94 ALDRIDGE ON POLLEN. 



membranes, several have only one; on the other hand, there 

 are plants whose pollen have three or even more membranes. 

 In far the greater number of cases, however, there are pre- 

 sent two well-marked transparent membranes, and these act 

 in such a manner towards reagents, as to show that the 

 kinds with three and four membranes, are produced solely 

 by the doubling of one or two of these. As it is necessary to 

 employ more exact language, than that of the inner and 

 outer membrane, and so forth, the author designates the 

 outer membrane Exine, the inner one Inline, the inner fold 

 of the outer membrane Intexine, and the outer fold of the 

 inner membrane Exintine, 



The Inline, or inner m inbrane of the pollen, wh ch in all 

 cases covers the foviila, immediately shows itself everywhere 

 under the same circumstances : it remains as clear as water, 

 an uncoloured, extremely delicate, perfectly transparent 

 membrane; solution of iodine does not colour it; it becomes 

 destroyed by concentrated sulphuric acid, and is as permeable 

 as cellular tissue over its whole surface, although destitute of 

 pores. When there is only one membrane present in the 

 pollen, it has always manifestly the characters of the Inline, 

 and it is thus seen to be for more indispensable than the 

 Exine; it is, though unfrequently, found alone, but is always 

 sure to possess the same characters. Only in two cases has 

 the author seen it thickened, namely, in the Carex precox and 

 in Nerium, in which it presented regular and defined spots. 



Notwithstanding the little importance which the Exine, or 

 outer membrane, appears to have in the business of impreg- 

 nation, yet it is nearly to it alone, that the peculiar forms of 

 the pollen are indebted. It is evident from our knowledge 

 of the foviila and Inline, that it would be impossible, if the 

 Exine were removed, to distinguish the pollen of a great 

 number of plants ; while by the Exine alone we are enabled, 

 in many instances, to recognise from what plants the pollen 

 is taken. 



The Exi7ie is not like the Inline, one single membrane, 

 but possesses a compound structure. Mohl, in his work on 



