19 1 9] SCHERTZ—SCROPHULARIA 443 



Tapetal layer 



The tapetal layer appears first as a ring or band of cells surround- 

 ing the nucellus (fig. 12). The cells of the tapetal layer divide only 

 anticlinally, and later come to form a layer of cells completely sur- 

 rounding the embryo sac. The cells of this layer lie with their 

 longer axes perpendicular to the embryo sac, while the cells of the 

 nucellus layer within them lie with their longer axes parallel with 

 the embryo sac. This layer of cells is more persistent than the 

 nucellus, and is observed even after most of the endosperm is 

 formed. When the embryo approaches maturity no traces remain 

 of the tapetal layer. These cells possess large nuclei and are well 

 filled with protoplasmic contents. In all of the species of this 

 group as figured by Balicka-Iwanowski, this tapetal layer is 

 shown. The extent to which the tapetal layer incloses the embryo 

 sac varies. In S. marylandica the tapetum almost surrounds the 

 embryo sac, or at least to a greater extent than it does in any of 

 the species as figured by Balicka-Iwanowski. 



Nutritive tissue 



In some species, as Scrophularia vernalis and Scoparia dulcis, 

 the micropylar end of the tapetal layer is composed of many cells 

 and is called nutritive tissue. In Digitalis purpurea, Linaria 

 Cymbalaria, and Torenia Deli no such tissue is found. At the 

 chalazal end, where tht tapetum leaves off, the nutritive tissue 

 completely encircles the lower extremity of the embryo sac in 

 Scrophularia marylandica (fig. 22) and in Linaria Cymbalaria. 

 In Scoparia dulcis no chalazal or micropylar nutritive tissue is 

 observed, and in Torenia Deli this tissue is very scanty. The 

 cell walls of the nutritive tissue stain deeply with Delafield's 

 haematoxylin, and the nuclei are less prominent than they are in 

 the surrounding tissues. This seems to indicate that these cells are 

 chemically different from the surrounding tissues. 



Embryo sac 



The embryo sac develops from the functioning megaspore, and 

 comes to occupy the space formerly occupied by the three poten- 

 tial megaspores. The "nucelle" layer of Balicka-Iwanowski, 



