EMBRYOGENESIS IN FLOWERING PLANTS 245 



admit of the two families being placed side by side, as if they were 

 derived from a common ancestral stock? Have they community of 

 origin with the Rhoeadales, all stemming from the Ranales, as 

 Hutchinson (1926) has suggested? Hutchinson regards the Archichla- 

 mydeae as diverging in two main lines, the Ranales giving rise to 

 predominantly herbaceous orders and the Magnoliales to predominantly 

 arborescent ones. He considers, however, that these two main phyla 

 'have often converged on similar lines and are associated in the same 

 family, as is probably the case in Rosaceae' (1926, p. 5). In his phylo- 

 genetic scheme, Rosales are derived from Magnoliales, and Rhoeadales 

 and Saxifragales from Ranales. Wettstein (1911), on the other hand, 

 views the Rosales as a comprehensive order, comprising Crassulaceae, 

 Saxifragaceae, Rosaceae and other families. Numerous affinities, in 

 fact, have been suggested for both Rosaceae and Saxifragaceae. 

 Because of the many close similitudes, vegatative as well as floral, the 

 separation of the two families is difficult. 



When comparative studies of embryos are undertaken in the 

 interests of taxonomy, it is essential to define criteria of primitiveness 

 and of advancement. Following Milne-Edwards (1867) and Soueges, 

 Lebegue maintains that in a group of related species the most highly 

 evolved is that which shows greatest differentiation and maximal 

 division of labour, i.e. greatest individualisation (Soueges, 1936). At 

 the embryonic level, the most evolved species will be that in which the 

 embryo shows the earliest differentiation ; in other words, evolutionary 

 advancement is based on what has been described as the principle of 

 precocity. 



Embryologically, the Cruciferae appear to stem from the primitive 

 Myosurus type in the Ranunculaceae, Fig. 56. In its possession of a 

 greatly swollen basal cell, Capsella bursa pastoris is quite exceptional in 

 the Cruciferae, all other investigated species having elongated filament- 

 ous suspensors. Fig. 60a-j. The principle of precocity is illustrated by the 

 fact that the hypophysis is formed in the fifth embryonic cell generation 

 in Cardamine, Fig. 60a-c, the fourth in Sisymbrium and the third in 

 Draba verna (the most evolved type). The systematic grouping based on 

 embryological criteria does not coincide with that based on the usual 

 floral criteria. Lebegue's explanation of this is that floral construction 

 is a considerably less ancient manifestation of evolution than the 

 primary development of organisms as revealed by the embryogeny. 



The Resedaceae, Fig. 60k, l, resemble the Cruciferae in their 

 embryology but are more precocious. Indeed, the Cruciferous type of 

 embryology is found in a considerable number of polypetalous and 

 gamopetalous dicotyledons, the relationship thus indicated being in 

 close agreement with Hutchinson's phylogenetic scheme. 



