10 PROF. MARGARET J. BENSON ON 



Directing our attention to the dome, we are very fortunately able to trace to some 

 extent the development of the pollen chamber. 



6. The Structure of the Pollen Chamber. 



In the older ovules there is a clear broad space seen in radial section on either 

 side of the central column and roofed over by a single layer of epidermal cells. 

 In longitudinal sections it will be observed that the column consists of regularly 

 arranged tiers of cells. In this latter feature Sphserostoma resembles Lag eno stoma . 



Tn transverse section the form of dome and column is found to be circular. 



In the nearly radial section of a young ovule, shown in PI. I. figs. 1 and 2, the 

 tissue in the annular region, to be later replaced by the pollen chamber, is seen to be 

 undergoing lysigenic degeneration. A difference between the younger and older domes 

 is seen also in the character of their epidermis. In the younger, the epidermis is 

 composed of thin-walled cells with flat peripheral walls. In the older, the epidermis 

 over the pollen chamber has entirely changed its character, while that over the column 

 has remained unchanged. Each cell over the pollen chamber has its walls sharply 

 differentiated, i.e. the outer and basal walls remain thin, and all the four walls vertical 

 to the surface are thickened in such a way that they appear of an opaque dark brown 

 colour, while the outer and basal walls are quite transparent. Thus the whole roof or 

 wall of the pollen chamber has much the appearance of a multiseriate annulus of such 

 a sporange as that of Senftenbergia. The resemblance is heightened by the obliquity 

 of the end walls of the cells (PI. II. figs. 7 and 10). 



The circumscissile dehiscene, by means of which presumably the pollen grains entered 

 the chamber, is effected in a very definite manner. The central column becomes severed 

 entirely from the roof of the pollen chamber along a line just within the margin of the 

 epidermis of the column. 



Thus marginal epidermal cells of the column remain attached to the roof-cells 

 (vide PI. 1. fig. 6). There is no loss of surface cells, the dehiscence is schizogenic, and in 

 some respects the whole apparatus is curiously reminiscent of the peristome in the 

 sporogonium of Polytrichum, which opens around a persistent, circular diaphragm. 

 The mechanism, however, of the closure in the two cases must have been very 

 different, and the resemblance between them merely adventitious. 



There are tetrahedral spores found in the pollen chambers of seeds in slides 217, 

 241, 270*1, and 304 - 4. Occasionally they occur still arranged as tetrads. These spores, 

 which are presumably the pollen grains, are from 27-29m in their widest dimension, 

 and thus much smaller than those * attributed to Lyginodendron, which are from 50- 

 70/j- in diameter. 



Traces of a species of microsynangium do, however, occur with the Sphwrostoma, 

 and its spores resemble in size and form those in the pollen chamber. Description of 



* Kiuston, "On the Microsporangia of the Pteridospennece," Trans. Boy. Soc, 1906, vol. cxcviii. p. 423. 



