THE ANGIOSPERMAE 



1459 



A 



Fig. 1337. — Myoporiim serratinn. A, Mature embryo sac. B, Later stage showing 

 massive micropylar haustorium formed of many cells belonging to the 

 endosperm. (After Billings.) 



part of the haustorium throws out numerous tubular branches into the 

 tissue of the integuments. 



The micropylar haustorium, in certain families, is cut off from the main 

 endosperm at a certain stage of development by the ingrowth of the integu- 

 ment to form a conspicuous constriction called the isthmus. Examples are 

 common in Labiatae, Scrophulariaceae and Bignoniaceae. In other cases a 

 suberized separation layer may be formed. 



The chalasal haustoria (Fig. 1339) are most frequently traceable to the 

 antipodal segment which is cut off by the first transverse wall, in cellular 

 endosperm formation. This cell may remain undivided and grow out into 



