272 MORPHOLOGY OF THE ANGIOSPERMS 



nucellus. In tenuinucellate ovules, there are often no parietal cells; the 

 spore mother cells lie directly below the epidermis. In small ovules, 

 the few parietal cells may degenerate as the sporogenous tissue ma- 

 tures. Massive nucelli may be built up largely by the parietal cells, as in 

 Sassafras, Cahjcanthus, and many of the Ranunculaceae, or by both 

 hypodermal and epidermal initials, as in the Rosaceae, Urticaceae, 

 Vitaceae. Sequence in development of ovules on the placenta is ap- 

 parently correlated with the ontogeny of the carpel. 



Time of Ovule Development. Development of the ovule is usually 

 closely correlated with that of the flower, especially the stamen, but 

 there are marked departures from this. Ovules may not have begun to 

 develop at time of pollination, as in Qiiercus and Fagus; in some of the 

 orchids, the ovules are not mature for weeks or months after pollina- 

 tion. In Quercus (black oaks), pollination occurs a year before the 

 ovule is mature; similar conditions obtain in some of the conifers and 

 cycads; in the Araucariaceae, there is no evidence of the ovule at the 

 time of pollination, not even when the pollen tube reaches the position 

 where the ovule will develop. 



Anatomy of the Ovule 



Ovule Traces and Venation. The distribution of vascular tissues in the 

 ovules of the lower seed plants has received considerable attention, be- 

 cause of its possible phylogenetic significance. The absence of vascular 

 bundles in the ovules of the conifers — with the exception of Torreija 

 and Cephalotaxus — together with the supposed general absence of vas- 

 cular tissue in the ovule integuments of most angiosperms, led to the 

 theory that the vascular system of angiosperm ovules is greatly re- 

 duced, as a part of reduction from ancient gymnosperm seeds. But in- 

 tegumentary vascular systems are now recognized as fairly character- 

 istic of angiosperms. These systems seem to be in process of reduction; 

 the smaller and more delicate ovules of many families have none, and 

 vestigial bundles have been described in others. Vestigial ovules usu- 

 ally have traces, and the placental position of lost ovules is often 

 marked by the presence of their traces within the carpellary tissues. 

 These traces are a well-marked feature in the anatomy of follicles in 

 many families (Fig. 42B) and are part of the evidence that the follicle 

 is the primitive type of carpel, rather than the achene, as is urged under 

 the peltate theory of the nature of the carpel. 



The ovule-trace supply in angiosperms is commonly a single bundle, 

 derived at various places on the vein system of the carpel lamina. In a 

 few taxa, the ovule has two traces — Magnolia, Michelia, Schisandra, 

 Austrohailei/a; more than two traces are rare. The origin of the traces 

 varies with placentation type — from ventral veins, the laminar mesh- 

 work of small veins, the midrib vein (rarely); the place of origin is 



