278 LYCOPODIALES [CH. 



a feature suggesting comparison with seed-structure. Some 

 megaspores have been found filled with a prothallus. The 

 longitudinal section shown in fig. 219, A, illustrates the charac- 

 teristic horizontal position of the megasporophyll, as also the 

 relation of the ligule, I, to the sporophyll with its single vascular 

 bundle, and to the hairy integument, which overarches both 

 sporangium and ligule; the line m shows the position of the 

 megaspore-membrane, detached from the sporangial wall on the 

 upper side but in contact with it below. The microsporophyll 

 shown in 219, E, was originally referred to Miadesmia but has 

 since been recognised by Watson ^ as that of a Bothrostrobus. 



Miadesmia affords an example of a Palaeozoic plant com- 

 parable with Isoetes and Selaginella; it agrees also with 

 Lepidocarpon in possessing true seeds, and with Watson's 

 Bothrodendron cone in the shape of the sporangia, which are 

 more like those of Selaginella than the radially elongated 

 sporangia of Lepidostrobus. Miadesmia agrees with Selaginella, 

 e.g. S. spinosa, in its stelar structure,, in the form of the 

 sporangia, and in the presence of a ligule. It is distinguished 

 by having only one instead of four megaspores in a sporangium, 

 in the possession of an integument which formed a close invest- 

 ment to the spore and served as a stigma (comparable with 

 the stigma-like integument of the male flower of Welwitschia), 

 and in the shedding of the megasporophylls, which have been 

 aptly compared with winged seeds. 



On the ground of their general anatomical features Lepido- 

 carpon and Miadesmia are clearly entitled to be included among 

 extinct representatives of the Pteridophyta. These plants had, 

 however, crossed what it has been customary to regard as the 

 boundary between Pteridophytes and Phanerogams: they 

 possessed megasporangia with the attributes of seeds. It has 

 been suggested by Lester Ward'' that Pteridophytic seed-bearing 

 plants shall be recognised as a distinct phylum for which he 

 proposes the name Pteridospermaphyta, a designation implying 

 exclusion from the Spermatophyta as usually understood. For 

 seed-bearing Lycopodiaceous genera he suggests the name 

 Lepidospermae. As knowledge of the Palaeozoic seed-plants 



1 Watson (08-) p. 12. 2 Ward (04). 



