1400 A TEXTBOOK OF THEORETICAL BOTANY 



divides once periclinally into a parietal and a primary sporogenous cell. 

 The former may be further divided, the latter develops without division 

 into a mother cell. 



Type III. There is usually only a single sub-epidermal, archesporial cell, 

 which divides once periclinally into a parietal cell and a sporogenous cell. 

 The former undergoes further divisions, the latter becomes directly the 

 mother cell. 



Type IV. There is only a single sub-epidermal, archesporial cell. This 

 divides periclinally into a parietal cell and a sporogenous cell. Both of these 

 undergo further divisions. One of the innermost cells becomes the mother 

 cell, though other, outer cells may show a tendency to develop into mother 

 cells, which usually remain abortive. 



Type V. There is only a single sub-epidermal, archesporial cell, which 

 does not divide but passes directly into a mother cell. 



Type VI. There is a complex of mother cells. Parietal cells are not 

 formed. The complex of mother cells may arise from a single archesporial 

 cell but this is uncertain. 



The parietal cells may divide both periclinally and anticlinally, chiefly 

 the former, up to three successive divisions. This produces quite a massive 

 tissue, which naturally forces the sporogenous cells downwards, away from 

 the surface. In the Rosaceae there is not only the formation of regular 

 periclinal files of parietal cells but the epidermal cells also divide 

 and add to the tissue complex (Fig. 1297). The greatest development is in 

 the Malvaceae, where as a rule there are 7-12 rows of cells, and in Althaea 

 sulphurea as many as 18 rows of cells separate the mother cell from the sur- 

 face of the nucellus, all derived by the division of a primary parietal cell. 

 (Type IV above.) In this family some of the accessory mother cells men- 

 tioned above, which usually remain abortive, may come from cells of the 

 parietal complex. There is, therefore, not the clear-cut functional differen- 

 tiation which normally accompanies the division of parietal from sporo- 

 genous cells. Similar occurrences have been observed in Anthurium, 

 Butomus and Riippia. 



The parietal cell very rarely remains undivided ; there are usually 

 several rows of cells formed and their subsequent histories may be very 

 different, but we never find among them the high degree of histological 

 differentiation which is shown by the parietal layers in the anther. 



It is scarcely possible to say definitely whether the development of a 

 parietal tissue in the ovule is a primitive feature or not. The strongest 

 argument in its favour is the almost universal absence of parietal cells in 

 the Metachlamydeae, where it is associated with the tenuinucellate condi- 

 tion. Among Archichlamydeae the presence of parietal cells is inconstant, 

 sometimes even within a single genus, and their suppression is unrelated to 

 the size of the nucellus. It is noteworthy that they are absent from all the 

 Ranunculaceae with the exception of Helleboriis and Thalictrum, in which 

 there is a single, undivided cell. Apart from this family, the evolutionary 



