276 MORPHOLOGY OF THE ANGIOSPERMS 



the sporogenous cells. The function of these tracheids and their mor- 

 phological significance are unknown. Resemblance in form and structure 

 to the elaters of liverworts has been suggested. Correlation of these 

 tracheids with the central core of a massive nucellus, like that of 

 Casuarina, might be used to support the hypothesis that the massive 

 nucellus is a surviving remnant of an ancestral, stalked sporangium, 

 with elaters among the spores, but this suggestion is farfetched. The 

 histological nature of these "tracheids" needs critical study; they may 

 be neither tracheids nor elaters — in the strangely sclerenchymatous 

 flower of Casuarina, perhaps merely supporting cells. 



The Hypostace. In the ovules of many taxa, there is a platelike or cup- 

 shaped area of specialized cells below or around the base of the embryo 

 sac, long ago termed the hypostace (Figs. 115A and 138A). Histo- 

 logically, it consists usually of sclerenchymatous cells and is considered 

 supportive or protective in nature, but layers of secretory and apparently 

 nutritive cells in this position have also been called a hypostace. The 

 hypostace has been confused with the area of branching vascular tissue 

 at the base of the integuments. The term epistace has been applied to 

 layers or groups of thick-walled cells at the top of the nucellus. A 

 thickened cuplike tip of the integuments around the micropyle has 

 been called an operculum. These three terms have no apparent mor- 

 phological significance. 



A projection of nucellar or chalazal tissues, not always structiually 

 distinct from the hypostace, may extend into the base of the embryo sac. 

 These projections are sometimes prominent features of embr3/o-sac 

 structure, as in Zostera, Ribes. Various terms have been given to them: 

 podium, pedestal, postament. The term podium is sometimes restricted 

 to projections that are nucellar. Morphologically, these projections may 

 be remnants of basal nucellar tissue, where disorganization accompany- 

 ing embryo-sac growth has progressed on the sides more rapidly than 

 below, or they may be an upgrowth of chalazal tissue. The antipodal 

 nuclei or cells are sometimes sunken in the top of this projection. 

 Embryo sacs with these basal intrusions are found in many families 

 throughout the angiosperm system. 



Reduction in the Ovule 



Reduction of the integuments to one — by union of the two or loss of 

 the inner one — and of the nucellus to a single layer of cells above the 

 sporogenous tissue is common in the more specialized families. Greater 

 reduction, accompanied by loss of the integuments and the nucellus, is 

 characteristic of taxa with greatly reduced body habit — parasites, 

 saprophytes, and some hydrophytes and epiphytes. In extreme reduc- 

 tion, the ovule itself is lost, and the archesporial tissue is borne in the 



