48 EMBRYOGENESIS IN PLANTS 



Various observations recorded by Farmer and Williams suggest that 

 rapid and probably complex metabolic changes are associated with, or 

 induced during, the period when the spermatozoids are being attached 

 to the ova and the subsequent fertilisation and post-fertilisation phases. 

 The evident repulsion of spermatozoids from the ovum as soon as 

 fertilisation has taken place is probably a biochemical phenomenon. 

 Levring (1952) has suggested that a substance acting like dupunol is 

 released from the egg at the moment of penetration by a spermatozoid. 

 This causes the spermatozoids in proximity to lose their mobility. 

 Oospheres passing out from the conceptacles may become fragmented, 

 and these fragments, which are of variable size and mostly enucleate, 

 become rounded-off, attract spermatozoids like normal ova, and form 

 an enveloping wall. Their further development and segmentation have 

 not, however, been observed. Farmer and Williams have called 

 attention to the fact that whereas the unfertilised ovum has a micro- 

 scopically indeterminate, somewhat frothy homogeneous protoplasm, 

 the zygote has a distinctly alveolar cytoplasm with spindle-shaped 

 aggregates of chromatophores, or striations, disposed in a radiate 

 pattern in relation to the nucleus. 



Levring has shown that the wall which is formed soon after fertihsa- 

 tion consists of two layers : (i) the outermost is the remnant of the egg 

 membrane covered with the gelatinous coat; (ii) the innermost is 

 strongly birefringent and comprises rod-shaped molecules tangentially 

 arranged on the surface of the egg. This layer contains cellulose and 

 polysaccharide sulphates (fucoidin). The cortical layer also contains 

 fucoidin. Various enzyme systems are involved in these surface and 

 sub-surface reactions. It appears that, at fertilisation, cell wall material 

 is released from the periphery of the egg, mainly in the cortical layer, 

 from whence it is forced through the plasma membrane. It reacts with 

 the egg membrane, the new wall being formed on its inner side. As 

 wall formation begins before the spermatozoid has fused with the egg 

 nucleus, it is not determined by the diploid nucleus. 



In the eggs of Fucus, as in the meristematic tissues of vascular 

 plants, auxin (indole-3-acetic acid) is closely associated with growth 

 and morphogenesis. Olson and DuBuy prepared extracts of growth- 

 regulating substances from (1) egg cells, (2) spermatozoids, (3) fertile 

 branch tips and (4) thallus of Fucus vesiculosus, and showed, by using 

 the Avena test, that high concentrations are present in (1) and (2) and 

 lower concentrations in (3) and (4). This work, which was undertaken 

 to explore a possible relationship between the presence of a growth- 

 regulating substance and polarity, was taken a stage further by these 

 investigators when they were able to demonstrate, by using the neat 

 experimental technique illustrated in Fig. 3c, that the polarity of the 



