102 Mr. Hassall on the Structure of the Pollen Granule. 



pentapftylbim, Bessera elegans and Ly thrum Salicaria ; in this 

 last it is curious to notice, that while the pollen of the upper 

 stamina is green, that of the lower ones is of a bright yellow. 



Pollen tubes are prolongations of the intine, and are filled 

 with the fovilla, which passes into them by gravitation. They 

 vaiy much in number, size, and arrangement ; they issue 

 either through fissures or apertures in the extine, and where 

 there are three envelopes, through similar provisions in the 

 second also. Except in some few instances, I have not been 

 able to discover any provision in the extine for the escape of 

 the pollen tubes. These exceptions occur in the genera Canna, 

 Strelitzia, Roscoea and Hedychium, all of which belong to 

 closely-allied orders, and in which the extine, which is of great 

 thickness, forms a shut-sac. If water, and more quickly if 

 dilute nitric acid, be applied to the pollen, the granules are 

 seen to enlarge a little, and the extine to crack irregularly, 

 but often separating into three unequal pieces ; while the 

 intine, having undergone no change except a slight increase 

 of size, and still containing the fovilla, frequently disengages 

 itself from its envelope and floats away from it. This cracking 

 of the extine is assuredly the natural means by which the 

 pollen tubes are afforded an outlet ; and it is not necessary 

 that the intine should be dermded at any particular spot, for 

 wherever it is so it possesses the power of elongation, or ra- 

 ther growth. In one or two cases the apertures are provided 

 with valves, as in the different species of Passijlora, as first 

 noticed by Purkinje. Fritzsche has described one valve in 

 the grasses, two in the nettle, four in the orange, and six in 

 the primrose. Although I have searched with care for these 

 valves, I have not been able to detect any trace of them, and am 

 strongly inclined to deny their existence in any of these plants. 



The primary form of the pollen of the orange precludes the 

 possibility of its being furnished with regular valves, the 

 pollen tubes issuing through fissures and not circular aper- 

 tures 5 while had a valve been present in the grasses, I think 

 I must have detected it in the pollen of the Zea Mays, 

 which is of a very large size, and in which the apertures for 

 the escape of the pollen tubes are very visible. The nettle I 

 believe to possess three pollen tubes. 



The emission of the pollen tubes is produced, artificially, 

 by the action of the mineral acids and water, and naturally, 

 partly by the rarefaction of the contents of each pollen gra- 

 nule by the sun's heat * (the rarefied fovilla distends the shell 



* The effect of heat upon the pollen has occasionally been demonstrated 

 when I have been examining it under a strong reflected light of the sun, 

 by the emission of the pollen tubes, and even rupture of the intine. 



