XXIX] . LAGENOSTOMA 59 



by regular and deep furrows on the inner face. Enclosed by the 

 canopy, with its outer surface fluted as the result of the partial 

 collapse of the outer wall of each compartment due to the decay 

 of the filling tissue, is the flask-shaped apex of the nucellus; 

 between the apical cone of nucellar parenchyma and the super- 

 ficial layer is an annular cavity which Williamson^ called the 

 lagenostome. The parenchymatous core tapers to a narrow 

 summit which slightly overtops the integument and is constricted 

 at the broad base (fig. 493, A, B; page 311). The bottle-shaped 

 apical tissue is separated by an annular space, c, fig. 493, B, 

 from the limiting layer of the nucellus : this space is the pollen- 

 chamber formed in the living seed by the disorganisation of the 

 subepidermal cells of the nucellar apex. The pollen-chamber is 

 a feature characteristic of recent cycadean ovules (see p. 6). 

 In Lagenostoma the annular form of the pollen-chamber is a 

 peculiarity distinguishing this type of seed from those of recent 

 Gymnosperms and most other Palaeozoic seeds. As Oliver says, 

 it marks an 'advance in precision'^ over other forms as the micro- 

 spores which fall into the chamber are brought direct to the surface 

 of the underlying megaspore and presumably to the archegonia 

 which, it is reasonable to believe, were disposed in a circle at the 

 base of the annular crevice. Microspores frequently occur in 

 the pollen-chamber and some have been discovered apparently 

 in the act of hberating male gametes^. 



The outer wall of the nucellus is bounded externally by a 

 similar circular space {d, figs. 408, C ; 493, B) which separates it 

 from the domical canopy. In the great majority of specimens 

 the central tissue of the seed is not preserved and an empty sac 

 supported from the base of the nucellus-apex occupies nearly 

 the whole of the interior : the shrunken wall of the sac is all that 

 remains of the large megaspore. It would seem, then, that the 

 nucellus was almost completely destroyed as a consequence of 

 the growth of the megaspore or embryo-sac, which eventually 

 occupied nearly the whole of the seed-body. 



In an exceptionally well preserved specimen recently described 

 and admirably illustrated by Mr McLean* part of the parenchy- 

 matous tissue of the prothallus which originally filled the megaspore 

 1 WilKamson (76). ^ Oliver (03) p. 462. = Benson (08). ■• McLean (12). 



