428 SPORANGIOPHORIC PTERIDOPHYTES 



On the grounds thus stated it appears probable that in the Sporangio- 

 phoric Pteridophytes the sporangiophore is a non-foliar structure, arising 

 either on axis or on leaf: that it is of placental origin, and bears outwards 

 as it grows the sporangia, which may be regarded as the results of 

 disintegration of a single primitive loculus : that the synangial condition 

 of these was the prior state, but that in most cases the sporangia have 

 shown independent growth, and now project as separate sacs, often 

 becoming inverted during development, a change which brings advantages 

 of protection while young. 1 



The general features of the shoot common to the Equisetales and 

 Sphenophyllales may be summarised as follows : 



1. The axis is the predominant part of the shoot: it is traversed by a 

 monostele, which frequently shows the protostelic state, with solid xylem- 

 core. All known types of vascular system in these phyla are referable in 

 origin to that primitive structure. 



2. The lower part of the plant is vegetative : there is a more or less 

 definite and compact strobilus above, usually terminal : but in some there 

 is an indefinite " Selago " condition, characterised by being more lax, and 

 often also more effective for assimilation. 



3. The leaves are simple or branched, in whorls (Equisetales and 

 Sphenophylleae), or alternate (Psilotaceae). 



4. The sporangia in variable number are disposed radially on the 

 sporangiophores, which may be inserted either on the axis (Equisetales), 

 or on sporophylls (Sphenophyllales), but still have an essentially uniform 

 character in both types. They are held to be organs sui generis, of the 

 nature of placental growths. 



5. The sporangia are eusporangiate, and dehisce by slits radially 

 disposed and structurally defined. 



6. In all cases which have been examined developmentally a consider- 

 able portion of the sporogenous cells distributed through the mass are 

 disorganised without undergoing tetrad-division. 



The plants thus characterised lend themselves readily to interpretation 

 on a hypothesis of a strobiloid origin. The predominance of the axis in 

 the embryology of Equisetum (the only sporangiophoric type in which it 

 has been followed), as well as in the mature shoot of them all, is very 

 striking, while the sporangiophores and leaves figure only as minor 

 appendages : the primitive monostelic structure of the axis, with more or 

 less definite cauline xylem-core, and insertion of the leaf-traces upon its 

 periphery, also supports a strobiloid hypothesis from the point of view of 

 internal structure. The existence of the undifferentiated " Selago " con- 

 dition brings these plants into line with the Lycopods : the facts showing 

 the relation of the sterile to the fertile regions would here be insufficient 



1 With the above paragraphs, compare Scott, Studies, pp. 496-500 ; also Progressits, 

 vol. i., p. 163, etc. 



