214 EMBRYOGENESIS IN PLANTS 



GENERAL COMMENT ON CONIFER EMBRYOLOGY 



Buchholz (1950) has summarised the general state of knowledge of 

 conifer embryology as follows. Of the 50 or more genera of living 

 conifers, the embryogeny of about 80 per cent has been investigated to 

 some extent. The same general pattern of development is found in all 

 species of a genus, and in closely related genera the same pattern may 

 also be observed. Podocarpiis is exceptional. In this large genus many 

 diverse types of embryogeny occur; but within the sections of the 

 genus the relationships are comparable with those that obtain in the 

 genera of other families. The Podocarpaceae which have been described 

 as separate genera, Acmopyle, Pherosphaera, Dacrydium, Saxegothaea, 

 etc., have embryogenies that fit into the series shown by the different 

 sections of Podocarpiis. Thus, the embryological data may be used as 

 criteria in the taxonomiic diagnosis of genera, though several genera 

 may have embryogenies that are essentially alike. In the Araucariaceae, 

 all members of the family have the same type of embryogeny, with 

 simple polyembryony. As far as is known, the Podocarpaceae are 

 characterised throughout by the presence of peculiar binucleate cells 

 in the early embryo. Extensive embryological investigations have shown 

 that most conifers, formerly supposed to develop through simple poly- 

 embryony, have some form of cleavage polyembryony. Only the 

 Araucariaceae and certain scattered genera such as Thuja (excluding 

 Biota), several genera in Pinaceae, in Podocarpaceae and the Taxaceae, 

 develop with simple polyembryony. Cleavage polyembryony has not 

 afforded a precise taxonomic criterion as intermediate conditions are 

 known to occur. The origin and explanation of this phenomenon have 

 prompted considerable discussion and some disagreement, especially on 

 the question of whether it is indicative of a primitive or advanced 

 condition. 



EPHEDRA, WELWITSCHIA AND GNETUM 



The three genera Ephedra, Wehvitschia and Gnetum have been 

 placed in one order and one family by some taxonomists because they 

 share a number of important characters in common, namely, the 

 presence of vessels in the secondary wood, a compound strobilis in 

 both male and female, the extension of the inner integument into a long 

 micropylar tube, opposite leaves, absence of resin canals, and embryos 

 with two cotyledons (Chamberlain, 1935). In the contemporary view, 

 however, the Gnctales (or Ephedrophyta) of earlier classifications do 

 not constitute a natural group (Eames, 1952), a view expressed by 

 Bertand nearly eighty years ago. There is justification for raising them 

 to ordinal rank. Each genus is probably the contemporary survivor of 



