the existence of the sporangiophore 

 as the immediate sporangium-bearing member will supply a valid basis 

 on which to trace affinity : the decision must rest on the degree of 

 correspondence of the sporangiophoric types in other characters, such as 

 the external morphology and anatomy of the vegetative organs, and the 

 details of the gametophyte. Unfortunately these are often so imperfectly 

 known that we are thrown back in great measure upon the spore-producing 

 members : but on grounds previously explained these are held to be the 

 most important of all. 



The Equisetales, which are taken first of the sporangiophoric types, 

 are distinguished from the rest by the fact that their sporangiophores are 

 inserted directly upon the axis, not on appendicular parts : in some 

 cases they show a definite relation to the bracts which subtend them : in 

 others no such relation exists. Other less distinctive characters of the 

 vegetative organs are the constantly radial construction of the shoot : 

 the elongation of the internodes which are longitudinally striated, the 

 verticillate arrangement of the leaves, a high degree of branching, and a 

 structure of the stele with a ring of isolated vascular strands ; these 



