310 LAGENOSTOMALES [CH. 



rib and tentacle. The single integument consists of a few 

 layers of cells, those of the epidermis being prolonged into clavate 

 mucilaginous hairs, fig. 494, I, h, that may reach a length of 

 5 mm. and in the living seed almost covered the whole surface 

 of the testa, being especially long on the ribs and tentacles. 

 There is no special development of sclerous tissue, the vascular 

 bundles, v, being embedded in parenchyma in the inner portion 

 of the integument. The nucellus is represented by a zone rich 

 in secretory cells, and internal to this is a tapetum. Integument 

 and nucellus are coalescent up to the apical region where the 

 former splits into 10 tentacles. The nucellar apex has the form 

 of a tall dome surrounded by a bell-shaped pollen-chamber (fig. 

 494, I, pc; fig. 493, C, D, c) into which it projects like the base 

 of a wine-bottle. The circular opening of the pollen-chamber 

 overtops the roof of the dome formed of the secretory tissue of 

 the nucellus and the carbonised remains of the tapetum: this 

 dark band surrounds the large megaspore-cavity (fig. 494, I). 

 Physostoma is the only member of the Lagenostomales in which 

 the megaspore projects into the free nucellar apex: in other 

 genera intercalary growth has produced a more or less prominent 

 plinth, the name given to the free portion of the nucellus between 

 the megaspore and the pollen-chamber. Williamson 1 described 

 the mammillated apex of the nucellus as pushed up into the 

 base of the lagenostome which 'looks like a bladder half full of 

 fluid resting upon and overhanging the end of a soda-water bottle' : 

 it was this appearance that suggested the name Physostoma. 

 The section reproduced in fig. 493, D, shows in the centre the 

 limiting tissue of the nucellus surrounded by the pollen-chamber, 

 c, and external to this are the tentacles with their groups of long 

 hairs: the vascular bundles are represented by spaces in the 

 more internal small-celled tissue (see also fig. 494, 1). A character- 

 istic feature is the presence of a tapetum or megaspore- jacket 2 

 in the nucellus: immediately internal to the vascular bundles 

 stretching from the chalaza to the apex of the megaspore-cavity is 

 a layer of delicate cells with secretory sacs, and this is succeeded 

 by a broad black layer of rather larger cells, a tissue which was 

 probably in full activity in a younger stage of development. 



1 Williamson (76) p. 160. 2 Oliver (09) p. 106. 



