ALDRIDGE ON POLLEN. 95 



the pollen, originated the opinion that the Exine was formed 

 out of undeveloped cells, cemented together by the gelatinous 

 matter which fills up the intervals between them. In the 

 beautiful work just mentioned, wherein Mohl has made 

 known his important inquiries concerning this gelatinous 

 matter, to which be has given the name of intercellular sub- 

 stance, there are three possible cases stated, by which the 

 formation of the Exine can be explained. Mohl declares 

 himself for the last, which is identical with the view already 

 mentioned, but admits that a more enlarged experience may 

 decide in favour of one of the other two cases. These other 

 suppositions are, first, that the Inline may form on its outer 

 surface the Exine; and secondly, that the same inner mem- 

 brane, may secrete, on its outer surface, an external skin, 

 which, adhering to the surrounding undeveloped cells, may 

 constitute the Exine. The first case Mohl dismisses wholly : 

 but the author states that, unfortunately, there are, in Mohl's 

 delineations, no sufficient proofs of the new view which he ad- 

 vances; that the drawings on which the decision of the 

 question rests, are either merely of portions, or too small for 

 constructing a generalization. The author considers that 

 his own observations have brought him much nearer to the 

 explanation of the origin of the Exine; but he finds that no 

 one of the already mentioned cases, uncomplicated and alone, ' 

 is sufficient, but that this interesting envelope presents a 

 great variety in the kinds of its formation. 



The first results of his researches, contradicts Mohl's be- 

 fore-mentioned opinion in favour of one of the former cases, 

 as it undoubtedly exemplifies the existence of a simple mem- 

 brane as the origin of the Exine. In the usual way, by mere 

 treatment with water, the isolated membrane, may occasion- 

 ally, but with difficulty be demonstrated in the Ruellia for- 

 mosa, and Barhria longifolia. We possess in concentrated 

 sulphuric acid, however, a means of showing with facility, in 

 many cases, a distinct membrane in the Exine^ together with 

 investitures of different kinds upon its surface. With the 

 Ruellia formosn and nniisophyUa, Beloperone oblongata, Eran- 



