REPRODUCTION 355 



its capacity of germinating. The period of viability depends on 

 the kind of pollen, as well as on the conditions in which it is being 

 preserved. The capacity of germination is preserved considerably 

 longer in a dry and cool place. Under favorable conditions it is 

 possible to preserve fertile pollen for several months. 



The pollen of cereals, and of some other plants, represents 

 another physiological type. The membranes of these pollen grains 

 are readily permeable to water. In dry air such pollen rapidly 

 desiccates and perishes. It must, therefore, be kept in a moist 

 atmosphere. But even under these conditions its life is of short 

 duration. 



The question as to what factors help to prolong the life of pollen 

 is also of great importance in practical work of cross-fertilizing 

 varieties that blossom at different times of the year. 



Having reached the ovule or, more exactly, the embryo sac, 

 the pollen tube ruptures and the two generative nuclei contained 

 in it effect fertilization. One of them unites with the nucleus 

 of the egg cell, the other with the nuclei of the primary endosperm 

 cell. When this has occurred, the process of the division of the 

 fertilized cells begins. The embryo develops from the fertilized 

 egg and the endosperm from what was the primary endosperm 

 cell. If fertilization has not taken place, this division usually does 

 not occur, except in cases of parthenogenesis, which will not be dis- 

 cussed here. Hence the assumption has been made that some 

 inhibiting factor prevents further development in a mature egg 

 cell, but is removed by fertilization. 



What this inhibition factor is and how it is removed by fertiliza- 

 tion are problems that have not been sufficiently studied. Experi- 

 ments with animals and with free-swimming egg cells of certain 

 algae have shown that artificial parthenogenesis, or the division of 

 non-fertilised egg cells, may be produced by means of various chem- 

 ical stimulants, especially certain mineral salts. This leads one to 

 the conclusion that in higher plants the generative nucleus exerts a 

 chemical stimulus which leads to further development of the egg cell. 



Fertilization not only results in the development of the embryo 

 and the endosperm, but also in a number of other changes, par- 

 ticularly in the ovule and other parts of the flower. The integu- 

 ments of the ovule commence to grow and finally form the seed 

 coat. The walls of the ovary develop and form the pericarp, while 

 parts of the pedicel also frequently are involved in its formation. 



