352 GYMNOSPERMS 



be more than the group of greatly elongating cells which emerges 

 from the proembryo. It is a special part of the suspensor system 

 which elongates very greatly, and in such forms as Sciadopitys, may 

 precede the formation of the primary suspensors. Each embryo of 

 Sciadopitys, which appears later, is borne on its own primary sus- 

 pensor. Thus, Pinus may be considered as having no prosuspensor, 

 but a group of four primary suspensors placed parallel to each other, 

 followed by the embryonal tubes, which together constitute the sec- 

 ondary suspensor. Dacrydium has a prosuspensor, but omits the for- 

 mation of the primary suspensor, and forms, instead, a massive sec- 

 ondary suspensor on one or more of the several embryos. The very 

 long suspensors which appear on the early embryos of podocarps 

 would therefore be described as prosuspensors. Prosuspensors seem 

 to be recognizable in conifers above the level of the Abietaceae. 



There can be no doubt that the highly developed suspensor pushes 

 the growing embryo down into the gametophyte, which is richly 

 stored with starch and other foodstuffs. As the foodstuffs become 

 liquid, the tissues surrounding the embryos break down, so that a 

 large cavity is formed. The elongating suspensor, as it kinks and 

 coils, fills most of this cavity, thrusting the embryo deeper and deep- 

 er into the gametophyte. After the embryo has reached its maximum 

 length, the suspensor, together with the remains of the egg and nu- 

 cellus, forms a dry cap which may protect the root end of the embryo 

 as it breaks through the testa. 



In the early embryogeny there is frequently a definite apical cell, 

 segmenting like the apical cell of many pteridophytes (fig. 339). In 

 B and C of this figure, the apical cell and its segments are indicated 

 by stronger lines, while the subdivisions of the segments are indi- 

 cated by lighter lines. At the stage shown in D there is no longer any 

 segmentation of the higher fern type, but a growth by a group of 

 initials. 



There are apical cells in other conifers, Sciadopitys, Thuja, Junip- 

 eriis, and others, but the apical cells do not cut off so many seg- 

 ments. Embryos with caps, and many embryos without cleavage 

 polyembryony, like Picea, Larix, and Abies, have no apical cells. 

 In Thuja occidentalis and Taxus, there seems to be an apical cell. 



