CH. XXXV] COLOSTOMA 315 



The nucellus is coalescent with the integument, as in Physostoma, 

 as far as the level of the domical free apex of the nucellus where 

 the tapetal tissue that Unes the seed-caAdty passes across the 

 almost flat top of the central region originally occupied by the 

 megaspore. In some sections prothallus-tissue was found with an 

 apical 'tent-pole' protuberance. A striking feature of Conostoma 

 is the mechanism for the reception of the microspores. The free 

 part of the nucellus consists of the phnth, a broad tapering 

 region originally filled with parenchyma but in most cases repre- 

 sented only by its epidermis : the pUnth, p, is seen in fig. 494, B, 

 to be two-storeyed, the upper and narrower storey being a space 

 formerly filled by a pad of tissue suspended from the floor of 

 the superposed lagenostome (pollen-chamber)^. The greater de- 

 velopment of the domical plinth is a feature in which Conostoma 

 differs from Physostoma. At the apex of the pUnth and resting 

 on a sUght depression is a small lagenostome, bowl-shaped in 

 section, and like the pollen-chamber of Lagenostoma, formed as 

 the result of enzyme-action on the apical papilla of the nucellus 

 (fig. 494, B, B', I'). The mouth of the lagenostome engages with 

 the micropylar tube by a projecting flange (fig. 494, B', /) of 

 tissue fining the micropylar canal and by a second flange (/') at 

 the base of the lagenostome where the roof of the plinth (fig. 494, 

 B', p) bends downwards and inwards. The walls of the lageno- 

 stome are formed by strong cells with thickening bands giving 

 them the appearance of tracheids (V), but the floor of the lageno- 

 stome is made of thinner cells which become disorganised, al- 

 lowing the microspores to fall into the large plinth-cavity below 

 {p, fig. 494, B), an arrangement comparable with the two-storeyed 

 pollen-chamber of Bowenia^ and, to a less extent, with the micro- 

 pyle of the Conifer Tsuga. The microspores are multicellular 

 and elhpsoidal measuring 75/i x 65jli. 



The species Conostoma anglo-germanicum agrees closely with 

 C. oblongum in general form and organisation, but it has eight 

 ribs, four more prominent than the others, and differs also in other 

 minor characters from the rather shorter seeds of the type-species. 

 Conostoma differs from Lagenostoma in the absence of the tubular 



1 For a detaUed account of the plinth see Oliver and Salisbury (11). 



2 Page 27. 



