Chapter XIV 



EMBRYOGENESIS IN FLOWERING PLANTS: 

 SPECIAL SECTION 



THE embryonic developments in flowering plants may be studied 

 collectively, either by referring them to a number of embryonomic 

 types, or by considering them in the orders and families to which the 

 species belong, as Johansen (1950) has done. In this book no com- 

 prehensive systematic treatment of embryos will be attempted : rather, 

 a kind of sampling will be undertaken to see whether or not, in different 

 groups, or categories, there are associated embryogenic phenomena of 

 special or general interest. 



EMBRYOGENESIS IN PRIMITIVE ANGIOSPERMS 



Contemplation of the angiosperms prompts an inquiry into the 

 nature of the embryonic development in what taxonomists regard as 

 the most primitive orders and families : Do the embryos of the species 

 that have been investigated show any special features which, for 

 example, might be regarded as primitive ? Do they afford a clue to the 

 probable ancestry of the flowering plants ? Do they yield any evidence 

 that a classification based on embryology is ever likely to be feasible ? 

 These and other questions readily suggest themselves. 



The whole subject of the phylogeny of the angiosperms bristles 

 with difficulties: the origin of the flowering plants is still a mystery. 

 As to which are the most primitive families of dicotyledons there may 

 still be some differences of opinion. According to Arnold (1947), the 

 evolution of the angiosperms may have begun to take place during the 

 Mesozoic era, and various seed-bearing predecessors have been sug- 

 gested as possible ancestors. (See Arnold (1938) for a short review of 

 Palaeozoic seeds.) But no fossil forms are known by which the flowering 

 plants can with certainty be related to lower groups. The angiosperms 

 of the Lower Cretaceous are very like those of today, some of them 

 having, indeed, been referred to living families, even genera. But the 

 more primitive types, or prototypes, which are of paramount interest 

 to the phylogenist, are still quite unknown. Species intermediate in 

 character between angiosperms and gymnosperms or pteridophytes are 

 also unknown.^ Walton (1940) has summarised the situation by saying 



^ For a recent discussion of this matter, see Walton (1953). In this review he suggests that such 

 evidence as we possess points to the pteridosperms as the most likely progenitors of the angio- 

 sperms. 



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