SEXUALITY OF PLANTS 



43 



Pollen-grains germinated in a nutritive 

 medium, under a cover glass, of which 

 the margin is shown. The tubes curve 

 away from the margin, that is, away from 

 the supply of oxygen. (After Molisch.) 



FIG. 23. 



Result of culture of pollen tubes of 

 Xarcissus Tazetta, in the neighbour- 

 hood of the style and stigma, in 7 p.c. 

 sugar, after sixteen hours ; diagram- 

 matic. ( - : 10. After Molisch.) 



it, and especially towards the cut surface (Fig. 23). They 



also tend when grown exposed to the air to follow a 



moist surface. These three fac- 



tors all influence the growth of 



the tube in the same way, when 



pollen germinates on the stigma. 



Turning away from free air, ad- 



hering to the moist surface, and 



attracted by the tissue of the 



style, the tube penetrates it. 



Sometimes there is a channel 



down the style filled with mucil- 



age ; in which case the tube 



grows down it, and does not 



penetrate the tissue. In other 



,, ,, r ., , . 



CaSeS tlie CellS Ol tile Stigma 



, r , , . ., 



are themselves perforated by the 



FlG - 



Pollen-grains of the Corn Cockle 

 (Agrostemma) germinal! g on the 

 stigma, and the pollen-tubes pene- 

 trating its tissue. ( After Strasburger.) 



