1396 A TEXTBOOK OF THEORETICAL BOTANY 



Gymnosperms, with the exception of the Gnetales, have only one integu- 

 ment, and, except in Taxales, no cupules, unless the epimatium be inter- 

 preted as a vestigial cupule, or, more probably, the basal aril in Ginkgo and 

 the basal disc in Bennettitales. Either, therefore, the majority of Gymno- 

 sperms are descended from non-cupulate Pteridosperms or they have lost 

 their cupules, or alternatively the cupule has become united to the integu- 

 ment, forming the outer sarcotesta. Only in Gnetales and in the Angio- 

 sperms does the cupule appear to have remained as a second sheath around 

 the ovule. 



There are certainly difficulties in equating the outer integument with a 

 cupule. For example, in Sphaerostoma the abscission of the seed took place 

 above the cupule, leaving the latter attached to the plant, and this may have 

 been the case in other types also. Further, the cupule in Gnetopsis, Cala- 

 thospermiim and in Caytonia enclosed a group of ovules and was therefore 

 not an integument but an involucre. The first objection may be dismissed, 

 since it refers only to a relatively primitive stage. The second also is not of 

 serious weight, for these cases are exceptional and are not, in any case, 

 involved in the direct ancestry of the Angiosperms, so far as we know. The 

 further difficulty that many Angiosperms themselves have only one integu- 

 ment is not conclusive, for there is evidence in several cases that the uniteg- 

 minous condition may result from the abortion of one of the two and we may 

 reasonably consider the bitegminous condition as the original one. 



There are a number of indications to be gleaned among the Pterido- 

 sperms, the Taxales and the Gnetales, which serve to confirm the idea that the 

 cupule, like the seed-integument, is a compound structure and that it repre- 

 sents either a megasporophyll or a laminar portion of a megasporophyll. 

 The homology of the cupule with the outer integument, although the evi- 

 dence is far short of proof, remains, despite all difficulties, as the most 

 probable solution to the question of its morphological nature. It must be 

 realized, however, that its acceptance is bound to throw considerable doubt 

 on the foliar character of the carpel. It must also cause speculation on 

 whether the ancestors of the Angiosperms can have been related to the 

 non-cupulate Cycads. 



The Archesporium 



The origin of the embryo sac can be traced to the archesporium, but 

 the significance of this latter term is not quite precise, owing to many varia- 

 tions in ovular development. At a very early stage a sub-epidermal cell at 

 the apex of the young ovule may become conspicuous by its size and the 

 density of its contents and this is usually designated as the archesporial 

 cell. This cell usually divides, cutting oflF a primary parietal cell and forming 

 a primary sporogenous cell, from which the megaspore mother cell develops, 

 either directly or after further division of the sporogenous cell. Such a 

 sequence is closely parallel to that in the microsporangium and presents 

 little difficulty. 



