POLLINATION AND FERTILISATION 



303 



pollen of the Aconite, while the food which the flower offers may in 

 some measure react in determining the distribution of the bee. The 

 methods of transfer of the pollen may thus be varied. But the essential 

 feature of them all is the same, viz. the conveyance of an immobile 

 body essential for propagation from the pollen-sac, where it is pro- 

 duced, to the surface of the stigma 

 where it can germinate. (For numerous 

 instances of the methods of pollination, 

 as illustrated by various examples de- 

 scribed in detail, see Appendix A.) 



The germination of the pollen-grain 

 takes place normally on the stigma. 

 (Figs. 221, 222.) But it can be induced 

 in a nutritive medium, apart from the 

 stigma, such as a solution of cane sugar 

 of suitable strength. This makes it 

 possible to observe the origin and 

 behaviour of the pollen-tube. The 

 germination may be very rapid. From 

 fresh pollen of the Wild Hyacinth, 

 placed in a 7-10 p.c. solution, pollen- 

 tubes will be produced at a normal 

 summer temperature in about 15 min- 

 utes, and in an hour will have grown 

 to a length several times the diameter 

 of the grain. In some cases the struc- 

 ture of the wall of the grain does not FlG - •"• , , 



Ovary of Polygonum Convolvulus at 



indicate where the tube will be formed, time of fertilisation. /s=baseof ovary. 



... . /«>== wall of ovary. /i = funiculus. cha = 



But in Others its Origin IS determined chalaza. «u = nucellus. tfiz" = micropyle. 



,, T , „ r11 11 1 ii = inner integument, ie = outer mtegu- 



StruCturally. In the WlllOW-nerbS and ment. * = embryo-sac. ek = central fusion 



~ . . . r • • nucleus. ei = egg-apparatus. an = anti- 



Geraniums three points 01 origin are podai ceils. g=styie. n=stigma. p= 



. . t-, ,, pollen-grains. ps= pollen tubes. ( x 48.) 



present on each grain. frequently (After strasburger.) 



their number is greater, as in the Corn 



Cockle (Fig. 222, A) ; but of the 40 or 50 points of exit there seen, only 



one gives rise to a tube. A curious exception is seen in the Mallow, 



where numerous tubes emerge, which firmly anchor the grain. (Fig. 



222, B). 



The effect of external influences upon the growth of the tube can be 

 studied in culture-experiments. If grains be germinated in a suitable 

 solution under a cover-glass, the tubes, as they first issue, point indis- 

 criminately in all directions. But soon those near the margin turn 



