362 



POLLEN 



is perforated, or rendered thin, by one or more pores or pits 

 (Fig. 211, C, 0.), through one of which the pollen tube is sub- 

 sequently protruded. In this respect the pollen of Angiosperms 

 is more specialised than that of Gymnosperms, where the outer 

 layer of the membrane is merely burst open in germination. 

 As the pollen matures the waUs of the cells composing the 



~^^ G. 



Fig. 



211. — Diverse types of pollen. A, Willow-herb {Epilobiuni), with 

 three thin spots in the outer coat. B, Hollyhock (AlthcBa), with 

 numerous pores. C, Small part of same enlarged, showing the pores (0.). 

 D, Pollen-tetrads, a-c, of Bird's Nest Orchid (Neottia nidus avis), 

 d, of Bilberrj' {Vaccinium inyrlillus). E, ^ixisV (Mimnlus moschatiis). 

 F, PoUinium of Orchis niorio. G, Pollen grain of Gourd (Cucurbita 

 pepo), showing the embossed lids {/.) of the outer coat, which become 

 pushed off by the growing pollen tubes. (A-D after Sachs ; d and F 

 after Wettstein ; E after Mohl ; G after Schacht.) 



fibrous layer become strengthened hy a network of thickening 

 bars, and simnltaneousljf the cells loose their living contents 

 (Fig. 212, en.) ; many of the cells in the region of the connective, 

 moreover, develop similar thickenings. At this stage the two 

 pollen sacs, in each half of the anther, become confluent through 

 the dr\''ing up of the intervening thin-walled septa, and it is this 

 same process of desiccation that ultimately leads to the setting 



