Mr. Hassall on the Structwe of the Pollen Granule, 103 



of the pollen grain, which produces pressure upon its contents, 

 and assists in forcing them out in the only direction in which 

 they can issue, namely, through the openings provided for 

 the purpose), partly by the granules imbibing a portion of 

 the stigmatic secretion, some of which gets between the outer 

 and inner membranes, producing pressure upon the latter, 

 and partly by the stimulating effect of that secretion. 



Pollen tubes are supposed to be elongated by means of the 

 distension or stretching of the inline. This opinion appears 

 to me to be erroneously entertained. It is difficult to con- 

 ceive any membrane to be endowed with such immense 

 powers of expansion as would be required for it to afford a 

 covering to the whole pollen tube, lengthened as it often is to 

 such a great extent in its passage through the style. Thus 

 in Criniim amabile the pollen tube has to pass through a space 

 18 75 times the diameter of the pollen grain before reaching 

 the ovarium. In Cleome spinosa 2719 times. In Oxyanthus 

 speciosa 4489 times the diameter of the pollen grain, and in 

 Colchicum autumnale 9000 times. Pollen tubes are frequently 

 met with of considerable length on ])arts of the flower distant 

 from the stigma, proving that the stigmatic fluid is not essen- 

 tial for their growth, although it doubtless favours it. 



I am of opinion, therefore, that pollen tubes are growths and 

 not mere elongations of the intine ; and that as they grow a 

 vacuum is formed within them, into which the fovilla passes. 

 At the same time, I do not mean to deny the fact of the in- 

 tine being extensible ; it is no doubt highly so, as proved by 

 the great length of the pollen tubes emitted under the action 

 of dilute nitric acid in some cases. 



Were pollen tubes but mere extensions of the intine, it 

 might with reason be expected that the size of the granules 

 and consequent extent of the intine would bear some relation 

 to the distance which they have to travel through the stig- 

 matic tissue before reaching the ovary, and that the greater 

 the distance the larger would be the pollen grains ; but no 

 such relation exists. 



I am at present inclined to regard the active molecular 

 bodies in the fovilla, which have attracted so much the inter- 

 est and attention of physiologists, as nothing more than par- 

 ticles of that fluid which have become separated into little 

 masses or globules. The fluid nature of these particles is 

 proved by their great diversity of form and size, by their 

 transparency, and by their traihng, or " tailing *," as they 



* " Tailing " is a term used by druggists to signify the elongation of 

 form which t]\o globules of impure quicksilver undergo in passing over 

 paper. 



