EMBRYOGENY OF THE PTERIDOPHYTES 667 
Fig. 3561.); it has been shown that the latter type exists initially in all 
cases where a suspensor is present, e.g. Lycopods (Figs. 183, 186, 187), 
and presumably in Botrychium obliquum (Fig. 264). In Jsoeces the 
orientation may vary between wide limits even in the same species.! But 
a still more interesting case is that of the genus Sotrychium: in 
B. Lunaria and virginianum the orientation of the primary axis is 
towards the archegonial neck (Figs. 261, 262, 263). In B&B. obliguum, 
however (Figs. 264, 266), where a suspensor is present, it is at first 
turned away from the archegonial neck, as in other embryos with a 
suspensor. Thus within the old genus Bosrychium there are two types 
of opposite orientation. An inversion of polarity must have occurred in 
descent. Probably in more than one case such an inversion of polarity 
Fic. 356. 
Diagrams to show the relation of the basal wall, B, B, and hypobasal (dotted) and 
epibasal (clear) hemispheres to the archegonial neck, which is indicated by an arrow ; x,y 
shows polarity, + being the apex; S=stem; ZL=leaf; R=root; A=foot. I. shows 
the orientation seen in Marattiaceous Ferns. II. that for Leptosporangiate Ferns. 
III. that for Aguisetui and Uphioglossaceae. 
has taken place, not by any rotation of the embryo, but by change in 
the way in which the zygote has itself initiated its organisation. It is 
necessary in this connection to realise that the zygote is at first without 
any determinate polarity: that this may be initiated in various relation 
to the axis of the archegonium, in different types of plant or even in 
different individuals; and that its position is controlled, not by external, 
but by internal causes at present unknown.? But whatever those causes 
may be, and whatever the orientation, a comparative study of embryos 
shows that when the direction of polarity is once indicated, as it is 
by the first segment-wall, the apex of the axis of the first shoot 
is initiated in a definite position relatively to it: occupying, in fact, the 
epibasal pole. ; 
1Campbell, Mosses and Ferns, pp. 545-547; compare Fig. 191 B above, p. 359. 
2 Goebel, Organography, i. p. 219, and ii. p. 246. 
