CHAPTEK IV 



THE MEGASPORANGIUM 



THE megasporangium, just as the microsporangium, is hy- 

 podermal in origin, being derived from the outermost layer of 

 the periblem. Although strictly an embedded organ, it becomes 

 superficially very distinct through the growth of cells beneath 

 and around it, the whole structure constituting the ovule. Al- 

 though in a strict morphological sense the ovule is more than a 

 megasporangium, just as the ordinary anther is more than four 

 microsporangia, the distinction is theoretical rather than prac- 

 tical, and in the following discussion will be disregarded. 



Although the carpels are concerned in forming all or a part 

 of the encasement of the ovules, they do not always produce 

 them. Just as in the case of the microsporangia, and in the 

 same sense, there are cauline as well as foliar ovules, and the 

 former are much more common than are cauline microsporan- 

 gia. This is probably due to the fact that the ovules are much 

 more closely associated with the growing point of the axis than 

 are the microsporangia, and hence the former are much more 

 likely to be borne by a lateral member than the latter. 



Cauline ovules are either terminal or lateral. In the former 

 case the apex of the axis becomes the nucellus, as is probably 

 true of most orthotropous ovules, certainly in_Naias, Zanni- 

 chellia, Lilaea, Piperaceae, Juglandaceae, Polygonaceae, and 

 others. In the case of laterally cauline ovules apical growth of 

 the axis may be checked, so that the lateral ovule appears to 

 arise in a terminal position from the bottom of the sporangial 

 chamber, as among the Compositae ; or the apical growth may 

 be continued into the sporangial chamber as a dome-shaped 

 (Anagallis arvensis} or columnar (Spergularia mibra, Amaran- 

 taceae, etc.) structure upon which numerous lateral ovules are 

 46 



