1898] ORIENTATION OF THE PLANT EGG 317 
sect the Hymenophyllacez directly with the bryophytes, as was 
the custom of the older pteridologists. 
There is naturally no difficulty presented by the embryogeny 
of Marsilia, nor by that of the closely related Pilularia or of the 
wot distant Salviniaceze. In all of the Hydropteridez the semi- 
inverted leptosporangiate fern embryogeny persists, except that 
the angle of variation from the original position is here quite 
sactly go°. This perpendicular position of the basal wall may 
‘ein some way an adaptation to heterosporous conditions, and 
may be connected with the positions of the nursing foot and of 
the emergent cotyledon and stem-apex. At any rate, it charac- 
terizes Isoetes, as explained above, and may be said to be the 
mark of all heterosporous pteridophytes of which the embryo is 
fot provided with a suspensor. Two subclasses, therefore, may 
% defined under class B above: 
B. Semi-inverted orientation. 
I. Sequence of segmentation planes normal, ¢. g., Pteris. 
2. Sequence of segmentation planes abnormal, ¢. %: 
Isoetes. 
In the case of a plant so strongly isolated as is Iscetes specu 
8 ition is Scarcely profitable in an attempt to explain the origin 
me Perpendicular basal wall. It may have been derived 
fon either an Equisetum-like or Angiopteris-like, or even 
‘like ancestral form. Certainly it is of all pteridophytic 
| [Pes the most disconnected, as it is in some respects the most 
7 a Whether the original tip-to-one-side began to charac- 
: aa embryo because a resistant prothallial mass lay between 
| se embryo and the outer world, or because homosporous con- 
EB Were gradually changed to heterosporous, and the nutri- 
t ey and the megaspore wall exerted an influence upon 
7 : — of the young hemispheres, in any case the position 
: ition “garded as one of adaptation. Just how the adaptive 
itsted, ‘tose in this plant will possibly never be certainly sug 
The j : 
% verted egg of the Lycopodine is very easily —* 
“Tas the nature of the adaptation is concerned. Plainly 
