94 



Mr. Ilassall on Mohl's views 



nous are not really at all cellular, and that the oil which is 

 found in the cells does not escape by visible apertures, but 

 transudes through the walls of the cells." 



In making the above statements J)r. Mohl and M. Bron- 

 gniart are both in part in error. Mohl errs in asserting, in op- 

 position to M. Brongniart, that the kinds of pollen which are 

 spinous are not at the same time cellular. In the paper al- 

 ready referred to, I have declared that the majority, if not all 

 (and I am inclined to think that all are so), of pollen granules 

 which are furnished with spines have a cellular extine ; and 

 that on each cell, and probably formed by it, either a spine is 

 situated, or where this is absent, its walls are perforated by 

 an aperture, which aperture is intended for the passage of the 

 pollen tubes. On the other hand, M. Brongniart is doubtless 

 at fault in the belief that he has found, in each cell of the re- 

 ticulated external membrane, an excretory duct under the 

 form either of a hair, spine, or aperture. In the pollen of 

 Lilium, Pancratium, and many other plants whose external 

 membrane is most manifestly cellular, neither apertures nor 

 any other visible outlets, save the longitudinal furrow for the 

 pollen tube, are discernible ; in these cases, therefore, the oily 

 matter must escape (if it be secreted by the extine) by exu- 

 dation from the general surface of the membrane, and not 

 through apparent openings. The spines themselves are for 

 the most part open at their extremities, and may, where they 

 are present, perform the office attributed to them by M. Bron- 

 gniart. M. Brongniart refers to the pollen of Mirabilis Jalapa 

 in support of his views. It is very doubtful whether the ex- 

 tine of this be cellular or not, and although it is indeed per- 

 forated with numerous apertures, these all undoubtedly are 

 designed to permit the escape of the pollen tubes. 



That the external membrane of most pollen which is hispid 

 is really cellular, any one may satisfy himself by a careful ex- 

 amination of the pollen of Ipomcea purpurea, Cynara Scolymus, 

 Pavonia prcemorsa, Hibiscus annuus, &c. 



Mohl thus speaks of the granular appearance sometimes 

 presented by the extine, and as his observations on this point 

 are highly interesting, I need make no apology for introducing 

 them at length in his own words. iS In many cases the cel- 

 lules of the external membrane become contracted to such a 

 point, that one knows not whether we ought still to give them 

 the name of cells, or whether the membrane is only made up 

 of very large grains. In these instances one might still admit 

 that the granular aspect of the membrane arose from the fact 

 of its being composed of extremely small cells, although our 

 means of observation permitted us not to recognise these 



