THE OVULE 283 



come elongate, filling the micropylar canal in D; as a last step, the 

 nucellar cells and the micropylar canal are lost. 



Nature of the Axgiosperm Ovule 



The angiosperm ovule has been interpreted morphologically in many 

 ways. It was long considered a bud, because, teratologically, a shoot 

 sometimes develops from it; because it seemed to resemble an axis with 

 a growing point and leaves; because it was borne on placentae which 

 were considered axial; because, from it, as a seed, came the axis of a 

 new plant. It has more commonly been considered of foliar nature — a 

 sporophyll, a leaflet, a lobe of a leaf (simple or three-part), the tip of 

 a leaf, an emergence, a trichome. It has been defined as a "mono- 

 sporangiate megasporophyll"; under this interpretation, it has been sug- 

 gested that "phylogenetic connections seem to be possible between 

 some angiosperms and the Lycopodiales through the conifers and 

 Gnetales." All difficulties in interpreting the ovule were supposedly 

 avoided by calling it a "new structure, an organ stii generis" "a com- 

 plete and independent organ inserted on the stem." In most of these 

 interpretations, it is recognized that, fundamentally, it is a structure 

 that bears sporogenous tissue in which megaspores develop. In the 

 twentieth century, it has nearly always been called a megasporangium 

 or megasporangium with accessory tissues. But there have been occa- 

 sional returns to the bud theory, and the interpretation of the integu- 

 ments as leaves. The position of the ovule — on the surface of the carpel 

 — and its form and structure have formed the basis for the theory that 

 it is a sorus, with or without an indusium. 



Similarly, the parts of the ovule have been variously interpreted. The 

 nucellus has been considered the growing point of the bud, the "core" 

 of a sorus, the axis of the bud, the megasporangium wall. The integu- 

 ments have been called the leaves or scales of a bud, the indusium of 

 a sorus, an "involucre," lobes of the sporangium wall, "protection about 

 the sporangium." In an interpretation of the ovule as a leaf, the inner 

 integument is identified as a terminal leaflet, the outer integument as 

 two lateral leaflets. 



Homology of the ovule with the stamen has been suggested; the 

 nucellus and integuments were interpreted as equivalent to the anther, 

 the funicle to the filament. Homology of the ovule with the fern sorus 

 has been perhaps the most common interpretation. In this theory, the 

 many sporangia of the sorus are considered reduced to one, and the 

 sporogenous tissue of this surviving sporangium reduced to one spore 

 mother cell. (Stages in reduction of the spore mother cells are common 

 in living ferns.) The integuments represent the indusium of the sorus. 



Many of these concepts of the nature of the ovule were shown to be 



