1426 A TEXTBOOK OF THEORETICAL BOTANY 



diverticula (Fig. 13 16) are also characteristic of Boraginaceae and some 

 other families. 



Fig. 1 3 16. — Fouquiera boiirragei. Mature 

 fi\"e-nucleate embryo sac with lateral 

 caecum. Epistase stippled, with median 

 channel leading towards micropyle. {After 

 Johamen:) 



In Stylidiiiiu it is the micropylar end of the sac which forms a diverti- 

 culum. There is a prominent endothelium around the sac, and the latter 

 spreads out over the upper end of this sheathing tissue and invades the 

 integument on all sides, presenting the appearance of a pileus of which the 

 rest of the embryo sac is the stipe. 



A general downward extension of the embryo sac at the antipodal end 

 occurs in a number of genera. In Casuarina, for example, it may begin to 

 form as early as the binucleate stage of the embryo sac, and it penetrates the 

 chalaza. Later, the primary endosperm nucleus and sometimes the anti- 

 podals may pass into it. A special peculiarity in this genus is that chalazal 

 haustoria are formed not only by the embryo sac but also by the numerous 

 sterile sporogenous cells, so that a remarkable complex of tubes is created. 



In Tropaeolum ma jus, the sac grows downwards into the massive tissue 

 of the chalaza, which it largely absorbs, forming a big caecum in which the 

 embryo develops (see p. 1473 for the suspensor haustoria of Tropaeolum). 

 The most exceptional chalazal outgrowths of the embryo sac, apart from 

 those formed by the endosperm, are to be found in Loranthaceae and 

 Santalaceae. We have already referred to the micropylar outgrowths of the 

 embryo sac in these families. Their chalazal development is sometimes 

 {e.g., Theshim) almost as remarkable. The extreme case is that of the related 

 genus Myzodendron (now placed in a separate family) where the chalazal 

 growth of the embryo sac penetrates the placenta and extends down into the 

 Horal receptacle, where it ends in a swelling among the diverging vascular 



