THE AXGIOSPERMAE 1397 



All the hypodermal cells of the nucellus may, however, be potentially 

 archesporial and two or more may develop equally, giving a multicellular 

 archesporium, which again recalls the condition in the microsporangium. 

 The number of cells involved is not always clear, for gradations may 

 occur between obviously archesporial and unchanged nucellar cells. Multi- 

 cellular archesporia are widespread in the Rosaceae, the Ranunculaceae 

 (where the number of cells involved is very irregular) and in the " Amenti- 

 ferae ". Isolated cases are found in many other families, but ver\' rarely in 

 Monocotyledons. Some of the cells composing these massive archesporia 

 are probablv not hypodermal, and this is almost certainly true of the large 

 central mass of sporogenous cells which occurs in Casuamia and in certain 

 species of Carpiniis, Oiiercus and Jiiglans. 



The massive archesporia of these genera might be interpreted as another 

 of the primitive features of the Amentiferae, but single-celled archesporia 

 are found in other genera of the same group, notably in Betula and Alnus. 



Difficulties of interpretation arise in two opposite ways: either by the 

 intercalation of a phase of development between the appearance of the 

 apparent archesporial cell and the differentiation of the sporogenous cell; 

 or by the suppression of one or more of the normal stages of development 

 whereby the distinction of the various cells involved is lost. 



The first condition is shown by Arisaema (Araceae) where the primary 

 archesporial cell divides anticlinally into three or four cells, each of which 

 behaves as an archesporial cell. Each cell, however, divides repeatedly 

 and forms a row of cells, the apical cell of which again enlarges and functions 

 as a sporogenous cell, turning indeed, owing to one of the puzzling sup- 

 pressions which are frequent, directly into an embryo sac. What, in this 

 case, is to be called the true archesporium ? 



There is a sequence of events, which may be regarded as normal or 

 fundamental, on which we may try to base a judgment. An archesporial cell 

 divides to cut off a parietal cell. The remaining cell then becomes the 

 primary sporogenous cell, which is also the megaspore mother cell. This 

 divides meiotically and produces four megaspores. So far there is a close 

 parallelism with microspore formation, but in the megasporangium three of 

 the potential megaspores abort and only one develops to form the embryo 

 sac. 



There are manv departures from this sequence. No parietal cell may 

 be cut off from the archesporial cell, whereby the differentiation of a dis- 

 tinct sporogenous cell is lost. This occurs most frequently in tenuinu- 

 cellate ovules, while in crassinucellate species parietal cells are generally 

 formed, though many exceptions are known in both cases. The archesporial 

 cell itself may function as the megaspore mother cell, as we have just 

 pointed out. It may also function, as may also the sporogenous cell where 

 one is differentiated, as the embryo sac mother cell, making only one step 

 from archesporium to embryo sac. Where the archesporium is multi- 

 cellular there may, therefore, be a production of several megaspore mother 

 cells, or of several embryo sacs. 1 his multiplicity is usually only temporary 



