186 EMBRYOGENESIS IN PLANTS 



a motile spermatozoid. Proembryo development then follows, the 

 phase of free nuclear division continuing, with a progressive diminution 

 in nuclear size, until about 256 nuclei have been formed. These nuclei 

 become uniformly distributed in the cytoplasm and after one or more 

 general nuclear divisions — the eighth or ninth — wall formation takes 

 place simultaneously throughout the proembryo. One nucleus is 

 enclosed in each of the approximately equal cells, Fig. 41. This cellular 

 homogeneity in the spherical embryo, however, masks an underlying 

 polar differentiation. The cells at the basal end of the archegonium 

 soon begin to divide actively and continue to function as meristematic 

 cells, whereas those towards the central and upper regions divide 

 infrequently, become relatively large and vacuolated, and may even 

 undergo some dissolution. At this stage the embryo is characterised by 

 a marked gradient of cell size. According to Johansen (1950), no true 

 suspensor is formed in this species, but Radforth (1936) states that the 

 proembryo elongates into a conical suspensor. The actively dividing 

 cells at the base of the archegonium now become organised as an apical 

 meristem and in due course two protuberances, the cotyledons, can be 

 distinguished and other leaves are subsequently formed. In the com- 

 pactly cellular region below the shoot apex the primary root has its 

 inception. Fig. 41. Each cotyledon has an apical grov/ing point con- 

 sisting of a number of small meristematic cells. Like the shoot apex, 

 this meristem is of the eusporangiate, or bulky, type of organisation. 

 Embryos with three cotyledons are of frequent occurrence. Anomalous 

 developments have also been recorded. Thus some differential growth 

 has been observed as between the two cotyledons : one may be longer 

 than the other and notched at the apex, while the other is deeply bifid ; 

 but at maturity the two are of equal length. 



The first procambial strands originate in the cotyledons and can 

 be traced basipetally into the young shoot or axis. The next strands 

 are those of the first two leaves, which are decussate with the cotyledons, 

 the constitution and form of the shoot stele being determined by these 

 two sets of strands. In embryos with two cotyledons the stele is 

 elliptical in transverse section; with three cotyledons it is triangular. 

 In the root meristem in the former the plerome is wedge-shaped; in 

 the latter it is bluntly pyramidal. The fully developed embryo usually 



Fig. 41. Embryogeny in Ginkgo biloba 



A, Proembryo with sixteen free nuclei, of which seven can be seen; a, archegonium 

 neck. B, Cell formation in the proembryo is completed. In A and B, as in the other 

 illustrations, the shoot apex is uppermost (x 70; redrawn from Johansen). C, 

 Young embryo showing gradient of cell size from the basal into the apical region 

 (x 120, after Lyon). D, Upper part of older embryo in l.s. showing the shoot apex, 

 the cotyledons and the beginning of tissue differentiation ( x 40). E, Nearly mature 

 embryo (x 7-5). (D, E, after Coulter and Chamberlain.) 



