of the Structure of the Pollen Granule. 103 



external membrane does really form a closed cell. The ap- 

 pearance of a thin film stretching across the apertures may 

 be sometimes produced by the watery medium in which they 

 are generally viewed. 



The following remarks of Mohl are somewhat opposed to the 

 opinion expressed by him, that the internal membrane should 

 always be regarded as a homogeneous structure, thin and 

 transparent as water. " In all pollen in which the external 

 membrane is altogether smooth, in all those which possess 

 but a single plait, in a great part of those which have three 

 plaits, in those which have spiral plaits, and in a great many 

 of those which are provided with pores, the internal mem- 

 brane forms a cell altogether spherical or ellipsoidal. On the 

 contrary, in pollen which, like that of the Onagrarice, have 

 a pore at their three truncated angles, or as that of the Dip- 

 sacece, have one upon their three sides, or as among very many 

 species of the Solanea', Gentiaiice, Synantheracete, Umbelliferte, 

 Apocynacece, Papilionacece, &c, have one upon their three 

 longitudinal furrows, or as among many species of the Bora- 

 ginacece, have a great number, the structure of the internal 

 membrane is not absolutely similar in all its parts ; but there 

 exist often, even in the dry granule, papillae, which are as lit- 

 tle blind appendages of the internal membrane. The line of 

 union of these blind appendages with the cell formed by the 

 internal membrane is ordinarily well marked ; sometimes, as 

 in the Onagrarice, it presents a thickening, which gives to it 

 the appearance of a white band/' I have already explained, 

 that the papillae, wherever met with, are formed in one of two 

 ways, either, where there are but two membranes, by the pro- 

 trusion of the internal through fissures or pores in the exter- 

 nal, or, where there are more than two membranes, by the 

 protrusion of the one next the external, called by Fritzsche, 

 intextine. When produced in the latter way they are always 

 permanent, that is, they are to be met with in every form and 

 condition of the granule, as they are only in the Onagrarice, 

 and perhaps Umbellifercs, of all the examples of their pre- 

 sence cited by Mohl ; but when they are formed in the first 

 way, as they are in all the other instances referred to by Mohl, 

 they are only to be observed where the pollen has undergone 

 either a partial or complete change of form, and are to be re- 

 garded as the commencement of pollen tubes just emerging 

 through the fissures in the external membrane, and formed 

 by an elongation or growth of the internal with which they 

 are continuous, without any line of demarcation similar to 

 that described by Mohl. Such therefore is their origin in all 

 cases where there are but two coatings to the pollen granule. 



