286 EMBRYOGENESIS IN PLANTS 



inhibiting substance has been dissolved out or removed, the embryo 

 enlarges within the seed and becomes differentiated into a small-celled, 

 embryonic, distal region and a large-celled basal region. At this stage 

 the germ is still uninfected by the symbiotic fungus. On being released 

 from the testa the germ can only continue to grow if it is infected by the 

 appropriate fungus (Francke, 1934). In the saprophytic mycorrhizic 

 species Sarcodes sanguinea (Ericales: Monotropaceae), the embryo 

 and the endosperm in the mature seed are also poorly developed 

 (OUver, 1890). 



In Zeuxine sulcata (Orchidaceae : Neottinae), a holosaprophytic, 

 rootless species exhibiting an extreme case of absence of chlorophyll 

 and feebly developed vascular tissue, Swamy (1946) has described a 

 very much modified embryogeny. Fig. 78c-e. In the ovule, the mega- 

 spore mother-cell begins to degenerate so that few, if any, ovules form 

 a functional embryo sac. Diploid embryos arise adventitiously 

 (apomictically) from the cells of the nucellar epidermis, polyembryony 

 being common. As these embryos are not formed in the normal 

 environment of an embryo sac, it is of some interest to note that while 

 the very young embryo may exhibit some evidence of polarity, older 

 embryos consist of a somewhat irregular mass of quite undifferentiated 

 meristematic cells. 



APOMIXIS 



By apomixis is meant the substitution of the normal sexual repro- 

 duction by an asexual process. Apomixis may include the formation of 

 an embryo from an unfertilised ovum (haploid parthenogenesis), or 

 from some other cell of the gametophyte (haploid apogamy); but as 

 the resulting haploid sporophytes are usually sterile, the process is not 

 repeated from one generation to the next. This has been referred to as 

 non-recurrent apomixis. In recurrent apomixis, the nuclei of the 

 embryo sac are usually diploid. Hence we may have diploid partheno- 

 genesis (from a diploid ovum) or diploid apogamy (from some other 

 diploid cell of the gametophyte). Yet other variants are known. In 

 what has been called adventive embryony, or sporophytic budding, the 

 embryo is formed from a cell of the nucellus or the integument and its 

 nuclei are therefore diploid; in these cases, the gametophytic generation 

 has been by-passed. Lastly, in viviparous reproduction, the flowers 

 may be replaced by bulbils or propaguies, this being simply an unusual 

 form of vegetative reproduction. (For discussions and data on the 

 cytology of apomixis, the reader is referred to Gustafsson, 1935, 1946, 

 1947.) 



The occurrence of haploid parthenogenesis may be of considerable 

 value in genetical studies in that it enables true-breeding homozygous 



