312 LAGENOSTOMALES [CH. 



A comparable tapetal layer is described by Lang 1 in the ovule of 

 Stangeria: the majority of the sporogenous cells surrounding the 

 megaspore become disentegrated and are absorbed, but the 

 outermost zone forms a more definite tapetal layer: as already 

 suggested, this tissue in Physostoma may be a group-character. 

 No archegonia have been found, but in a few cases some of the 

 delicate prothallus-tissue occurs in the interior of the seed. Micro- 

 spores are often abundant in the pollen-chamber (fig. 493, C, c) ; 

 in one seed 80 are recorded. The occurrence of so many micro- 

 spores suggested to Oliver that insect-agency may have been 

 responsible for the precision in pollination that is greater than 

 one would expect in anemophilous plants. The spores are smaller 

 than those of Lagenostoma (55/A x 45/*) and in several of them 

 the remains of a cellular tissue are preserved (fig. 494, N), also 

 some sub-reniform bodies (fig. 494, M) similar to those described 

 as spermatozoids by Dr Benson in Lagenostoma (fig. 408, D). 



The most interesting features of Physostoma are : the absence 

 of a continuous micropylar tube and its replacement by a circle 

 of integumental lobes; the apical prolongation of the nucellar 

 apex into the pollen-chamber, and the presence of long mucila- 

 ginous hairs on the integument. The large pollen-chamber is 

 a character which distinguishes Physostoma from Conostoma and 

 its form is very different from that in Lagenostoma. 



The tentacles of the integument and the form of the nucellar 

 apex are features consistent with Oliver's view that Physostoma 

 is the most primitive of Palaeozoic seeds though, as Burlingame 2 

 says, the elaborate form of the encasing envelope marks a con- 

 siderable advance beyond the earliest type of megasporangium 

 integument. 



A new type of Physostoma has been briefly described by 

 Gordon 3 , without a specific designation, from the Lower Carboni- 

 ferous beds of Pettycur (Fife) : it was found in association with 

 Heterangium and Sphaerostoma ovale. 



We have no knowledge of the plant to which Physostoma 

 belonged, but the general plan of organisation of the seed points 

 to a near relationship to Lagenostoma and presumably, as regards 

 the parent-plant, to a genus related to Lyginopteris. 



1 Lang (00) p. 288. 2 Burlingame (15) p. 19. 8 Gordon, W. T. (10). 



