1398 



A TEXTBOOK OF THEORETICAL BOTANY 



and unity is generally established by the abortion of all but one of the super- 

 numerary structures (see Fig. 1283). 



So various are the processes of development in the ovule that one may 

 doubt whether the term archesporium has any value in this connection. 

 Schnarf gets over the difficulty by applying the term only to the megaspore 

 mother cell, but this of course is quite different from its application in the 

 microsporangium. 



l^-\ 



1 



Fig. i2q6. — Rumex crispus. A, Nucellus showing archesporial cell 

 terminatiriK an axial row. B, Two megaspore mother cells each 

 with a pair of parietal cells. C, Mother cell with primary parietal 

 cell. D, T-shaped group of megaspores, parietal cell divided 

 anticlinally. (After Dudgeon.) 



The parietal cell, when one is formed, becomes part of the tissue of the 

 nucellus. It rarely remains undivided and its division may be anticlinal or 

 periclinal or in both directions (Fig. 1296). 



We have said that this happens commonly in crassinucellate species 

 and indeed this multiplication of parietal cells is often, though not always, 

 responsible for the crassinucellate condition, as the nucellar epidermis only 

 occasionally divides periclinally, e.g., in Rosaceae. In some species the 

 parietal cells divide repeatedly, and strikingly regular files of cells may be 



