102 Mr. H assail on MohVs views 



the same error as M. Meyen, considers the action by which 

 the acids occasion the pollen to burst and coagulate its con- 

 tents as of the same nature as the action of water and the 

 stigmatic secretion. M. Fritzsche does in truth distinguish 

 between natural tubes and tubes produced artificially^ and he 

 refers to the last those which are formed in consequence of 

 immersion in an acid, while he ranges under the first denomi- 

 nation those which are developed from the effect of moisture 

 upon the stigma or upon the corolla when the grains of pollen 

 fall there ; but he attributes to them the same origin, in ad- 

 mitting that they are formed by the mucilaginous part of the 

 fovilla, and that they issue by breaking through the internal 

 membrane of the pollen granule. This certainly occurs in 

 the formation of the tubes which he considers as produced 

 artificially; but the natural tubes differ absolutely, in that 

 they are immediate prolongations of the internal membrane, 

 of which we may be convinced by detaching the external 

 membrane. It is indeed true, as M. Fritzsche says, that 

 these tubes pierce a membrane ; but that membrane is not the 

 internal, it is the external, which is not pierced with holes, as 

 M. Fritzsche thinks he has observed, but, as I have above 

 explained, lines the pores, sometimes under the form of a fine 

 membrane, sometimes under that of an operculum.^^ 



These observations of Mohl are in part only correct. 

 Fritzsche is doubtless in error in supposing that the pollen 

 tube which is to convey the fovilla through the tissue of the 

 stigma and style to the ovary, is formed by the coagulation 

 and hardening of the surface of the cylindrical mass, and not, 

 as it really is, by the continued growth of a portion of the in- 

 ternal membrane ; but I cannot see that there is any essential 

 difference between the mode of action of water or the stigmatic 

 secretion in the production of pollen tubes, and that of any of 

 the dilute mineral acids ; the only difference which I can de- 

 tect being, that the latter, from the force with which it causes 

 the principle of endosmosis to operate, most frequently, but 

 not invariably, occasions the rupture of the internal mem- 

 brane and consequent effusion of its contents, a thing which 

 the former does sometimes, but much less frequently. If di- 

 lute sulphuric acid be used to the pollen of Scabiosa cauca- 

 sica, true pollen tubes will be emitted covered by the internal 

 mem'brane, and differing in no way from those the result of 

 natural processes. The difference is not in the modus ope- 

 randi, but in the effects of the agents. 



If my views of the nature of the folds and apertures be cor- 

 rect, no membrane is ruptured, not even the extine, as stated 

 by Mohl, save in the comparatively rare cases in which the 



