306 MORPHOLOGY OF THE ANGIOSPERMS 



its protoplasts are filled with starch and other food substances, and its 

 cell walls built up similarly of materials used in the development of 

 the embryo. Modifications of its usually simple structure are frequent, 

 especially in the outer layers, and the function of these outer layers is 

 probably diflFerent from that of the inner. An aleurone layer, filled with 

 aleurone grains, an outer layer of the endosperm in a few families, 

 especially the Gramineae, consists of cells usually larger than those 

 below. (Similar layers are found in other storage organs in which the 

 underlying cells are filled with starch.) 



Ruminate Endosperm 



Endosperm that is irregularly ridged and furrowed, often very deeply, 

 occurs in a few families, most of them primitive — the Eupomatiaceae, 

 Annonaceae, Myristicaceae, Rubiaceae, and some of the Palmae. The 

 method of origin of this condition is uncertain; it has been reported to 

 be, in some taxa, the result of the expansion of the developing endo- 

 sperm into furrows in the integument and, in other taxa, the result of 

 invaginations into the endosperm of outer tissues, nucellar or integu- 

 mentary. 



Nature of the Endosperm 



The history of the morphological nature of the endosperm goes back 

 to the middle of the nineteenth century and has been much discussed.* 

 The endosperm was early interpreted as a second, but abortive, embryo 

 — at first, because the union of the polar nuclei was considered fertiliza- 

 tion and, later, when union of the second male nucleus with the polar 

 nuclei was discovered. Still later, the endosperm was considered a 

 delayed, complex type of nutritive gametophytic tissue, not an abortive 

 structure resulting from a fertilization. The discovery that the endo- 

 sperm, in early stages, exists as markedly different types (cellular or 

 nuclear) and varies in nature and number of constituent cells has 

 greatly complicated interpretation of its nature. It can probably best be 

 termed — as it has been several times — a "new structure," one of complex 

 morphological nature, characteristic of the angiosperms only. Interpre- 

 tation of the endosperm parallels that of the extraordinary bisporic and 

 tetrasporic gametophytes, also "something new," found only in angio- 

 sperms. 



BIBLIOGRAPHY 



Artschwager, E.: Development of flowers and seed in the sugar beet, Jour. Agr. 

 Res., 34: 1-25, 1927. 



* An excellent detailed discussion is presented in P. Maheshwari's "An Introduc- 

 tion to the Embryology of Angiosperms," 1950, pp. 424-425. 



