94 SIZES AND GERMINATION OF POLLEN-GRAINS [CH. 



In Rhododendron, Arbutus, and other Ericaceae they 

 are united in tetrads of four grains each. 



As regards size, while the pollen-grains of Clematis, 

 Rhamnus Frangula, Ribes nigrum, &c., average about 20 jx 

 in diameter, they reach an average of 50 55 /a in Acer, 

 Ampelopsis, CraUegus, Prunus Avium, &c. ; but much 

 smaller and larger grains occur, e.g. those of some species 

 of Cucurbita, Mirabilis, Malva, &c., are particularly large, 

 up to as much as 250 fx in diameter. 



When the pollen-grain has been carried by wind, 

 water, insects or other agency to the viscid surface of 

 the stigma, in the case of Angiosperms, or to the micro- 

 pyle, in the case of Gymnosperms, the protoplasmic 

 contents of the vegetative cell absorb fluid and feed 

 on the sugary solution there exuded. 



That this is so can be proved by sowing the pollen- 

 grain in a drop of sugar solution under the microscope 

 and observing directly the changes to be described. 



The swollen vegetative cell soon bursts the exine, 

 either by irregularly rupturing it or by boring through 

 one of the specially thin places, and is protruded as a thin 

 tube the poUeu-tube the extremely delicate cell-wall 

 of which is au extension of the intine. Into the oily, well- 

 nourished protoplasm of this pollen-tube we now see the 

 vegetative nucleus wander, and pass towards the free- 

 growing end ; and soon afterwards the generative cell 

 is found to have its wall disorganised and its nucleus 

 following the vegetative nucleus into the pollen-tube. 

 When, as in Gymnosperms, there are other cells in the 

 pollen-grain, they usually remain behind and soon suffer 

 dissolution. 



The functions and fate of the vegetative and generative 

 cells are entirely different. The vegetative nucleus sooner 

 or later undergoes disorganisation and disappears in the 



