PHYSIOLOGICAL PROCESSES DURING FLOWERING 439 



effect on fruit setting. The harm done by heavy rains and fogs 

 is due to the fact that pollen subjected to protracted wetting 

 with water will burst before germination. Besides, sugar and 

 other substances necessary for the germination of pollen may be 

 washed off the stigma. 



Since pollen is a male gametophyte in a resting condition, it 

 will preserve for a certain period its ability to germinate. The 

 period of viability depends both on the specific properties of the 

 plant and on the conditions of storage. The ability to germinate 

 is preserved considerably longer in a dry, cool chamber. Under 

 favorable conditions, for instance, when placed in desiccators, 

 fertile pollen of some plants can be preserved for several months. 

 The pollen of cereals and of some other plants represents a 

 different physiological type. The membranes of these pollen 

 grains are readily permeable to water. In dry air, such pollen 

 rapidly desiccates and dies. It must, therefore, be kept in a 

 moist atmosphere. But even under these conditions, its life 

 is of short duration. The question of what factors help to 

 prolong the life of pollen is also of great importance in the 

 practical w^ork of cross-fertilizing varieties that bloom 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 fertiUzation. One of them unites with the egg 

 nucleus, the other with the nuclei of the primary endosperm. 

 When this has occurred, the division of the fertilized nuclei 

 begins. The embryo develops from the fertilized egg, and the 

 endosperm from the triple fusion nucleus. If fertilization has not 

 taken place, this division usually does not occur, except in cases 

 of parthenogenesis, which will not be discussed here. Hence 

 the assumption has been made that some inhibiting factor 

 prevents further development in a mature egg cell, which is 

 removed by fertilization. 



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

 ization are problems that have not been sufficiently studied. 

 Experiments with animals and with free-swimming egg cells 

 of certain algae have shown that artificial parthenogenesis, i.e., 

 the division of nonfertihzed egg cells, may be produced by means 

 of various chemical stimulants, especially certain mineral salts. 

 This leads one to the assumption that in higher plants the 



