POLLINATION "AND FERTILISATION 351 



Fucus, of which, it will be remembered, all are func- 

 tional in some species, while in other species four, 

 two, or only one are functional eggs. 



Pollination. When the pollen is ripe the anthers 

 open in various ways. Very commonly a longitudinal 

 split runs down the side of the anther between the two 

 pollen sacs whose walls break through along this line 

 and fold back, exposing the pollen grains (Fig. 58, B). 

 The split is actually caused by the contraction on 

 drying up of the wall of the sac, whose cell walls have 

 fibrous thickenings, the tension which develops eventually 

 tearing apart the weak unthickened tissue between the 

 two sacs on each side of the anther. 



The pollen grains are conveyed to the stigma in 

 various ways. Sometimes the stigma touches the 

 opened anther, rubbing off the pollen (self-pollination) ; 

 but very often the pollen is conveyed, generally 

 by insects or the wind, to the stigma of another 

 flower of the same species (cross-pollination). The 

 stigma is papillose and sticky, or hairy, so that 

 the grains, which are often rough coated, readily 

 stick to it. 



Germination of the Pollen Grains. Fertilisation. 

 The cells of the stigma secrete a sugary solution which 

 is absorbed by the grains lying among them, and these 

 germinate, a tube (pollen tube) being pushed out from 

 the thin spot on the wall. Into this tube pass first the 

 pollen grain nucleus, and then the independent cell, 

 which divides to form the two male gametes during 

 the growth of the tube (Fig. 58, D, E). The tube 

 grows into the stigma (Fig. 58, F) and down through 

 the loose tissue of the style till its tip reaches the cavity 

 of the ovary. The growing tip is positively chemotropic 

 to sugar, or some other substance secreted by the tissue 



