THE OVULE 287 



its nature as an outgrowth of the sporophyll lamina is accepted, it is 

 morphologically unlike any other known ovule. As has often been sug- 

 gested, it has resemblances, in position and structure, to a fern sorus 

 and, in its eusporangiate development and embedded sporogenous tis- 

 sue, to the eusporangiate ferns. The sporangia of the Ophioglossaceae 

 especially suggest the angiosperm ovule in some characters: they are 

 modified distal parts of the fertile leaf, and tlie sporogenous cells are 

 embedded in sterile tissues that are not set apart as a "sporangium 

 wall"; the vascular supply is a single strand, which branches below the 

 sporogenous tissue; development of the sporangium is eusporangiate. 

 (These resemblances suggest possible relationship between eusporangi- 

 ate fern stock and an ancestral angiosperm stock.) 



Since the seeds of tlie pteridosperms have become well known, re- 

 semblances have been seen between the ovules and seeds of the angio- 

 sperms and these ancient structures. Emphasis has been placed on the 

 presence of two integuments in both pteridosperms and angiosperms. 

 But these resemblances seem superficial, consisting merely of protect- 

 ing structures, two in number, with vascular supplies. And the in- 

 teguments of the pteridosperms, like those of the cycads and Ginkgo, 

 have strong vascular supplies in definite positions, in contrast to those 

 of the angiosperms, which are weak and indefinite in form and posi- 

 tion. Under one interpretation of the integuments, the outer one con- 

 sists of bracts; only the inner is a true integument. The morphological 

 nature of the angiosperm ovule is still uncertain. 



The nucellus of the pteridosperms is structurally complex; that of the 

 angiosperm is simple. There seems to be little morphological resem- 

 blance between the ovules of the angiosperms and those of the pterido- 

 sperms, cycads, and Ginkgo. The ovule has probably developed inde- 

 pendently in several lines, with sporogenous tissue borne in ovules of 

 different natme; those of the pteridosperms, cycads, and Ginkgo repre- 

 sent, at least in part, lobes of the fertile leaf; those of the angiosperms, 

 parts of lesser rank, emergences. 



BIBLIOGRAPHY 



Bechtel, A. R.: The floral anatomy of the Urticales, Am. Jour. Bot., 8: 386-410, 



1921. 

 Benson, M.: Contributions to the embryology of the Amentiferae. I, Trans. Linn. 



Soc. London, set. 2, 3: 409-424, 1894. 

 Berlese, A. N.: Studi sulla forma, struttura e sviluppo del seme nelle Ampelidee, 



Malpighia, 6: 482-531, 1892. 

 Blaser, H. W.: Studies in the morphology of the Cyperaceae. I. Morphology of 



flowers. A. Scirpoid genera, Am. Jour. Bot., 28: 542-551, 1941. 

 Cooper, G. O.: Microsporogenesis and development of seed in Lobelia cardinalis, 



Bot. Gaz., 104: 72-81, 1942. 



