24 EMBRYOGENESIS IN PLANTS 



There is evidence, which needs to be confirmed and extended, that 

 polarity and the orientation of the first wall dividing the zygote are 

 affected by gravity, Leitgeb (1878), in experiments with Marsilea, in 

 which the large megaspores could at will be variously oriented, rotated 

 on a kleinostat, and so on, found that the first partition wall of the 

 zygote was always formed in the axis of the archegonium, no matter 

 what conditions obtained. But the wall could occupy any rotational 

 position round this axis, and, in fact, its position was such that the 

 embryo was always disposed with an upwardly directed shoot-forming 

 half, and a downwardly-directed foot- and root-forming half. This 

 admittedly limited evidence suggests that while gravity may have some 

 effect on the initial phase of development in some species, it is to factors 

 in the fertihsed ovum or in the adjacent gametophyte tissue that we 

 should look to explain the observed developments. So far as the 

 writer is aware, it has not been suggested that there is a protoplasmic 

 differentiation in the unfertilised egg or in the zygote in plants com- 

 parable with that in certain mozaic eggs known to zoologists. In 

 encapsulated ova it may, however, be inferred that immediately after 

 fertilisation, if not before, protoplasmic differentiation begins to take 

 place. This, indeed, is to be expected when we consider the configura- 

 tion of the archegonium and its relation to the nutritive prothallial 

 tissue. Somewhat similar considerations will apply to the embryo sac 

 in flowering plants. 



While the growth of the encapsulated zygote may be ascribed to 

 biochemical stimuli resulting from the fusion of the male nucleus with 

 that of the ovum, its polarised development seems likely to be deter- 

 mined by auxin or other physiological gradients in the adjacent tissue. 

 In pteridophytes the uptake of nutrients from the prothallus — a process 

 which has been in operation during the maturation of the ovum — is 

 maintained until the embryo becomes a free-living sporophyte; in 

 bryophytes, it continues until the maturation of the sporophyte. 

 Contemplation of the structural details in archegoniate plants suggests 

 that the path of nutrition to the zygote is more likely to be by way 

 of the base and sides of the archegonium than through the region 

 adjoining the neck. If so, the metabolic gradients may determine both 

 the polarity and the position of the first partition wall. 



Much importance has been attached to the presence of a suspensor 

 in pteridophytes by comparative morphologists. By them, it is regarded 

 as a primitive feature that has been retained in some modern survivors 

 and it has been suggested by Jeffrey that it is 'useful in pushing the 

 developing embryo deeper down into the prothallial tissue' (Campbell, 

 1911-40). But, if we look at the matter objectively, two points may 

 be noted: (i) the suspensor undergoes a rapid parenchymatous 



