78 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



1935), Potentilla (Rutishauser, 1945), and some members of the Ru- 

 biaceae (Fagerlind, 1937), in which the megaspores give out lateral 

 tubes which subsequently begin to grow upward and are in a state 

 of competition with one another (Fig. 54). In Putoria and Galium 

 the tubes enter the tissues of the integument. In Rosularia two 

 to three megaspore tetrads may be formed, and as every megaspore 

 can give rise to a haustorium, the upper part of the nucellus often 

 shows quite a tangle of haustorial processes competing with one 

 another. 



Failure of Wall Formation during Meiosis. The functioning of 

 only one megaspore out of four is the commonest condition in 

 angiosperms, but in several plants only the first of the two meiotic 

 divisions is accompanied by wall formation, so that after the meiotic 

 divisions are over each of the dyad cells is binucleate. In others 

 wall formation fails altogether, or if walls are laid down they soon 

 disappear, so that all the four megaspore nuclei lie within the same 

 cell. Such differences in the mode of origin of the megaspores 

 form the basis for a classification of the types of embryo sacs in 

 angiosperms. 



References 



Afzelius, K. 1916. Zur Embryosackentwicklung der Orchideen. Svensk Bot. 



Tidskr. 10: 183-227. 

 . 1918. Zur Entwicklungsgeschichte der Gattung Gloriosa. Acta Horti 



Bergiani 6(3): 3-12. 

 . 1924. Embryologische und zytologische Studien in Senecio und ver- 



wandten Gattungen. Acta Horti Bergiani 8: 123-219. 

 Archibald, E. E. A. 1939. The development of the ovule and seed of jointed 



cactus (Opuntia aurantiaca Lindley). South African Jour. Sci. 36: 195-211. 

 Bambacioni-Mezzetti, V. 1935. Ricerche morfologiche sulle Lauraceae. Lo 



sviluppo dell'ovulo e dei sacchi pollinici nel Laurus nobilis L. Ann. di Bot. 



21(1): 1-19. 

 Banerji, I. 1940. A contribution to the life history of Costm speciosus Smith. 



Jour. Indian Bot. Soc. 19: 181-196. 

 Baranow, P. 1926. Zytologische und embryologische Untersuchungen an Drimi- 



opsis maculata Lindl. Ztschr. f. Zellforsch. und Mikros. Anat. 3: 131-148. 

 Benson, M. 1894. Contribution to the embryology of the Amentiferae. I. 



Trans. Linn. Soc. London, II. Bot. 3 : 409-424. 

 , Sanday, E., and Berridge, E. 1906. Contribution to the embryology of 



the Amentiferae. II. Carpinus betulus. Trans. Linn. Soc. London, II. Bot. 



7: 37-44. 

 Berg, 0. 1898. "Beitrag zur Kenntnis der Entwicklung des Embryosackes der 



Angiospermen." Diss. Erlangen. 



