SEX AND HEREDITY 



syngamy in Flowering Plants. They will now be con- 

 sidered in detail. 



The germination of the pollen-grain takes place normally 



on the stigma, and the course of 

 the pollen-tube can be followed, 

 as in Fig. 21. But germination 

 can also be induced in a nutri- 

 tive medium apart from the 

 stigma, such as a solution of 

 cane sugar of suitable strength. 

 This makes it possible to ob- 

 serve the origin and behaviour 

 of the pollen- tube. The ger- 

 mination may be very rapid. 

 From fresh Pollen of the wild 

 Hyacinth placed in 7-10 p.c. 

 solution pollen-tubes will be 

 formed in about fifteen minutes, 

 and in an hour will have grown 

 to a length several times the 

 diameter of the grain. The 

 effect of external influences 

 upon the growth of the tube 

 FlG - 2I - can be studied in such cultures. 



Ovary ot Polygonum during fertiliza- 

 tion, containing one straight ovule. fs= por instance, if grains be 

 base of ovary; //< = fuuiculus ; cha = 



chalaza; = nucellus; ;*' = micropyle ; Urinated Ulldcr a CO 

 tt = inner, and ;> = outer integuments; 



= embryo sac; ft=cential nucleus {he tubes filSt isSUC 

 of sac; ei = egg-apparatus ; an = antipodal 



ceiis; g=st y ie; n= stigma; />=poik-n- indiscriminately in all direc- 



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



su-asburgcr.) tions. But soon those near the 



margin turn inwards from the free air, that is, they grow 

 away from the source of oxygen (Fig. 22). If a similar 

 culture be prepared, and a piece of the style and stigma 

 of the same species be introduced, the tubes curve towards 



