SEXUALITY OF PLANTS 



43 



V 



o 



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 

 Xarcissns 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 \vhich case the tube 

 grows down it, and does not 

 penetrate the tissue. In other FIG. 24. 



,1 11 c i Pollen-grains of the Corn Cockle 



CaSeS the CellS OI tlie Stigma (Agrostemma) germinating on the 



r 11 . -, stigma, and the pollen-tubes pone- 



are themselves perforated by the trating its tissue. ( After Strasbuvger.) 



