110 EMBRYOGENESIS IN PLANTS 



a transverse wall. A two-tiered quadrant stage is thus constituted, 

 Fig. 25c, D. The more distal or epibasal tier on further development 

 gives rise to the shoot apex and first leaves, and the lower or hypobasal 

 tier to the hypocotyl and first root. Meanwhile, the suspensor may 

 have divided by a second transverse wall and by an obliquely transverse 

 wall. On further growth, the epibasal region is divided by periclinal 

 walls, except for the most centrally placed, distal cells. It can already 

 be seen that these are the nacent apical cells of the developing axis. 

 Fig. 25f. Further cell divisions take place in all three planes in both 

 the epibasal and hypobasal regions and the short embryo begins to 

 curve upwards and to form its organs and differentiate its tissues. A 

 distal group of superficial prism-shaped, embryonic cells constitutes an 

 apical meristem, on the flank of which the first leaf, with its ligule, is 

 formed. A second leaf is later formed approximately opposite to the 

 first. Fig. 25g. Meanwhile the active growth in the hypobasal region 

 has resulted in the elongation and curvature of the embryo which is 

 now deeply embedded in the prothallial tissue. The central cells of the 

 hypocotyl, from below the apical meristem down to a lateral and basal 

 outgrowth which can be recognised as the first root (rhizophore), have 

 become elongated and narrow and constitute the primary vascular 

 strand or stele. Thus, from the outset, the stele is of axial and not of 

 foliar origin. 



According to Bruchmann, the first leaf may not be formed for some 

 time: it may still be absent even when a considerable axis, curved 

 relative to the suspensor, has already been formed. The formation of 

 the second leaf may be still further delayed ; in some specimens it only 

 appears after the embryo has emerged from the prothallus. However, 

 tv/o nearly opposite but unequal leaves can eventually be seen at the 

 shoot apex; these leaves which become green and of approximately 

 equal size are sometimes described as cotyledons. The apical meristem 

 of the young shoot consists of a number of equal cells, i.e. there is no 

 single conspicuous apical initial. Bruchmann states that no large foot 

 is formed in S. spimdosa, nor in S. apus, as in the well-known 5. martensii. 

 In those species in which a foot is present it usually occupies a position 

 basal to the hypocotyl and in continuity with the narrow suspensor. 



SELAGINELLA DENTICULATA; S. MARTENSII; AND OTHER SPECIES 



In these species the zygote also elongates in the axis of the arche- 

 gonium and divides by a transverse wall. The next conspicuous 

 development consists in the considerable elongation of the suspensor 

 which, in species such as S. deuticidata, is divided by many transverse 

 walls. Fig. 25h-l. Meanwhile the hemispherical embryonic cell has 

 enlarged and undergone divisions, leading to the inception of the shoot 



