472 Mr. Valentine on the Development of the Theca, 



although it follows the law above stated, is very anomalous in other respects. 

 It is a horizontal membranous ring, formed between the opercular membrane 

 and the horizontal portion of the columellar membrane. This situation pre- 

 cludes the possibility of its having been formed by the separation of the 

 internal layer of the operculum. 



It is now necessary to describe the development of the sporules. The 

 period at Avhich this process commences is rather uncertain ; most probably it 

 begins at the time of the separation of the columellar membrane from the 

 columella. Dr. Hooker in the Flora Londinensis, vol. iv. fasciculus i., under 

 " Diphyscium foliosum," has this passage : " It would be curious to ascertain, 

 were it possible, what becomes of the substance forming the cellules in the 

 early state ; for the ripe seeds are quite free and unconnected, yet not sepa- 

 rated by any membranous substance such as the walls of the cellules appear 

 to have been formed of. On the contrary, they occupy a cavity around the 

 columella, which appears evidently to be nothing more than the remains of 

 the cellular and pulpy substances in which the seeds have not been per- 

 fected, and which, as we may consequently expect, when dry, shrinks up into 

 an angular axis or columella, as it is called by Hedwig and other muscolo- 

 gists." Mr. Brown, in the Linnean Transactions, vol. x. p. 315, says, in 

 speaking of what he names the placentation of the seeds : " That in some 

 cases the seeds may be formed in a much greater portion of the columella 

 than in others : and it is even not improbable that in certain cases its whole 

 substance may be converted into seeds : or, to speak more accurately, that it 

 may produce seeds even to the centre, and that the cells in which they were 

 probably formed may be reabsorbed." From these passages it appears that 

 their authors consider the seeds or sporules to be formed in the columella, 

 and even of its very substance. Dr. Greville and Mr. Arnott, in their Memoir, 

 object to the opinion that the columella, in the ripe theca, is merely a con- 

 traction of the debris of the sporular mass, from the regularity of figure which 

 it often retains, and also from its being sometimes tubular ; a fact which, they 

 say, is irreconcileable with the notion of contraction. My observations have 

 convinced me that the sporules are formed from a gummy fluid, which is 

 secreted either by the columella or columellar membrane (most probably by 

 both), and that this secretion becomes cellular by the gradual separation of 



