THE ANGIOSPERMAE 1401 



tendency seems to be towards suppression and, as the parietal tissue does 

 not seem to have any important function in the ovule, this is reasonably 

 to be expected. 



A similar difficulty arises with regard to the pluricellular archesporium, 

 which is sometimes regarded as a primitive character, in contrast with the 

 usual unicellular type. Pluricellular archesporia occur regularly, or at 

 least with some frequency, in families at all levels of advancement, e.g., 

 Cruciferae, Rosaceae, Papilionaceae, Cornaceae, Umbelliferae, Rubiaceae 

 and Compositae, and one must reject the idea of a primitive character having 

 been retained in all these cases. Furthermore, the later families in this list 

 are tenuinucellate and have no parietal cells, two secondary characters that 

 are incompatible with the retention of a primitive archesporium. The pluri- 

 cellular archesporium cannot therefore be regarded as having a phylogenetic 

 significance. 



Although the term tapetum is sometimes applied to the parietal tissue, 

 it does not undergo the same kind of changes which are shown by the tape- 

 tum in the anther. The term should be avoided, ahhough it is morpholo- 

 gically justified, for the reason just stated and also because it has been 

 applied to the " nutritive jacket " or endothelium around the enlarging 

 embryo sac in many species, which is generally formed by the inner in- 

 tegument, as we have already described (p. 1383), and sometimes also from 

 the nucellar epidermis. This may function as a tapetum but it is in no way 

 homologous with that in the anther. 



Some disintegration of cells around the embryo sac always occurs, but 

 to a very variable extent. In some cases all the parietal tissue is involved and 

 in many tenuinucellate ovules even the nucellar epidermis disappears by 

 the time the embryo sac is mature. No general comparative study of these 

 changes seems to have been made. 



Megaspore Formation 



The megaspore mother cell develops either directly from the primary 

 sporogenous cell or, if the latter divides, or if there are several sporogenous 

 cells, then normally one of them becomes the mother cell. At this point 

 we reach the end of the life-history of the sporophyte, for the first division 

 of the mother cell is the reduction division and the nuclei thereafter are 

 haploid. 



The procedure which is called " typical ", i.e. that which probably 

 occurs in the majority of Angiosperms, consists of two successive divisions 

 transversely to the long axis of the nucellus giving a row of four similar 

 megaspores, of which the lowest, namely that nearest to the chalaza, 

 generally develops into the embryo sac. Although a tetrad is usually formed 

 there is much greater variability in megasporogenesis than in microsporo- 

 genesis. The extent of the variation in this procedure may be judged from 

 the following details of conditions in the Araceae. In this family a distinct 

 sporogenous cell is not formed, but it is represented by the primary arche- 



