1382 A TEXTBOOK OF THEORETICAL BOTANY 



or by abortion. The former has without doubt occurred in unitegminous 

 Rosaceae, as Juel has shown, and this is probably true also of Betulaceae, 

 Ranunculaceae and some Papilionaceae. Abortion of the inner integument 

 is indicated in Salicaceae, where Popuhts species may possess a small and 

 late-developing inner integument, while Salix has consistently only one. 





• t.. 



«r 





A- 



m -f 



%* 

 "^^f 



1 



I1 



Fig. 12S4. Hypei uiini (uidrusdenium. Young ovule, with nucellus and two 

 integuments, curving to assume the anatropous position. 



Complete absence of an integument is now only accepted for the Santa- 

 laceae (apart from the anomalous case of Crinum, mentioned above). For- 

 merly a considerable number of families and genera were held to possess 

 naked ovules, but it has been shown that earlier observers were deceived 

 by appearances and that in these plants it is, in fact, the nucellus which has 

 disappeared, and the single integument, enclosing the embryo sac, was 

 mistaken for it. This does not apply to the Loranthaceae and Balano- 

 phoraceae (see p. 1386), in which there are not only no integuments, but the 

 whole ovular structure has been reduced or suppressed. 



Supernumerary coverings to the ovule are found in a few cases. A 

 third integument or aril arises from the base of the ovule and overgrows 

 the others but remains distinct from them. During the ripening of the seed 

 it does not form part of the seed coat but develops as an outer, detached 

 covering which usually has a distinctive colouring and texture. A different 

 sort of outer covering is sometimes provided by an outgrowth from the lip 



