Chapter XI 



EMBRYOGENESIS IN GYMNOSPERMS: 

 CYCADALES AND GINKGOALES 



GENERAL INTRODUCTION 



IN the evolution of plants, the gymnosperms occupy a unique position 

 between the still amphibious pteridophytes and the flowering plants. 

 The gymnosperms are seed plants and include members of great 

 antiquity. In their organisation they exhibit characters which remind 

 us of their archegoniate or pteridophyte ancestors, on the one hand, and 

 of the more highly evolved flowering plants, on the other. In the 

 gymnosperms, we shall be concerned with the development of embryos 

 borne within seeds and nourished by the parent sporophytic plant; 

 we shall also have to survey a considerable and varied assemblage of 

 species of which the systematic and phylogenetic relationships are by 

 no means clear. That the Gymnospermae is a class of Pteropsida (or 

 Filicopsida) is generally accepted by contemporary systematists ; but 

 the origins of the class are still obscure and uncertain. Some taxono- 

 mists, indeed, have suggested an origin from Lycopsida. At the other 

 end of the scale a not dissimilar difficulty presents itself: there is no 

 clear and generally accepted view as to how the angiosperms may have 

 originated from gymnospermous ancestors, if that indeed was the 

 evolutionary sequence. It is against this general background that this 

 survey of embryogenesis in different gymnosperm orders, families and 

 genera has been undertaken. 



Contemplation of the facts of reproduction in a gymnosperm such 

 as Piims, in a lycopod such as Selaginella, and in a heterosporous fern 

 like Marsilea, suggests that the gymnosperms may have originated from 

 some early pteridophyte stock, or still more ancient proto-pteridophyte 

 stock. In their somatic organisation and their reproduction, however, 

 the gymnosperms have made important advances : they are true seed 

 plants, adapted to a land habitat, widely distributed, and in some 

 regions constituting the predominant vegetation. Considerable and, 

 thus far, unbridged phyletic gaps separate the gymnosperms from the 

 pteridophytes on the one hand, and from the angiosperms on the other. 

 Nevertheless, comparison with these groups seems natural. The 

 essential features in the life cycle are common to them all. In the 

 assumed evolutionary sequence from pteridophyte to gymnosperm to 



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