EMBRYOGENESIS IN FLOWERING PLANTS 281 



In the Loranthaceae, Balanophoraceae, and Santalaceae (Santalales) 

 the species of the first two famihes are parasitic, or semi-parasitic, while 

 those of the third include some parasites. Maheshwari (1937) regards 

 these as being among the most difficult angiosperms to investigate 

 embryologically, Fig. 74. Since the Loranthaceae exhibit many curious 

 morphological and anatomical features, some of which are associated 

 with their parasitic habit, it is of interest to ascertain if the embryogeny 

 is characterised by any unusual features and, if so, when these appear. 

 Singh (1952) has described some very remarkable developments of the 

 female gametophyte and embryo in Dendrophthoe falcata {Loranthus 

 longiflorus). Fig. 76. In this and related genera and species, the whole 

 embryonic environment is a very unusual one. No well-defined 

 nucellus or integument can be distinguished, but there is a considerable 

 mass of megaspore mother cells, each of which gives rise to a linear 

 tetrad of megaspores. Greatly elongated filamentous embryo-sacs, 

 22-28 mm. long, and with eight nuclei, are formed from the uppermost 

 megaspores. These penetrate upwards into the style with the result 

 that the egg apparatus may eventually be situated close to the stigma. 

 After fertilisation, the several embryo sacs become fused and a com- 

 posite endosperm is formed. The evidence, sometimes incomplete, of 

 several investigators (Griffith, 1844; Treub, 1881; Ranch, 1936; 

 Schaeppi and Steindl, 1942; and Singh, 1952) is that the first division of 

 the fertilised egg is by a longitudinal wall, this being followed by rapid 

 polar elongation of the embryo and a succession of transverse divisions. 

 The suspensor of the proembryo now undergoes rapid elongation and 

 thereby thrusts the distal embryonic cells down through the style into 

 the endosperm in the ovary region below. If this rapid elongation did 

 not take place, the young embryo would be shed along with the style- 

 like organ which soon falls by abscission. Only one embryo of the 

 initial polyembryonic system reaches maturity. The fully developed 

 embryo has a club-shaped appearance with a single cotyledon and an 

 enclosed sunken plumule. Singh, however, has shown that at an 

 earlier stage there are two distinct and equivalent cotyledons which 

 become fused on further development. A flat, cushion-like radicle 

 constitutes the base of the embryo. The mature fruit is a complex 

 structure consisting of four tissue layers. The seed has no testa. 

 Within the outermost layer of the fruit there is a viscid layer which is 

 responsible for the ejection of the seed and its subsequent adhesion to 

 the host plant. (Fig. 76a-f, from Singh, 1952). Tupeia antarctica. 

 Fig. 76g, h, the New Zealand mistletoe (Loranthaceae), which closely 

 resembles Viscinn and Loranthus but belongs to a distinct genus, is also 

 characterised by some quite extraordinary developments of its embryo 

 sac and embryo (Smart, 1952). 



