THE MEGASPORANGIUM 



65 



the inner surface of the endothelium becomes cutinized and this 



layer seems to take up a protective instead of a nutritive function. 



Hypostase. 6 Just at the level of origin of the two integuments 



and directly below the embryo sac, there is often a well-defined but 



Fig. 46. Stages in the formation of integumentary tapetum in Lobelia trigona. 

 A, two-nucleate embryo sac with remains of degenerating megaspores; nucellar 

 epidermis still intact. B, Four-nucleate embryo sac, showing degeneration of 

 nucellar epidermis and formation of integumentary tapetum from inner layer of 

 integument. C, mature embryo sac bounded by cells of integumentary tapetum. 

 (After Kausik, 1935.) 



irregularly outlined group of nucellar cells which are usually poor 

 in cytoplasmic contents but have partially lignified or suberized 

 walls composed of a highly refractive material. Van Tieghem (1901), 



6 Dahlgren (1940) has reviewed the literature on the occurrence of the hypostase 

 in angiosperms and also recommended some changes of terminology. Reference 

 should be made to this paper for fuller information on the subject. 



