248 EMBRYOGENESIS IN PLANTS 



formed. This cell persists for some time, but later it is subdivided into 

 small cells which yield the elements of the dermatogen and periblem. 

 On embryological grounds the woody Rosaceae are held to be the 

 more primitive. Malus, Fig. 61e, Pyrus and Crataegus, Fig. 61b, D, are 

 low in the evolutionary scale; Fragaria, Potentilla, Fig. 60s-u, and 

 Geum are high; and Spiraea, Fig. 60m-r, is the most advanced of all. 

 Rosa and Rubus occupy a middle position. In Lebegue's view, the 

 subdivisions of the Rosaceae might well be raised to higher rank, e.g. 

 to familial rank. In embryological comparisons, there is a greater 

 difference between Spiraea and Crataegus than between the Cruciferae 

 and Resedaceae, or between Saxifragaceae and Crassulaceae. Lebegue's 

 studies do not lead him to suppose that the Polycarpicae, based on the 

 type o^Myosurus, have any embryological community with the Rosaceae. 



In the Saxifragaceae, e.g. in S. caespitosa, Fig. 61f-h, both the 

 body of the embryo and the greater part of the suspensor are derived 

 from the terminal cell of the two-celled proembryo. The early embryo- 

 geny is characterised by a linear series of cells, and would be assigned 

 to Soueges second period. In this and related species. Fig. 61j, the 

 basal cell of the suspensor becomes enlarged and divided by transverse 

 and longitudinal or obliquely longitudinal walls. Peltiphyllum peltatum, 

 at one time included as a species of Saxifraga, is characterised by a 

 curiously irregular suspensor. Fig. 61k-n, and Penthorum sedoides. 

 Fig. 6I0-Q, by a distended basal suspensor cell. The former species is 

 considered by Lebegue to be exceptional among the Saxifragaceae in 

 the matter of its embryogeny, in that both the basal and terminal cells 

 divide by transverse walls; and unlike Saxifraga, almost the whole 

 of the suspensor in Peltiphyllum is derived from the basal cell. 

 The basal cell, which has first divided transversely, subsequently 

 divides again in a variable manner, yielding a multicellular and some- 

 what asymmetrical suspensor. Certain irregular embryonic develop- 

 ments in Peltiphyllum are like the normal developments found in Geum. 

 From these comparative studies Lebegue concludes that Peltiphyllum 

 peltatum 'appears to have evolved secondarily from the floral point of 

 view in the direction of the Saxifragaceae,' but, as the embryological 

 investigations show, its origins are profoundly different. 



With regard to the possible affinities of Rosaceae and Saxifragaceae, 

 Lebegue points out that although very close anatomical and floral 

 similarities are known (e.g. sections of the petioles of Saxifraga, 

 Spiraea and Alchemilla are practically identical, and Spiraea and 

 Astilbe show closely comparable floral development), the embryological 

 developments are quite different in the two families. Embryos of 

 Saxifragaceae belong to Soueges second period, those of Rosaceae to 

 the first period. The saxifrage embryonomic type, however, shows 



