54 EMBRYOGENESIS IN PLANTS 



The further growth of the embryo is characterised by abundant 

 anticHnal divisions and occasional periclinal ones in the peripheral 

 layer; the inner cells, on the other hand, tend to enlarge with fewer 

 divisions. An incipient differentiation of tissues thus takes place. The 

 embryo as a whole is now elongating and widening distally, i.e. it 

 shows obconical development. 



Naylor (1953) has followed the development in culture of embryos 

 of Marginariella urvilliana, a submerged, endemic New Zealand 

 member of the Fucales. The early development of zygotes took place 

 while they were still attached to the receptacle; the germlings sub- 

 sequently dropped off and became attached to the bottom of the culture 

 dish. The oospore is enveloped in a firm wall. The formation of the 

 rhizoidal protuberance is attended by the aggregation of denser cyto- 

 plasm in that locus. The first partition wall is now formed in an 

 approximately transverse or median position, but obliquely disposed 

 walls were also observed. The rhizoid, which typically bursts through 

 the oospore wall, is separated off from the body of the embryo by a 

 transverse wall. The further development of the embryo is in general 

 like that of Fucus. 



As noted above, the early development in Sargassum is somewhat 

 different. Blomquist (1944) has given a general account of the embryo- 

 geny of the Sargassaceae. The discharged oval oogonium is anchored 

 on a stalk and lies outside the conceptacle and the developing embryo 

 usually remains attached in this position for 2-3 days until it has 

 developed into a several-celled structure. The zygote is first divided 

 across the shorter diameter into two approximately equal cells, if the 

 egg is ellipsoidal. At the second division, a small lenticular cell is cut 

 off at one end. Sargassum Unifolium {see above, and Fig. 11k, l) may 

 be an exception in this respect. The embryo proper develops by a 

 regular quadrate method of cell division until it has become many- 

 celled and an apical cell is established. There is considerable variation 

 among the different genera, and even among the species of one 

 genus (Sargassum), in the number of rhizoids and their mode of 

 formation. 



At an early stage important developments begin to take place in 

 the most distal region of the embryo. The details for Fucus are as 

 follows. Of the distal group of superficial cells, one — the initial cell — 

 develops a conspicuous nucleus and grows out as a hair. As it lengthens, 

 what has been described as a meristem {see Fritsch, 1945; and the 

 authors cited by him) appears above the initial cell (Fig. 11h). The 

 hair, in fact, undergoes a kind of intercalary or trichothallic growth. 

 The initial cell, however, remains unchanged during this and the sub- 

 sequent course of events. As the hair develops, the adjacent cells grow 



