104 EMBRYOGENESIS IN PLANTS 



factor affecting the further behaviour of the embryo.'' (1935, p. 528). 

 He further suggests that the suspensor, a primitive organ which has 

 been 'retained' in this genus, as also in Selaginella, is a highly in- 

 convenient feature except where the archegonium points downwards, 

 and that its presence has necessitated marked curvatures in the embryo 

 before an upright shoot can be estabhshed. 'It can hardly be assumed 

 that such shifts as these should have been attained in the course of 

 opportunist evolution, unless there be some initial disability. . . .' 

 ... 'In more than one evolutionary line the simplified form of the 

 embryo may be explained by its having broken loose from the tie of 

 the suspensor.' As against these views, advanced chiefly in relation to 

 the embryos of Lycopodium and Selaginella, it may be noted that in 

 different Marattiaceae, all with endoscopic embryos, the suspensor, 

 which may or may not be present, would appear to have very little 

 effect on the embryogeny {see Chapter IX). 



The quotations given above reflect a typical approach from the 

 standpoint of the comparative morphologist. Here our chief interest is 

 in the processes that may be involved in the nutrition of the young 

 embryo. Where, as in Psilotum, Tmesipteris and Equisetum, the embryo- 

 geny is exoscopic, we appear to be on familiar ground — though it is, 

 as a fact, practically unexplored. The hypobasal cell, which remains in 

 contact with the prothallial tissue, behaves as perhaps might be ex- 

 pected : it becomes the absorbing region of the embryo, the nutrients 

 taken up by it being passed on to the epibasal region which becomes 

 the actively meristematic shoot apex, i.e. as in a fully organised shoot, 

 the distal region of the embryo depends on nutrients passing up from 

 below. We should not, indeed, expect the epibasal cell, which lies in 

 contiguity with the archegonial neck and may soon emerge from the 

 prothallus, to be the absorptive region of the young embryo. All this 

 seems to be in conformity with our general knowledge of axial growth. 

 Nevertheless, it is doubtful if such reasoning, based on a consideration 

 of the anatomical data, goes to the root of the matter; for in endo- 

 scopic species, such as Lycopodium, the suspensor and the nutrient- 

 absorbing foot are in contiguity with the archegonium neck and the 

 growing embryo becomes deeply embedded in the fleshy tissue of the 

 prothallus. In short, the uptake of nutrients by the embryo is not so 

 simple and direct as it at first appears. Thus we may ask: Do these 

 embedded endoscopic embryos absorb nutrients over their entire 

 surface, or is the uptake confined to the foot? While the embryo is 

 still very small, it may well be that nutrients are absorbed over the whole 

 of the embryonic surface; but, as it enlarges, the indications are that 

 the foot is the main absorptive region. Certainly, from an early stage, 

 there is a marked difference in the development of the basal cells as 



