FERTILISATION. 257 



structure: hollow within, they are filled with a very 

 granulous protoplasm, with many oil globules ; their 

 solid coat is formed in two layers, an inner and an 

 outer, called intine and extine. The pollen grains 

 are developed within the anther, by constant seg- 

 mentation of the contents of the latter. It appears 

 that, when the grains become isolated from each 

 other, the nucleus of each one divides into two un- 

 equal parts, the smaller of which attaches itself to the 

 wall of the granule. When the pollen grain is placed 

 upon the moist stigma, so that nourishment is given 

 to it, the interior parts grow, and burst the exterior 

 enveloping coat (the extine), at points where it is 

 curiously thinned down, while the intine is corre- 

 spondingly thickened— the blunt angles of the pollen 

 of epilobium, e.g. (D, Fig. 57), have a delicate pellicle 

 of extine only, but here the intine [ti) is extremely 

 strong and dense ; the latter, therefore, remains un- 

 ruptured, and holds the protruding interior, elastically 

 extending with it (E), so that, under favourable con- 

 ditions, a tube of extraordinary length is developed, 

 through which the larger nucleus at last passes. 

 Pollen grains, placed in soda-water sweetened with 

 a little sugar or honey, will, if kept in a genial 

 temperature, grow under the microscope, giving some 

 such forms as shown at C, Fig. 49. The Y ma y often 

 be found in the stomach of the bee with a tube 

 partly developed, while in the later food of the 

 larvae this phenomenon is quite common. 



During the curious sequences represented by 

 ABC, Fig. 50, the stigmas, usually covered by 

 little papillose bodies, coat their surfaces by secretion 



