212 EMBRYOGENESIS IN PLANTS 



Austrotaxus (Saxton, 1934), Taxus baccata (Mirbel and Spach, 1843; 

 Hofmeister, 1851; Jager, 1899; Robertson, 1904; Saxton, 1936), 

 Taxus canadensis (Dupler, 1917), several gametophytes may be formed, 

 i.e. as a result of the development of several megaspores. In Torreya 

 taxifoUa and other species, wall formation takes place at the four free 

 nuclear stage of the proembryo. In Austrotaxus nuclei in the basal 

 group become walled in about the 8-16 nuclear stage, while the others 

 remain as a free group in the cytoplasm above. Taxus baccata has a 

 similar proembryonal condition. In Torreya taxifolia the proembryo 

 completely fills the small archegonium; in other species it fills from one 

 half to two thirds but subsequently elongates to fill the whole space. 

 Except in Austrotaxus, this is the normal winter resting condition and 

 has been designated the hibernal embryo. On further development, the 

 uppermost cells, as in A. spicata, may constitute a rosette tier; the 

 adjacent cells elongate and form the suspensor whilst the distal end 

 consists of three multicellular tiers capped by a single apical cell. The 

 further embryogeny is not known. Taxus baccata shows comparable 

 developmental features, but the uppermost tier of cells degenerates 

 contemporaneously with the elongation of the suspensor (Jager, 1889; 

 Johansen, 1950). In Torreya nucifera. Fig. 50, and T. taxifolia, the 

 proembryo may consist of six to sixteen cells. On the renewal of growth, 

 apart from various anomalous developments, there is characteristic 

 cleavage polyembryony. The distal apex is typically multicellular and 

 produces many enlarging embryonal tubes, a thick secondary suspensor 

 being thereby formed. Fig. 51. The mature embryo has two cotyledons 

 (see Coulter and Land, 1905; Buchholz, 1940; Tahara, 1940; 

 Johansen, 1950). 



Torreya and Taxus are highly specialised genera: Buchholz (1940) 

 regards the mature ovule of Torreya as 'the largest and most specialised 

 of those of all the conifers.' Torreya taxifolia, with only one small 

 archegonium per ovule, and various distinctive embryonic features, is 

 the most specialised species of the genus. Saxton (1934), in a phylo- 

 genetic scheme of the Taxaceae, considers that the five component 

 genera were all derived from the same prototype, Cephalotaxus, Taxus 

 and Torreya being indicated at the ends of three separate branches; 

 Amentotaxus leads on to Cephalotaxus, and Austrotaxus appears as a 

 connecting link to Taxus. Taxus, Austrotaxus and Cephalotaxus are 

 characterised by simple polyembryony, though Cephalotaxus has 

 determinate polyembryony in its small rosette embryos. Only in 

 Torreya, has cleavage polyembryony been retained. The prototype of 

 the order was probably characterised by cleavage polyembryony. 

 Saxton (1934) regards the Taxaceae as a distinct and well-defined family 

 of conifers, not closely related to any other family. 



