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. 

 Ji\ l v l\m \ From fresh Pollen of the wild 



\ I | f w 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 

 can be studied in such cultures. 

 For instance, if grains be ger- 

 minated under a cover-glass, 

 the tubes first issue pointing 

 indiscriminately in all direc- 

 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 



FIG. 21. 



Ovary of Polygonum during fertiliza- 

 tion, containing one straight ovule. fs= 

 base of ovary; /w = funiculus ; cha= 

 chalaza; nw=nucellus; mi=micropyle ; 

 ti=inner, and j> = outer integuments; 

 = embryo sac; eA;=central nucleus 

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

 cells; g= style; n= stigma; p= pollen - 

 grains; ps = pollen- tubes. ( x 48. After 

 Strasburger.) 



