100 



Mr. Hassall oti Mold's mews 



cretion, it undergoes, as already mentioned, a singular change 

 of form, the cause of which has been explained : nor is this 

 change of form without purpose, for in it may be traced a very 

 beautiful little example of contrivance, it being designed to 

 facilitate the escape of the pollen tubes from the external co- 

 vering of the granule, which, in its dry state, for the sake of 

 security, imprisons them. The granule swells and contracts 

 in its long axis, the furrows become obliterated, and the mar- 

 gins separated as an inevitable consequence of this approx- 

 imation of the extremities of the granule, and a space is left 

 between them, through which there is a free and unimpeded 

 passage for the pollen tubes hitherto incarcerated for wise 

 purposes, but now that the fitting time has arrived, liberated 

 by the above simple but most effectual means. 



With reference to the apertures found in the external mem- 

 brane of numerous forms of pollen, which are either scattered 

 in no definite manner over the general surface of the granules 

 or are placed at certain angles of its extent, Mohl asks the fol- 

 lowing question: — "These pores, — are they really apertures, 

 or are they anything more than a very great thinness of the 

 external membrane, in certain points like the pores of cellu- 

 lar tissue ? It is a question which I cannot resolve for the 

 smallest of these pores ; but in pollen, in which they acquire 

 a more considerable size, I have been able to convince myself 

 in a manner the most evident, by the separation of the exter- 

 nal membrane, that these pores are not true openings, but are 

 closed by a fine membrane." 



This last statement I also consider to be untenable for the 

 same reasons given for regarding the bands as true solutions 

 of the continuity of the external membrane. 



From the brief exposition which has now been given of 

 Mohl's views of the structure of the external covering of pol- 

 len granule, it is manifest that he regards it as being in all 

 cases a compound organ, and as forming a shut sac, being 

 thinner in the situations of the bands and pores ; opinions 

 in which I cannot concur. 



Mohl thus concludes his account of the external membrane : 

 — " After the description given above of the cells, spines and 

 grains of the external membrane, it is clear that these parts 

 ought to be considered as the secreting organs and reservoirs 

 of the viscid oil ; from which it follows, that the secretion of 

 this oil ought not to be attributed to the papilliform emi- 

 nences covered by a prolongation of the external membrane 

 destitute of grains, and which are only found in some forms 

 of pollen, and that even w r hen the membrane, being furnished 

 with fine grains which cover the large papillary projections 



