182 EMBRYOLOGY 
walls of the epibasal hemisphere of the embryo. This position of the 
apex of the axis remains the same though the appendages may vary greatly 
in their number, their position, and their relative time of development. 
It is also important to observe that the cotyledons show a constant 
orientation to this point, as to a relative axis, whether or not the apex 
of the axis is early developed as an obvious cone. These relations appear 
to be the most constant among the other fluctuating features of the 
various types of Pteridophyte embryos: the theory of the strobilus con- 
templates the phyletic pre-existence of the axis: the early and constant 
definition of the polarity thus seen in the primary embryology is a material 
fact in relation to that theory. 
Goebel further enforces the point that differentiation of the primarily 
similar cells of the embryo takes place gradually, and that the actual 
distinction of the organs begins only late, even though the arrangement 
of the cells may allow of their position being recognised at an earlier 
period. The actual time of their distinctive development may vary in 
different cases, and it is often possible to correlate this with the biological 
requirements; for instance, the Fern-prothallus is a limited body, with 
small powers of nutritive supply: it is therefore essential that the young 
Fern-plant shall soon establish itself, and accordingly its parts, especially 
the leaf and root, are rushed forward comparatively early. In the case 
of bulky prothalli with large reserves of nutrition, on the other hand, such 
as those of the Lycopodiaceae and Ophioglossaceae, the parts of the 
embryo are differentiated relatively late, not being required for immediate 
action. But even within near circles of affinity there is considerable variety 
in the time of appearance of the organs of the embryo. Jeffrey points 
out how in Maratta and Angiopteris there is a precocious development 
of the cotyledon, while in, Danaea it is the root which first shows 
prominently.) A somewhat similar state of affairs is seen in the Equisetaceae, 
where £. arvense and hiemale have a precocious root, while in Z. “imosum 
and palustre the root is of later appearance. Among the Ophioglossaceae, 
in O. pedunculosum the cotyledon first emerges, while in O. pendulum, 
and wulgatum, and in Botrychium Lunaria and virginianum the root takes 
the lead. 
Lastly, there is variability in respect of the suspensor. It is present 
in Lycopodium and Selaginella but absent in Jsvetes. It is absent in all 
Equiseta and Ferns, and in all Ophioglossaceae hitherto observed, excepting 
Botrychium obliquum, according to H. L. Lyon.? It thus appears that 
within near circles of affinity there is usually constancy of the suspensor, 
but that exceptions may occur even within the single genus. 
There is thus a considerable latitude of detail in the development of 
the embryo in Pteridophytes, and even within near circles of affinity. In 
face of this, the whole conception of embryology should be more plastic 
1Gametophyte of Botrychtum virginianum, Toronto, 1898, p. 18. 
2 Bot, Gas. vol. xl., p. 455. 
