198 EMBRYOGENESIS IN PLANTS 



the distal side, Fig. 44. The initial simple polyembryony, involving 

 3-4 proembryos, is superseded by the strong development of one 

 embryo and the disintegration of the others. There is no evidence of 

 cleavage polyembryony, though seeds with two subjoined mature 

 embryos have been observed. After a period of inactivity, the central 

 embryonic cells begin to grow, this new phase being also marked by the 

 disintegration and crushing of the primary suspensor cells and cap cells. 

 At first all the cells of the small embryonic mass grow and divide about 

 equally, but soon those on the proximal side elongate and form a 

 secondary suspensor, while those on the distal side form a subspherical 

 mass of small-celled embryonic tissue — in fact, the morphogenetic 

 region of the embryo. The growth of the apical region now slows down 

 and organ formation begins. A root apex is differentiated near the 

 suspensor end, the adjacent region being recognised as the hypocotyl: 

 at the distal end the nascent shoot apex appears to be inert, but two 

 cotyledons are formed on its flanks. They continue to grow and 

 differentiate and become very large relative to the rest of the embryo. 



The origin and function of the proembryo cap have given rise to 

 some speculation. It has been suggested that it protects the embryonic 

 cells, provides the digestive enzymes, and so on. Johansen (1950) 

 notes that, immediately after free cell formation, elongation takes place 

 simultaneously in both the upper and lower groups of cells adjacent to 

 the embryonic cells : if polar gradients are present, it is not surprising 

 that the suspensor and cap cells react somewhat diff'erently. The embryo 

 apex in Araucaria is endogenous, a condition found in conifer root apices 

 and in the shoot apices of some ferns {see Chapter IX; and Wardlaw, 

 1953). It has a multicellular, massive construction from the outset. 



TAXODIACEAE 



The embryonic development in Sciadopitys verticillata, Fig. 45, is in 

 general like that in the Pinaceae (Buchholz, 1931). When the first four 

 free nuclei move to the base of the archegonium, they divide and form 

 32 nuclei which become arranged in three tiers, i.e. an embryonic, a 

 suspensor (described as a prosuspensor) and a rosette tier. Briefly, the 

 subsequent development is characterised (i) by the very great elongation 

 of the prosuspensor, its collapse, disorganisation and replacement by 

 a so-called primary suspensor derived from the embryonic initials ; (ii) 

 by subsequent additions to the suspensor of embryonal tubes ; (iii) by 

 marked cleavage polyembryony described as the most extreme example 

 of this phenomenon in the conifers ; (iv) by the abortion of the apical 

 cell in the individual embryo; (v) by the formation of some rosette 

 embryos; (vi) by the occurrence of occasional mature twin embryos; 

 and (vii) by the formation of occasional bud embryos which do not, 



