POLLINATION AND FERTILIZATION 



167 



mine which are the good pollen-grains in a sample by use of the 

 microscope, it is necessary first of all to know what is the nor- 

 mal size and shape of the grains. The normal appearance of 

 the grains differs in various species and 

 even in varieties. An examination of 

 the pollen-grains shown in the photo- 

 micrograph in Fig. 104 will make this 

 plain. It will be noticed that some of 

 the grains are large in size, others are 

 small, and some are medium and reg- 

 ular in outline. Whenever pollen- 

 grains depart from their usual size and 

 shape, it is reasonably certain that 



FIG. 106. Germination of 

 Black Heart sweet cherry 

 pollen. A , grains swelling ; 

 B, tube starting; C, later 

 stage of tube growth. 



they lack viability and will not germinate. Grains smaller 

 than normal or very much larger seldom germinate. Such 

 grains can be seen among the normal ones in the illustration. 



A microscopic examination of pol- 

 len-grains, then, will reveal those 

 that are "off-type" and will not 

 germinate. 



Although it is fairly certain that 

 the grains apparently normal will 

 germinate, this is not always the 

 case. On this account, it is neces- 

 sary to have accurate methods of 

 testing the viability of pollen- 

 grains. This is done by placing 

 them in artificial media. Pollen- 

 grains from most varieties of 

 fruit-trees germinate well in a 

 solution of sugar and water in which the sugar consti- 

 tutes 12 per cent of the mixture. That the pollen may 

 have as favorable conditions for germinating as possible, 

 it is mounted in the culture in a closed glass chamber 

 known as a Van Teighem cell, one of which is shown 



Fia. 107. Photomicrograph of Cal- 

 ifornia almond pollen-grains ger- 

 m i n a ti n g . These grains were 

 mounted eight hours before pho- 

 tographing. Notice the length 

 of the tubes and the high percent- 

 age of germination . 



