EMBRYOGENY OF THE PTERIDOPHYTES 665 



a difference of initiation of the embryogeny as will serve for a safe 

 taxonomic guide. Where a suspensor is formed, the first segment-wall 

 (i, i) divides the zygote, separating the parent-cell of the suspensor from 

 what has been styled the embryonic cell (Fig. 355 i.). As the position of 

 the first segment-wall in all Pteridophytes where a suspensor occurs is 

 approximately at right angles to the axis of the archegonium, the mother- 

 cell of the suspensor is directed towards the archegonial neck, and the 

 practical effect of biological moment is that the embryonic cell is thrust 

 downwards into the tissue of the nourishing prothallus. While the 

 suspensor is thus recognised as biologically important, it may, on the 

 other hand, be regarded as a means of deferring the actual constitution 



FIG. 355. 



Diagrams illustrating the segmentation of embryos. I. = where a suspensor is formed, 

 which is cut off by the first wall, /, /; the suspensor is cross-hatched ; , -5 = basal wall, 

 separating the hypobasal hemisphere (dotted) from the epibasal (clear). II. is the same 

 seen from above, x marking the pole. III. =an embryo where no suspensor is formed, 

 and the segmentation resembles that in the embryonic cell where the suspensor is present ; 

 the lettering corresponds ; x, y indicate the polarity. Each hemisphere divides into 

 four by quadrant walls {Q, Q in II.) and octant walls o, o. 



of the definitive embryo, which is entirely derived from the remaining 

 portion of the zygote. The formation of a suspensor is in fact a form of 

 meroblastic segmentation, comparable generally, though not in detail, with 

 that seen in many Gymnosperms. But a further analogy is to be found, 

 as already pointed out, in the sporogonia of the Jungermanniaceae (Fig. 

 125): here, however, it is the segment furthest from the neck of the 

 archegonium which takes no part in the constitution of the definitive 

 sporogonium. 1 In either case a part of the product of the zygote, 

 which has some more or less obvious biological use, may in certain 

 forms be set aside from partaking directly in the formation of the definitive 

 embryo. 



Passing now to the embryonic-cell in the Pteridophytes which have 

 a suspensor, it has been shown in several well-investigated cases that it 



1 It is interesting to note that this body is absent from the Marchantiaceae ; and the 

 inconstancy in the Liverworts may be compared with that of the suspensor in the 

 Pteridophyta. 



