THE OVULE 



265 



evidence of lost ancestral structure, of a very large sporogenous mass 

 enclosed in a heavy "megasporangium wall," originally vascular at its 

 base; the presence, in a few genera, of "tracheids" within this core was 

 believed supporting evidence for this interpretation. 



The massive nucellus is undoubtedly the primitive type (Fig. 103). It 

 is found chiefly in the more primitive families, some of which, like the 



Fig. 103. Inverted, anatropous ovule of Casuarina equisetifolia in longitudinal sec- 

 tion showing obturator x and course of pollen tube through obturator and chalaza 

 to developing embryo sac. (After Swamy.) 



Rosaceae, show stages in its reduction. The story of specialization of the 

 nucellus closely parallels that of the integuments; the massive nucellus 

 generally accompanies two integuments. The lower dicotyledons and the 

 monocotyledons are, in general, crassinucellate; the gamopetalous dicot- 

 yledons, tenuinucellate; but there are many exceptions in both groups. 

 Some of the exceptions are, perhaps, phylogenetically significant. The 

 presence of a massive nucellus in the gamopetalous Cucurbitaceae sup- 



