206 HO*\\lEiSTEK, ON 



embryo which attach themselves to the prothallium divide 

 by the repeated formation of transverse septa into groups 

 of almost tabular cells. By this means the way is pre- 

 pared for the subsequent not inconsiderable longitudinal 

 extension of the primary axis of the embryo which is the 

 result of cell- expansion. 



The action of concentrated sulphuric acid soon loosens 

 the connexion between the prothallium and the embryo. 

 If the latter is detached the outer surface of its primary 

 axis appears to be surrounded by a gelatinous envelope 

 with radial markings : this is the loosened adhesive mat- 

 ter by which the embryo and the prothallium were united. 

 The outlines of the cells of the latter are most clearly 

 marked upon it by a net-work of narrow band-like protu- 

 berances. 



The growth of the embryo is accompanied by an active 

 multiplication of the cells of the prothallium adjoining the 

 impregnated archegonium. This multiplication, which is 

 not confined to the cells immediately adjoining the central 

 cell of the archegonium, gives rise to the formation of a con- 

 siderable cellular protuberance, attached to the under side 

 of the prothallium, and which encloses the embryo. The 

 circumference of this excrescence is usually very consider- 

 able in Pteris aquilina. The increase in growth of this 

 cellular tissue usually keeps pace so completely with that 

 of the embryo, that the expanding cavity is always exactly 

 tilled up. The multiplication of the neighbouring cells of 

 the prothallium is not however caused by the pressure of 

 the growing embryo upon the side walls of the central cell 

 of the archegonium : this is manifest from the fact of the 

 occurrence of exceptional cases of imperfect growth of the 

 embryo, as has been observed, not only in many vascular 

 cryptogams, but even in mosses.* The embryo, probably 

 in consequence of imperfect impregnation, only occupies a 

 small portion of the enlarged cavity of the central cell of the 

 archegonium, as has been observed by comparing two im- 

 pregnated archegonia of the same prothallium in Pteris 

 aquilina and in Aspidium Jtliw-mas (PL XXVIII, fig. 2) ; 



* By Gol tsclie iu Calypogeia Trichomaiies, ' N. A. A. L. C.,' and by myself 

 in Frullania clilatata and Targionia hypophylla. ' Vergl. Unters/ p. 41. 



