268 



BOTANY OF THE LIVING PLANT 



cut surface (Fig. 213, 5). They are positively chemotropic. Their be- 

 haviour on the stigma, where they take a course in close contact 



^VvV,' 



Fig. 212. 

 ^ ^ Pollen-grains of Corn Cockle (A grostemma) showing numerous possible points 

 of origin for pollen-tubes, but only one tube, which penetrates at once a papillar cell 

 of the stigma. B = a similar condition in Mallow (Malva), but here numerous 

 small tubes are formed for attachment. (After Strasburger.) ( x 120.) 



with the moist cell-walls, shows that they are also positively hydro- 

 tropic (p. 128). These three factors are effective in deciding the 



Fig. 213, A- 



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 oxj^gen. (After MoHsch.) 



Fig. 213, B. 

 Result of culture of pollen-tubes of 

 Narcissus Tazetta in neighbourhood of 

 the style and stigma, in 7 per cent, 

 sugar after 16 hours. Diagrammatic. 

 (After Molisch.) ( >< 10.) 



course of the tube when it germinates normally upon the stigma. 

 They lead it to apply itself closely to the surface cells. 



On germination the contents of the pollen-grain pass over into 



