April 1892.] THE BOTANICAL SOCIETY OF EDINBURGH. 363 



plasm from the vacuole and thus form the basal part of the 

 sac. This membrane is not always laid down, but may, on 

 careful examination, be found in at least 50 p.c. of ovules, 

 and it is seen in figs. 18, 19, 21, 22, 23. Fig. 18 shows the 

 membrane m placed obliquely [the anterior margin a has 

 been represented as a darker line than the posterior one p\, 

 evidently separating the apical portion of the embryo-sac 

 from the basal one, and bounded on eitlier side by a vacuole. 

 The apical vacuole is brought about thus : — As the ovule elon- 

 gates, the embryo-sac keeps step with the elongation, and the 

 apical protoplasm gradually recedes from the membrane, which 

 latter, by retaining its position, indicates the basal limit of 

 the apical protoplasm soon after the division of the sporocyte- 

 nucleus. In fig. 19 the membrane exhibits a distinct double 

 outline, and fig. 21, although a later stage in the development 

 of the embryo-sac, shows a portion of the apical protoplasm 

 still adhering to this membrane. Once I found two distinct 

 membranes crossing the embryo-sac (fig. 22), and I believe the 

 lower membrane m^ to have been formed first, the mem- 

 brane m^ afterwards. 



The question arises : — What is the significance of this 

 membrane ? If the embryo-sac is nourished by food coming 

 from the plerome-elements on which the basal half of the 

 sac abuts, one would not expect that the apical half would 

 shut itself off from the supply of nourishment by a double 

 barrier, namely, the vacuole and this membrane, provided it 

 was not able to procure its food-supply from some other 

 source. Such another source seems to exist, for the con- 

 stantly increasing size of the embryo-sac results in the 

 compression and the degeneration of the periblem- cells 

 surrounding it, and I believe that by the death of these cells 

 albuminoid materials are set free which serve as food- 

 material for the apical cell. This suggestion would also 

 explain why in many ovules no membranous partition is 

 formed : — If food-material is not procured in suflicient 

 quantity from the degenerating periblem-cells, then the 

 apical nucleus and its protoplasm must rely for their supply 

 partly on the nutriment brought to the basal end by the 

 plerome-elements, and hence a protoplasmic communication 

 between the two halves of the embryo-sac becomes neces- 

 sary and no membrane will be formed. 



