192 EMBRYOGENESIS IN PLANTS 



which will survive. This distinction can be applied to the embryonic 

 development in several families of the conifers. Buchholz considers 

 that the evolutionary steps may have been as follows : (i) indeterminate 

 cleavage polyembryony; (ii) determinate cleavage polyembryony; 

 (iii) simple polyembryony, with vestigial traces of (ii) ; and (iv) simple 

 polyembryony without traces of (ii). The embryogenies ofPinus, Cedrus, 

 Tsuga, Cryptomen'a, Biota and Chamaecypahs exemplify indeterminate 

 cleavage polyembryony: that of Dacrydium illustrates determinate 

 cleavage polyembryony. 



To return to Finns: the four basally-situated embryonic cells, 

 each with its subjacent suspensor, become slightly separated from each 

 other, but the four short filaments so constituted remain attached to the 

 rosette tier. In each of the cleavage embryos, the suspensor now begins 

 to elongate rapidly, with the result that the embryo is thrust downwards 

 into the gametophyte tissue. The distal embryonic cell, or apical cell, 

 has meanwhile enlarged and divided by a transverse wall. The subapical 

 cell so formed begins to elongate conspicuously to form what has been 

 described as an embryonal tube or secondary suspensor. A second cell 

 cut off from the apical cell also elongates and is transformed into an 

 embryonal tube, and this process may continue till there are three or 

 more embryonal tubes in linear sequence, all adding to the length of 

 the suspensor region of the embryo, Fig. 37. As a consequence of 

 these developments, the embryo has been thrust deeply into the richly 

 stored cells of the prothallus. The production of the so-called embryonal 

 tubes, which constitute the secondary suspensor, is evidently due to 

 physiological processes similar to those which give rise to the primary 

 suspensor with which they are continuous. Johansen notes that the 

 elongation of the suspensors is so rapid that it is greater than the rate 

 of the dissolution of prothallus tissue in front of the embryo apex; 

 and hence the suspensor and embryonal tubes become coiled and 

 convoluted, the first-formed suspensor cells soon collapsing. 



Meanwhile, the apical cell has undergone divisions in other planes ; 

 a bulky meristematic tissue mass is gradually built up, the identity of 

 the original apical cell being lost. Fig. 37. The presence of a distal 

 apical cell in the early embryogeny in Pinus has attracted a considerable 

 amount of attention. This cell grows, divides, and gives rise to the 

 tissues of the young embryo in the same way as does the shoot apical 

 cell in many pteridophytes. As the embryo enlarges, however, the 

 apical cell disappears, the shoot meristem then consisting of an apical 

 cell-group. In P. banksiana the apical cell persists for a considerable 

 part of the embryonic development, and in P. montana it is stated to be 

 evident in the mature embryo. The usual condition in Pinus, however, 

 is that the apical cell becomes indistinguishable before the cotyledons 



