1836.] Some remarks on the development of Pollen. 735 



tanists, so far at least as may be judged of from their descriptions, 

 considering them to be solitary. M. Mohl remarks, that in monoco- 

 tyledones they are generally single, but that in dicotyledones the num- 

 ber is generally increased, and occasionally exceeds twenty. The 

 portion that is folded in has always a different structure from the 

 remainder, and is generally smooth and transparent ; and it rarely 

 ever happens that in cellular Pollen the inflected portion is itself 

 cellular. He supposes that in all cases the outer membrane forms a 

 perfectly closed sac, although in some Pollen the inflected portion has 

 more of a gelatinous than a membranous consistence, and is ruptured 

 by immersion in water. The apparent pores -visible in the Pollen of 

 many plants, M. Mohl states to be in all cases covered over by a thin 

 membrane — to the existence of this membrane over the larger pores 

 he speaks positively. In those cases in which the membrane covering 

 the pore separates in the form of an operculum, it is attenuated alone 

 along the margin of function or continuation with the remainder of the 

 outer coat. These statements accord with the author's views of the 

 nature of the supposed pores of cellular tissue ; views, however, which 

 have not been generally received, and which in the case of cellular 

 tissue are open to weighty objections. The inner membrane is i - epre- 

 sented as always having the same structure ; it is always completely 

 homogeneous, very thin, and hyaline, and always exists as a shut sac. 

 It is particularly remai - kable for the facility with which it absorbs 

 water ; this M. Mohl looks upon as a physical action and as attribu- 

 table to endosmosis. 



The production of tubes (boyaux) by immersion in water, (and 

 which are prolongations in all cases, except perhaps in Coniferce, of 

 the immediate covering of the fovilla) never takes place in those 

 Pollens, the outer membrane of which is perfectly closed, or the folds 

 or furrows of which are unprovided with pores. But in every Pollen 

 they are produced by stigmatic action. The action likewise exerted 

 on the grains by this portion of the female organ is more energetic 

 than that of water, producing twelve or fifteen times the diameter of 

 the grain ; while the longest, M. Mohl observed, produced by the ac- 

 tion of water only exceeded the grain in length once, or once and a 

 half. 



I may here state, that M. Mohl has understated the length of the 

 tubes arising from stigmatic action. The length will depend upon 

 the distance between the part of the stigma to which the grains are 

 applied and the foramen of the ovulum to which they have to be ap- 

 plied. Thus, for example, in Zea mays, the length of the tubes must 

 be enormous, since the style itself is about a span long. It may be 

 5 c 2 



