302 



THE EMBRYO 



[CH. 



and the shoot correspondingly delayed. On the other hand, in the embryos 

 which spring from the delicate prothalli of Leptosporangiate Ferns the 

 cotyledon is advanced quickly, so as to take up autotrophic nutrition early. 

 In the aquatic Salvinia, though the origin of the shoot is as in other Lepto- 

 sporangiate Ferns, the root is absent, a fact which accords with the habit 

 of the plant, and with the rootless state of the adult. 



Embryos having a suspensor, and others without one, are included within 

 the Filicales; and the Lycopodiales are in a similar position. In both the 

 hypothesis may be put forward that the suspensorless type has been derived 

 by elimination of the suspensor in the course of Descent: and the change 

 appears to have been related to the bulk of the nutritive prothallus. If this 

 were so, then embryos with a suspensor would be held as relatively primitive 

 in that respect, and those without it as relatively advanced (Lang, 256). Among 

 the Filicales it is only in certain of the Marattiaceae and Ophioglossaceae 

 that a suspensor is found, and both of these families are on other grounds 

 held to be primitive types of the Class. No case of a suspensor has been 

 recorded among the Leptosporangiate Ferns. Further there is no evidence 

 of the formation of a suspensor de novo. These facts are in themselves 

 prima facie evidence of the correctness of the 

 hypothesis. It will now be shown how it ap- 

 plies to the several families of Ferns. 



In the Marattiaceae the prothallus is of 

 the cordate type, but unusually fleshy, and 

 the archegonia are directed downwards. The 

 embryo develops with its basal wall cutting 

 the axis of the archegonium transversely, 

 and its polarity is consequently vertical from 

 the first, so that the apex of the axis points 

 upwards : the result is that the upper surface 

 of the prothallus is ruptured, and the sporeling 

 emerges with its cotyledon and apical bud 

 erect, while its root projects downwards into 

 the soil (Fig. 285). If a series of embryos of the 

 Marattiaceae be all orientated as they would ^'^g-^'^i,- MarattiaDougiasH. A = \o\\ 



be in nature, with their basal wall {b, b) hori- 

 zontal, they would appear as in Fig. 286. In 

 Angiopteris (a), Cliristensenia {b), and Marat- 

 tia {c) there is no suspensor, and the segmen- 

 tation, though less regular, follows in essentials 

 that seen in the simpler Leptosporangiate 

 Ferns (cf. Fig. 294). But in Danaea jamaicensis {d, e) a short suspensor 

 has been found, and a similar organ appears also in D. elliptica. This goes 



gitudinal section of a young embryo 

 (X225); b, /!'=the basal wall: the 

 arrow points to the neck of the 

 archegonium. B~2i similar section 

 of an older embryo, showing its 

 position in the prothallus; j/' = stem ; 

 /'r = prothallus; ar= neck of the 

 archegonium ( x 72). (After Camp- 

 bell.) 



