216 EMBRYOGENESIS IN PLANTS 



place, the nucleate proembryo initial being thrust deeply into the pro- 

 thallial tissue. The embryonic cell divides, the proximal cell giving 

 rise to the suspensor (the secondary suspensor, according to Johansen). 

 This cell divides by a longitudinal wall, and the distal embryonic cell 

 by two intersecting obliquely longitudinal walls, an apical cell being 

 thereby constituted. As the result of further cell division the distal 

 region becomes more massive, Fig. 52m, n, the apical cell seemingly 

 functioning for some time but eventually losing its identity. Cells at the 

 proximal end of the embryo elongate to form suspensor cells. At about 

 this stage the suspensor tube loses its turgor, collapses and disintegrates. 

 Johansen regards this point as marking the transition from the pro- 

 embryo to the embryo proper. While several of the embryos may grow 

 for some time, usually only one comes to maturity. It is a dicotyle- 

 donous embryo of which the root is differentiated in proximity to the 

 suspensor. Under favourable conditions there is immediate germination 

 and the seedling becomes established during the same season. 



In other species of Ephedra there are differences in the details of the 

 development and segmentation of the suspensor tube. In £". foJiata 

 only one suspensor tube is initiated, the nucleus remaining undivided 

 prior to tube formation. E. altissima is characterised by a very long 

 tube. 



The order to which Ephedra belongs is widely separated from the 

 Welwitschiales and Gnetales {semu stricto). It may have been derived 

 from Cordaite stock or from an ancestral stock common to both the 

 Cordaitales and Coniferales (Schoute, 1925; Florin, 1939; Fames, 

 1952). 



GNETACEAE 



The embryogeny in Gnetum, of which some 30 species are known, 

 presents technical difficulties and consequently differences in interpreta- 

 tion have arisen. Differences between species have probably contributed 

 to this state of affairs. In the ovulate flower, the nucellus is surrounded 



Fig. 52 



A, Gnetum fiiniculare. Branching of suspensor tubes (from Johansen, after Haining). 



B, C, E, Gnetum gnemon. Developing embryos (x 175; after Bovver). D, G. 

 moluccense. Several celled proembryo (after Thompson). F-J, W'elwitschla mira- 

 bilis. F, Three-celled proembryo. G, Nine-celled proembryo, with four initial 

 cells. H, Older stage; the cells of the inner ring are now considerably elongated, 

 and those of the outer cortical ring are beginning to grow. J, A slightly older stage 

 (see Text); p, pollen tube; ic, initial cells; icr, inner cortical ring; ocr, outer 

 cortical ring; c cap cells ( x 230, after Johansen, from Pearson). K-N, Ephedra tri- 

 fitrca. K, Proembryonic cell, with two nuclei and suspensor tube. L, The next 

 stage, showing a partition wall in the suspensor tube. M, Beginning of formation of 

 embryo proper; the distal regionof the suspensor tube has divided by transverse and 

 longitudinal walls. N, A later stage; the distal cells have divided by periclinal walls 



(K, L, X 950; M, N, x 470; after Johansen). 



