EMBRYOGENY OF THE PTERIDOPHYTES 667 



Fig. 356 I.) ; 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 Botrychinm obliquum (Fig. 264). In Isoetes the 

 orientation may vary between wide limits even in the same species. 1 But 

 a still more interesting case is that of the genus Botrvchinm : in 

 B. Lunaria and Virginia nn m the orientation of the primary axis is 

 towards the archegonial neck (Figs. 261, 262, 263). In B. obliijitiim, 

 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 Botrychinm 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 



I. 



X 



FIG. 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 ; i , r 

 shows polarity, .r being the apex; .S' = stem; = leaf; /t > = root; /'' = foot. I. show-. 

 the orientation seen in Marattiaceous Ferns. II. that for Leptosporangiate I'Yi:i 

 III. that for Eqwisetum and Ophioglossaceae. 



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. 



1 Campbell, Mosses and Ferns, pp. 545-547! compare Fit;. 191 u aUnv, p. 359. 

 2 Goebel, Organography, i. p. 219, and ii. p. 246. 



