1 82 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 Marattia and Angiopteris there is a precocious development 

 of the cotyledon, while in Danaea it is the root which first shows 

 prominently. 1 A somewhat similar state of affairs is seen in the Equisetaceae, 

 where . arvense and hiemale have a precocious root, while in E. limosum 

 and palustre the root is of later appearance. Among the Ophioglossaceae, 

 in O. pedunculosum the cotyledon first emerges, while in O. pendulum, 

 and' vufgatum, 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 Isoetes. It is absent in all 

 Equiseta and Ferns, and in all Ophioglossaceae hitherto observed, excepting 

 Botrychium obliquum, according to H. L. Lyon. 2 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 



1 Gainetophyte of Botrychium virginianum^ Toronto, 1898, p. 18. 



2 Bot. Gaz. vol. xl., p. 455. 



