218 EMBRYOGENESIS IN PLANTS 



by three tissue sheaths or envelopes: by some investigators these are 

 referred to as an (inner) integument and two perianth members; by 

 others, as three integumentary structures ; and by yet others, as perianth 

 and outer and inner integuments. The growth of the inner integument 

 is such as to make for a long micropylar tube, as in Ephedra. 



The gametophyte and embryogeny have been studied in some 

 detail in Gnetum gnemon. The megaspore (or embryo sac) nucleus 

 divides repeatedly within an enlarging cell, the free nuclei becoming 

 distributed in the wall layer of cytoplasm which encloses a large central 

 vacuole. This free nuclear condition persists, at least in the upper 

 region of the gametophyte, until after fertilisation, but some cellular 

 tissue may be formed at the basal end (Thompson, 1916; Chamberlain, 

 1935). An extensive nutritional tissue is formed below the base of the 

 gametophyte which lies deeply within the nucellus (Coulter, 1908). 

 There is no formation of archegonia in Gnetum but a number of nuclei 

 near the micropylar end become isolated within a cytoplasmic wall or 

 sheath and function as eggs. Male nuclei discharged into the female 

 gametophyte fuse with these egg nuclei, and this apparently acts as a 

 stimulus to the rapid formation of a cellular endosperm-Uke tissue 

 throughout the gametophyte, the cellular mass being particularly dense 

 at the basal end. There is no evidence that the 'endosperm' formation 

 is the result of a double nuclear fusion as in the formation of true endo- 

 sperm in angiosperms. The ensuing embryonic developments are still 

 insufficiently known, but a free-nuclear stage appears to be lacking. 

 The zygote develops into a two-celled body which grows out in several 

 conspicuous filamentous suspensors. Fig. 52a. Some of these may 

 branch and all have a densely protoplasmic apical region at which a 

 small distal embryonic cell is distinguishable. In parenthesis, it is at 

 about this stage that the seed is detached. The definitive embryo is 

 formed by the further development of one of the terminal embryonic 

 cells, but in the earlier stages a polyembryonic condition prevails. In 

 a species of Gnetum described by Thompson (1916), the fertilised egg 

 divides to give two or three cells, each of which grows out into a long, 

 usually uninucleate, downwardly penetrating filament. (To call these 

 long tubular structures 'suspensors,' as if they were suspensors and 

 nothing else, is misleading. The growing end of each such tube is 

 potentially embryonic; so each 'suspensor' is comparable in its morpho- 

 genetic potentiality with a cleavage embryo in coniferous embryology.) 

 In G. moJuccense the suspensors grow beyond the gametophyte into the 

 tissue at the base of the nucellus. Embryos, however, are developed 

 only within the endosperm. In G. gnemon and G. funiculare. Fig. 52a, 

 the filamentous embryos ramify widely throughout the gametophyte 

 tissue and are thus not in close association with each other, as are the 



