50 PISTILS AND STAMENS 



with dew or rain. Apple pollen and the pollen of many other 

 fruit trees, although not destroyed when immersed in water, will 

 not function nearly so well and for this reason rain or dew on 

 a stigma may hinder the pollen in its work. 



The pollen of many plants is quite sensitive to a low tempera- 

 ture, showing a decrease in vitality when exposed for a few hours 

 to a temperature only a little below freezing. Pollen, if not in- 

 jured by cold, will not germinate while the temperature is low. 

 In the Apple, Pear, Pliun, Peach, and Cherry ^ a temperature of 

 — 1°C. has been found to interfere with the proper functioning 

 of the pollen by injuring the stigmas and preventing the ger- 

 mination of the pollen. Cold during the blooming period may 

 be responsible for much failure in fruit-setting. 



The Results of Pollination. — The most immediate as well as 

 the most important result of pollination is the fertilization of the 

 egg cell and primary endosperm nucleus. Through the process 

 of fertilization the pollen stimulates the ovule and other struc- 

 tures to develop, and transmits factors by means of which the 

 embryo and the endosperm of the seed inherit the characters of 

 the pollen parent. 



The importance of the stimulative effect of fertilization in 

 the development of a seed is obvious, for unless fertilization 

 occurs, the egg, endosperm nucleus, and other parts of the ovule 

 rarely develop into their respective seed structures, and con- 

 sequently the ovule either disappears or remains as a small 

 withered body as often seen in fruits. Furthermore, the devel- 

 opment of fruit depends upon the stimulative effect of fertiliza- 

 tion, as shown in case of fruit trees. Melons, Alfalfa, etc., in which 

 the flowers wither and fall from the plant unless fertihzation 

 occurs in some of the ovules. There are, however, a few instances 

 in which the stimulative effect of fertihzation is not necessary, 

 as in seedless Oranges, seedless Persimmons, Bananas, and a 

 few other fruits known as parthenocarpic fruits, which develop, 

 although no fertilization occurs. There are a few plants, the 

 Dandelion being a common one, in which ovules develop into 

 seeds parthenogenetically, that is, without fertilization, but such 

 plants as well as those that develop seedless fruits are exceptional. 

 In most cases our harvest of seed and fruit depends upon the 

 stimulative effect of fertihzation. 



' Research BuUetin 4, Wisconsin Agr. Exp. Sta., 1909. 



