POLLINATION AND FERTILISATION 



267 



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 produced, 

 to the surface of the stigma where it 

 can germinate. (For further details on 

 pollination see Appendix A.) 



The germination of the pollen-grain 

 takes place normally on the stigma. 

 (Figs. 211,212.) 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 

 indicate where the tube will be formed. 

 But in others its origin is determined 

 structurally. In the Willow-herbs and 

 Geraniums three points of origin are 

 present on each grain. Frequently their 

 number is greater, as in the Corn Cockle 

 (Fig. 212, 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. 212, B.) 



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

 be studied in the cultures. 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 

 inwards from the source of free Oxygen : they are negatively aero- 

 tropic (Fig. 213, A). If a similar culture be prepared, and a piece of 

 the style and stigma of the same species be introduced with the 

 pollen-grains, the tubes curve towards it, and especially towards the 



FIG. 211. 



Ovary of Polygonum Convolvulus at 

 time of fertilisation. /s = base of ovary. 

 fw= wall of ovary. ff=funiculus. cha= 

 chalaza. nw=nucellus. nw'=micropyle. 

 M=inner integument, ie outer integu- 

 ment. e= embryo-sac. ek= central fusion 

 nucleus. '=egg-apparatus. fln=anti- 

 podal cells. g=style. =stigma. p = 

 pollen-grains. />s=pollen tubes. ( x 48. 

 (After Strasburger.) 



