RESULTS, PHYLETIC AND MORPHOLOGICAL 713 
character of the sporangiophore once acquired appears to have been more 
constant, and affecting as it does the production and dispersal of the spores, 
It is of much more biological moment than details of leaf-arrangement : 
consequently it deserves a prior place in our comparisons. The designation 
of the Equisetales and Sphenophyllales, including the Psilotaceae as 
sporangiophoric Pteridophytes, is to be preferred to any separation of the 
“Articulatae” on the ground of leaf-arrangement. 
The essential unity of the characters of the Sporangiophoric Pteridophytes 
is becoming more apparent as the knowledge of them widens: this indicates 
that they constitute a brush of phyletic lines sprung probably from a 
common source: the original characters of the common stock appear to 
have been not unlike those of a primitive Lycopodinous type where the 
whole shoot was fertile; but here the spore-producing members proceeded 
early to a more elaborate structure, the sporangiophore replacing the simple 
sporangium, while a capacity for fission of the leaves supervened, and 
often of the sporangiophores also. The stelar structure in many cases so 
closely resembles that of the more primitive Lycopodiales as to lend material 
support to this suggestion. Starting from such a central type as Spheno- 
phyllum majus, in which a “ Sedago” condition is seen, a departure from the 
whorled disposition of the leaves, such as the Lycopods show within the 
genus Lycopodium, would give the type of the modern Psilotaceae: a 
transition to a higher differentiation of the sterile and fertile regions, with 
fission of the sporangiophores and reduction of the number of sporangia 
borne by each would give the more complex state of S. Dawson and 
Roemeri: a similar fission of both bracts and sporangiophores would lead 
towards the type of Checrostrobus. It is not suggested that the species 
named were thus grouped in actual phyletic lines, nor would these accord 
with stratigraphical sequence; the intention is rather to indicate morpho- 
logical relationships of the different known forms to a probable primitive 
type, a primitive type to which Sphenophyllum majus retained a high degree 
of similarity. 
On the other hand, the structure seen in Sphenophyllum emarginatum 
(p. 694, footnote) connects the Sphenophyllaceous-type of strobilus with the 
usual Calamarian type: it has been shown above how the various other types 
of the Equisetales are related to this (pp. 694-6; also chapter XXVIII.). 
The analogy of the Lycopodiales, together with the facts seen in the 
sporangiophoric Pteridophytes themselves, points to their origin also from 
a strobiloid type with a general-purposes shoot, in which the axis was 
* dominant and protostelic, the leaves were whorled, and in which the spore- 
producing members early attained to the sporangiophoric structure. Zhe 
phyletic relationship of the Sphenophyllales and Equisetales has undoubtedly 
been a very close one; the distinguishing features are not to be found in the 
primary plan or construction of the shoot, so much as in the secondary modtfi- 
cations of number and relation of the appendages, and of their branching, 
together with changes in the originally protostelic structure of the axts. Such 
