210 EMBRYOGENESIS IN PLANTS 



a common ancestry. The more primitive order, the Cephalotaxaceae, 

 will be treated first. 



The archegonia of Cephalotaxus, usually four in number, are long, 

 narrow, and pointed at the basal end. At the thirty-two celled stage, 

 the proembryo of Cephalotaxus drupacea is long and narrow, is closed 

 in by walls at the upper end, and is without regular tiers, Fig. 5 Id, e. 

 The upper cells constitute a rosette; the adjacent cells function as 

 suspensor cells; and the distal embryonic region of the proembryo has 

 a longish sterile cap cell which subsequently degenerates. The embryo 

 apex is multicellular. There is no cleavage polyembryony, but the 

 rosette cells may give rise to small embryos, these being without 

 terminal caps. The presence of the distal cap cell has been associated 

 with the suppression of cleavage polyembryony. The later development 

 shows no exceptional features, two cotyledons being usual. The shoot 

 apex is the last region of the embryo to be differentiated {see Strasburger, 

 1897; Lawson, 1907; Coker, 1907, Buchholz, 1925). 



Taxus baccata, Torreya spp. and Austrotaxus are sometimes treated 

 as the Taxaceae, with Cephalotaxus in a distinct but related family. 

 In their gametophytic and embryonic developments, Taxus and Torreya 

 exemplify a high level in gymnosperm organisation. In both Taxus 

 baccata and T. canadensis the female gametophyte may or may not, in 

 different instances, have become cellular when the pollen tube reaches 

 it (Dupler, 1917; Saxton, 1936). The pollen tube may be in contact 

 with the prothallus when the free nuclei are still in the course of becom- 

 ing arranged round the megaspore membrane. A common but exceed- 

 ingly interesting development in this group consists in the upward out- 

 growth from the prothallus of a multicellular tubular organ — described 

 as the prothallial tube — which meets the downwardly advancing 

 pollen tube in the nucellar tissue. This curious structure is mentioned 

 or figured by various investigators, e.g. by Dupler (1917) in Taxus 

 canadensis, by Saxton (1936) in T. baccata, by Coulter and Land (1905) 

 in Torreya taxifolia, by Sahni (1921) in Cephalotaxus, and by Saxton 

 (1934) in Austrotaxus. In short, it is of occasional occurrence in all 

 Taxaceae. The interest of the prothallial tube lies in the fact that it 

 closely resembles the very curious and unique prothallial tubes in 

 Wehvitschia {see p. 219). It may further be noted that no trace of a 

 ventral canal nucleus has ever been observed in any species of Taxus, 

 or in Torreya taxifolia (Coulter and Land, 1905), but it has been observed 

 in Torreya californica by Robertson (1904). 



The archegonia of Torreya are very small, those of Taxus a little 

 larger, and those of Austrotaxus twice as large and comparable with 

 those of Cephalotaxus. The number of archegonia is small, only one 

 per ovule being present in Torreya taxifolia. On the other hand, in 



