80 PTERIDOSPERMEAE [CH. 



middle the seed is circular in transverse section and octagonal 

 near the base and apex. The free apical part of the integument 

 forms a frill (canopy) round the micropyle and extends beyond 

 the nucellar apex which consists of a relatively flat plinth sur- 

 mounted by a central dome or lagenostome (fig. 414). The 

 lagenostome is surrounded by an annular pollen-chamber on to 

 the lower surface of which abuts the large embryo-sac, and 

 remains of archegonia were noticed below the pollen-chamber. 

 The roof of the chamber in the young state consists of a layer of 

 thin-walled cells extending across the flattened apex of the 

 nucellus, n, but as the pollen-chamber becomes differentiated 

 from the nucellar tissue by the disorganisation of the zone of 

 cells its roof-cells thicken their vertical walls and assume the 

 structure of a multiseriate annulus, which acts as a mechanism 

 for opening the pollen-chamber by a circular dehiscence in such 

 a way that the edge of the ruptured roof of the pollen-chamber 

 slightly overlaps the periphery of the central column of nucellar 

 tissue after it has returned to its original position subsequent to 

 the entrance of the microspores. The micropylar region is sur- 

 rounded by eight lobes of the integument and each is characterised 

 by a crest of radially elongated cells, fig. 414, f, especially promi- 

 nent on the outer side. External to this is the slightly longer 

 cupular sheath (fig. 414, c) which may also have been lobed. The 

 surface of the integument below the terminal crests consists of 

 a layer of cells with small papillae which eventually ruptured 

 and discharged mucilage. Both integuments have a vascular 

 supply, that of the inner integument being represented by eight 

 vascular bundles, some of which were found to have mesarch 

 xylem, given off from the single strand in the pedicel. Fig. 414 

 shows the apical region of a seed of Sphaerostoma : the flat-topped 

 nucellar cap, n, is surrounded by the annular pollen-chamber, pc, 

 below which are indicated the archegonia : the wall of the mega- 

 spore (embryo-sac) is seen at m and external to this vascular 

 bundles, v, run up the inner portion of the integument accom- 

 panied by some large cells (aqueous tissue). The elongated 

 epidermal cells at the apices of the lobes of the integument form 

 the frill, /, and at a lower level the cells of the same layer are much 

 smaller and papillate (e) : the outer integument, c, forms the 



