THE ANGIOSPERMAE 1461 



ends of the sac (Fig. 1341)- In this family the primary division wall in the 

 sac is transverse. The lower segment becomes the chalazal haustorium 

 directly, the upper segment divides again, and of its two portions the upper 

 develops the micropylar haustorium and the middle segment alone forms 

 the main endosperm. 



Fig. 1341. — Rhiimnthus major. Well-developed haustoria at 

 both microp\ lar and chalazal ends of the embryo sac. 

 {After Schwi'd.) 



A distinction must be drawn between truly haustorial outgrowths of the 

 endosperm and chalazal enlargements or caeca of the embryo sac itself, the 

 formation of which precedes endosperm development and which later become 

 filled with endosperm which either invades the caecum or develops there. 



In Physostegia (Labiatae) for example (F"ig. 1342), a large chalazal tube 

 is formed by the embryo sac, connected to the upper portion only by a 

 narrow junction. The endosperm is cellular and fills the chalazal lobe, the 

 main sac containing only a few cells. The chalazal lobe invades and destroys 

 nearly all the tissue of the integuments and later absorbs the main sac as 

 well. The embryo is pushed down by its suspensor into the chalazal endo- 

 sperm which, in turn, is nearly all absorbed by it. 



