FERTILIZATION OF FLOWERS 



The pollen or life-giving principle and its action 

 are thus described: "The pollen cells are differen- 

 tiated into an outer cuticular layer or Extine, and 

 an inner layer or Intine. The former is a firm mem- 

 brane, sometimes smooth, sometimes covered with 

 minute hairs, points or projections. It is generally 

 yellow, and often covered with an oily or viscid 

 secretion. The intine is thin, transparent and pos- 

 sesses great power of extension. The pollen grains 

 vary from 355 to ^ of an inch in diameter ; they are 

 usually ellipsoidal, but sometimes spherical, cylindrical 

 and even triangular and polyhedral. 



Within the pollen grains is a granular semifluid 

 protoplasmic matter, the Fovilla, together with some 

 oily particles, and at times starch. The Fovilla con- 

 tains small spherical granules about 3^ of an inch 

 in diameter, and a few larger, elongated corpuscles 

 which exhibit molecular movements. Moisture has 

 a marked effect, causing the pollen grains to swell up 

 by endosmose. If long continued, the extine becomes 

 so distended as to split, or open in places. The 

 intine is more distensible and is often forced through 

 the pores or the ruptures of the extine in sac-like 

 protrusions. Ultimately the inner membrane gives 

 way and the fovilla escapes, often in tube-like pro- 

 cesses. To guard against injury from the premature 

 admission of water to the pollen cells, the stamens 

 263 



