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



readily from the extiue, to which it does not appear to be 

 organically united, as on the application of water it is seen 

 to curl up and fall off the granules. It is also met with in 

 Centaurium, Scolymus, Dipsacus, Scabiosa, and some allied 

 genera ; in these instances it sometimes has an attachment to 

 the extine. 



The extine, or protective membrane of the pollen grain, on 

 which its shape depends, is of much greater substance than 

 the intine, which is so fine, that rarely can any appreciable 

 thickness of it be detected. It is seldom, if ever, a simple ho- 

 mogeneous membrane, being most generally formed by the 

 apposition of a number of cellules held together by " organic 

 mucus,^' which give to it a reticulated appearance, very obvious 

 in many of the LUiacece, as well as in a great variety of other 

 plants. In most hispid pollen, where the same reticulated 

 structure may be traced, from the centre of each cell of the net- 

 work, either one of the numerous spines which cover this form 

 of pollen grain may be seen to arise, or, Avhere this is absent, 

 an aperture for the passage of a pollen tube will be apparent. 



If this view of the structure of the extine be correct, then 

 must it consist of two layers, as each cell forms a shut-sac. 



By some observers the external tunic of the pollen grain is 

 said to present a granular structure. It often presents a gra- 

 nular cqipearance^ which is deceptive, and which I have found 

 to arise from the circumstance of the particles in the fovilla 

 being visible through their transparent coverings. 



It has been matter of doubt whether the extine be exten- 

 sible or not ; that it really is so in some cases to a consider- 

 able extent, is apparent from the following fact, the only one 

 with which I am acquainted which distinctly proves it. The 

 surface of the pollen granule of Canna inclica is covered with 

 numerous points, having their free extremities perforated. 

 When the pollen is immersed in water the spines disappear, 

 leaving small apertures on the surface of the now smooth ex- 

 tine ; each granule at the same time having enlarged to about 

 thrice its original size. This change occurs with greater ra- 

 pidity if a dilute mineral acid be used instead of water. All 

 kinds of pollen appear somewhat larger in water, but this de- 

 pends upon the magnifying poAver of that liquid. The intine 

 does not expand in an equal proportion with the extine ; but 

 being pressed upon equally on all sides by the water which 

 is imbibed by the extine, is forced inwards, and a large space 

 filled with fluid is left between the two membranes. 



After diligent and repeated search, I have at last succeeded 

 in making out the plano-convex bodies [Zunschenorpern) de- 

 scribed by Fritzsche as being pai'ticularly visible in some Mai- 



