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.? 
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 “ Sedago” 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 suz 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 Lguisetwm (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 ‘* Se/ago” 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 Progressus, 
vol. i., p. 163, ete. 
