246 General Botany 



must be alternated in the field in order to secure abundant 

 production. 



Certain varieties of strawberries, which are usually propagated 

 by runners, must be alternated in culture in order to secure fruit, 

 because they are either self-sterile or produce no pollen. 



From the above statements it will be seen that cross-pollination 

 is an advantage to some plants. Many flowers have arrange- 

 ments that make self-pollination impossible. Often the anthers 

 do not shed their pollen at the time when the adjoining stigma 

 is in condition to receive it. The pollen may be shed either 

 before or after the ripening of the stigma. In such plants there 

 is httle possibiHty of the stigma's being pollinated from the 

 stamens of the same flower. So, as insects go from one flower 

 to another they transfer pollen from flowers in which the pollen 

 is ripe to flowers in which the stigmas are ripe. This favors 

 cross-pollination. 



It is exceedingly interesting to study the various other mech- 

 anisms that favor cross-pollination, but it should be done in the 

 field or with the flowers in hand. In the white lily the stigma 

 is out of reach of the insects when the pollen is shed. In other 

 plants the pistillate and staminate flowers may occur on different 

 individuals, or on different branches of the same plant. In 

 primroses and bluets the stigmas and stamens each have two 

 different lengths ; the flowers on one plant have long styles and 

 short stamens, while the flowers on another plant have short 

 styles and long stamens. 



The most remarkable cases of cross-pollination by insects are 

 those in which a particular species of insect is necessary for the 

 pollination of a plant. Such relations exist in the yuccas and in 

 some orchids. In the absence of the particular insect, polKnation 

 and seed production fail. Yuccas may be grown in our Northern 

 states, but in certain locaKties they fail to produce seeds because 

 the moth (Pronuba) needed to poUinate the flowers does not live 

 there. The Pronuba moth coflects pollen from the anthers of the 



