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 " Selago " 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. Dawsoni and 

 Roemeri: a similar fission of both bracts and sporangiophores would lead 

 towards the type of Cheirostrobus. 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. The 

 phyletic relationship of the Sphenophyllales and Equisetales has undoubtedly 

 been a very close o?ie ; the distinguishing features are not to be found in the 

 primary plan or construction of the shoot, so much as in the secondary modifi- 

 cations of number and relation of the appendages, and of their branching, 

 together with changes in the originally protostelic structure of the axis. Such 



