648 PATTERNS AND PROBLEMS OF DEVELOPMENT 



in others the axiate pattern, of the gametophyte. The gametophytes of 

 most ferns are flattened, axiate prothalha with an apical growing cell or 

 cell group, with dorsiventral differentiation, and with archegonia develop- 

 ing on the ventral side.^ The first division of the zygote is in a plane trans- 

 verse to the axis of the archegonium in most forms, but in Polypodiaceae 

 it is in a plane passing through the axis of the archegonium and transverse 

 to the long axis of the pro thallium, that is, in definite relation to its en- 

 vironment. When it is transverse to the archegonial axis, the second divi- 

 sion plane is at right angles to it; when it passes through the archegonial 

 axis, the second plane also passes through that axis at right angles to 

 the first, and the third is transverse. The four quadrants or four pairs of 

 octants resulting from these divisions are said to represent more or less 

 exactly four regions of the developing plant — cotyledon, stem, root, and 

 foot, the latter a temporary nutritive organ connecting the embryo with 

 the gametophyte. The fern embryo is then temporarily a bilateral form, 

 but the relation of the four regions to archegonial and gametophyte pat- 

 tern differs in different groups. For example, the diagrammatic Figure 

 206, B, indicates the relation believed to exist in certain forms (order 

 Marattiales), the foot developing from the two unshaded quadrants 

 next to the neck of the archegonium, stem, cotyledon, and root, from the 

 two shaded quadrants. The relations in the Polypodiaceae are indicated 

 in Figure 206, C, a lateral view of the eight-cell stage. The two dorsal 

 anterior cells (upper shaded quadrant of the figure) give rise to stem; 

 the ventral anterior pair, to cotyledon; the dorsal posterior pair, to 

 foot; the ventral posterior pair, to root. In Figure 206, B, the embryonic 

 pattern is apparently related to the archegonial axis, perhaps secondarily 

 to the longitudinal axis of the gametophyte; in Figure 206, C, the re- 

 lation is somewhat different, but since it is apparently constant it is 

 significant. In Equisetum the first division is transverse to the arche- 

 gonial axis, the cell next to the base forming the foot or the foot and root, 

 the other cell, stem and cotyledon. 



In Lycopodium and Selaginella the first division is transverse to the 

 archegonial axis; and the cell next to the neck, or a descendant of it, be- 

 comes the suspensor, an embryonic organ not present in other pterido- 

 phyte groups; and cotyledon and stem develop from cells next to the 

 base of the archegonium; but in later development the embryonic axes 

 undergo change in direction, apparently in relation to the gametophyte 



■' Archegonia are on the dorsal side of the prothallia in Ophioglossales, and in Marsilea 

 the single archegonium is apical on the apparently radial gametophyte. 



