7i2 CONCLUSION 



found at different heights on the same plant : or the plane of the whorls 

 may be set obliquely to the axis. It would appear probable from such 

 facts that the original type had whorled leaves, and that the spiral 

 arrangement was acquired by secondary disturbance of it, a point of some 

 considerable interest for comparison with the sporangiophoric Pteridophytes. 

 The other character is seen in Isoetes (p. 318), and in Lepidostrobus Brownii, 

 both of which had very large sporangia (p. 322). In these a partial 

 sterilisation of sporogenous tissue producing trabeculae meets a mechanical 

 and nutritive requirement following on their large size, and the structure 

 thus approaches a state of septation : such septation is indeed technically 

 completed in the megasporangia of Isoetes, but no Lycopod shows a septate 

 state of the sporangium as a permanent character. The interest in this is 

 in comparison of these sporangia with the similarly placed synangia of the 

 Psilotaceae and Sphenophyllaceae. 



These two series, together with the Equisetales, have been included 

 under the general designation of the Sporangiophoric Pteridophytes (p. 423). 

 Though differing in detail, the main plan of their sporophyte is similar to 

 that in the Lycopodiales, as regards axis and leaves, branching, and 

 anatomical structure ; but the sporangia of the latter are replaced by 

 sporangiophores, while the relations of these to the bracts is not 

 uniformly so regular as that of the sporangia in the Lycopodiales. More- 

 over, both bracts and sporangiophores show evidences of fission, sometimes 

 independently, sometimes together. These relations have been considered 

 above (p. 694-5), together with the similar variations of exact position of 

 the sporangia and sporangiophores relatively to the axis : such facts, com- 

 bined with the arguments already advanced in Chapter XXVIII. , lead to 

 the conclusion that the functionally identical parts designated sporangiophores 

 and sporangia are cognate parts ; it appears probable that the sporangiophore 

 is itself a consequence of elaboration of a simpler type of spore-producing 

 member, of which the sporangium of Lycopodium is an example, while the 

 trabeculae in Isoetes and Lepidostrobus Brownii suggest a mode of origin of 

 the septate state. If this were so, then the sporangiophore would have been 

 distinct in its phyletic origin from the bract-leaves, which habitually subtend 

 the spore-producing members, whether they be sporangia or sporangiophores. 



The Sporangiophoric Pteridophytes (which include the " Articulatae " of 

 Lignier together with the Psilotaceae) are primarily characterised by the 

 presence of the sporangiophore. The fact that the leaf-arrangement is often 

 whorled, which is a leading feature of the Articulatae, while that in the 

 Psilotaceae is alternate, is here regarded as a point of secondary moment. 

 The reasons for this are, first, that the leaf-arrangement varies from the 

 whorled to the alternate in the very natural phylum of the Lycopods, and 

 secondly, that a similar change appears from the ancient Sphenophylleae 

 to the modern Psilotaceae groups clearly related to one another. It 

 seems probable that the whorled arrangement was initially general for the 

 strobiloid types, but that the regularity has been secondarily abandoned. The 



