116 EMBRYOGENESIS IN PLANTS 



group of apical initials is present. During the embryonic development 

 of S. pouJteri the shoot apex has a three-sided (i.e. tetrahedral) apical 

 cell; but as the apex enlarges a gradual transformation takes place so 

 that several apical initials are present in older and stouter branches 

 (Bruchmann, 1909). Bruchmann (1909, 1910) also found that the 

 apical meristem in S. lyalUi and S. preissiana consists of a group of 

 similar meristematic cells, as in Lycopodium. In S. wallichii, Strasburger 

 (1872) found that there were two conspicuous apical initial cells; 

 similar observations have been recorded by Williams (1931) for 

 S. grandis. Wand (1914) has stated that in different species apical growth 

 may proceed from a three- or four-sided apical cell, from a divided 

 apical cell, or from a group of similar initial cells. This last condition 

 is characteristic of the primitive, radially symmetrical species S. spinu- 

 losa and is apparent at a very early stage in the embryogeny. In 

 dorsi ventral species on the other hand, a single apical cell is established 

 as a distinctive feature at a very early stage in the embryogeny, e.g. 

 S. denticidata. Fig. 25k. 



The distinctive histological organisation of the shoot apex in any 

 particular species is ultimately referable to its genetic constitution. 

 While we are not yet able to explain how particular genes, or gene- 

 groups, bring about this organisation, we may note that, as in certain 

 other features of the embryogeny, the genie effect can be detected at a 

 very early stage in the ontogenetic development. 



ISOETES 



At the outset it should be noted that this account of the embryogeny 

 in Isoetes, is in two parts : in the first part use has been made of the 

 descriptions given in standard works, e.g. those of Campbell and Bower; 

 in the second the account by La Motte, based on experimental investi- 

 gations, has been followed. 



In Isoetes the megaspore is filled with a uniform prothallial tissue, 

 the first archegonium being formed at the point of convergence of the 

 apical fissures, i.e. it is closely comparable with Selaginella. The 

 ovum is conspicuous and practically fills the venter. Fig. 29a. After 

 fertilisation, the first partition wall is obliquely transverse to the axis 

 of the archegonium, Fig. 29b ; no suspensor is formed and the embryo- 

 geny is exoscopic {but see below). The two segments of the divided 

 zygote may be referred to as the epibasal and hypobasal cells. They 

 have been likened to the two tiers seen in the embryogeny of some 

 species of Selaginella (Bower, 1935). The next two divisions are at 

 right angles to the first wall and to each other and yield an octant 

 stage. These walls are somewhat irregular and not always clearly 

 defined in Isoetes, Fig. 29c. On further growth and cell division the 



