268 



BOTANY OF THE LIVING PLANT 



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

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



FIG. 212. 



A = Pollen-grains of Corn Cockle (Agrostemma) 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. 



Pollen grains germinated in a nutritive medium, 

 under a cover glaso, of which the margin is shown. 

 The tubes curve away from the margin, that is, 

 away from the supply of oxygen. (After Molisch.) 



FIG. 213, B. 

 Result of culture of pollen-tubes of 

 Narcissus Ta:etta in neighbourhood of 

 the style and stigma, in 7 per cent, 

 sugar after 16 hours. Diagrammatic. 

 (After Molisch.) ( x 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 



