304 



THE EMBRYO 



[CH. 



the flattened prothallus, while the stalk which points towards the archegonium 

 is clearly a pluricellular suspensor. In this respect Danaea and Macroglossiim 

 may be held as retaining an archaic feature, which appears to be wanting 

 in the rest of the Marattiaceae. 



The difference between the embryology of the Marattiaceae and that of the Lepto- 

 sporangiate Ferns is striking. In the former the embryo is erect from the first, perforating 

 the prothallus upwards. In the latter it is prone, emerging from the lower surface of the 

 prothallus. Hitherto no sufficient explanation has been given how that difference may 

 have been bridged over. The erect is probably the more primitive type. The embryology 

 oi Macroglossum gives a clue : for here the embryo lies obliquely between the surfaces of the 

 prothallus. Its position suggests some obstacle to perforation of the upper surface, while 

 the body of the embryo is more nearly related to the lower surface than in other Marat- 

 tiaceae. A slight further modification would then suffice for its emergence with a still 

 prone position from the lower surface of the thallus, as it does in the Leptosporangiate 

 Ferns. They would in that case illustrate a later and derivative type of embryology, as 

 they are also held to be later and derivative in so many other features, including the loss 

 of the suspensor. 



Similar facts have been observed for the Ophioglossaceae. In Helmin- 

 thostachys the axis of the archegonium is usually horizontal or oblique. 

 Lang has shown that as the zygote develops, it extends obliquely down- 

 wards from the venter of the archegonium into the massive prothallus before 

 segmentation. Then follow two transverse walls, so that a row of three cells 

 is formed. The two cells next the venter supply the first and second tiers 

 of the suspensor: the distal cell is the embryo proper. This is at first straight ; 

 and it divides into a hypobasal half next the suspensor, which forms the 

 foot; and an epibasal half which gives rise to the stem and leaf, and probably 

 also to the first root. As it grows 

 larger the embryo, of which the axis 

 is at first approximately horizontal, 

 curves so that its apex points up- 

 wards ; but later the apex of the 

 adolescent plant bends over, and 

 growth proceeds again horizontally 

 {La7id Flora, Figs. io8, 109). The 

 form of the embryo at a time when 

 the apex is still vertical is shown 

 by Lang's drawing, from which the 

 curvature resulting in the upward 

 turn of the apex is clearly seen 

 (Fig. 288). This relation of the parts 

 of the embryo is very similar to 

 that shown by the photograph of Botrydiiiim obliqniim of similar age taken 

 by Dr Lyon. Here also the first direction of growth was found to be inwards 



Fig. 288. Yoxvi\o'iG:\v\:ixyool Helniiiitliostachys. a, in 

 a younger; b, in a more advanced state; j- — sus- 

 pensor ;_/= foot ; ;-, r- = roots; j-/ = steni; cot — 

 base of cotyledon ; //;'/ = hypocotyl. (After Lang.) 



