POLLINATION AND FERTILIZATION 163 



some effect on the viability and abundance of pollen. In 

 some cases very old trees produce pollen less viable than do 

 the younger individuals. (4) Weather conditions affect the 

 development and transporta- 

 tion of pollen. Sunshiny dry c =^ 

 weather which enables bees to Q^ ff$j& 

 work during blossoming time <^ ^ ^ 

 is favorable. Rain at the time K 



the blossoms open is unfa- 

 vorable, because it not Only p IG . 104. PoUen-grains of the Drake 



soaks the dry pollen which has almond - 



been shed by the anthers to a degree that injures it, but 

 frequently washes it away so that it is lost. Many flowers 

 have ways of protecting the anthers and pollen from rains, 

 such as by drooping so that the rain-drops will be shed, or 

 by curling up the stamens so that the anthers are drawn into 

 shelter. Hot dry winds are unfavorable on account of their 

 dessicating nature. Pollen-grains from most kinds of fruit- 

 trees, however, withstand considerable hot weather and 

 sunshine. Frost, when at all severe, may injure the anthers, 

 but in most cases the pistils are injured before the anthers 

 suffer. 



The relation of the pistil to pollination and fertilization 

 is important. (1) The stigma must be receptive. This 

 means that it must be of the proper age and condition to hold 

 the pollen-grain and to furnish the necessary stimulus to 

 cause it to germinate. The stigmas in flowers of common 

 fruit-trees usually remain receptive for several days. Re- 

 ceptive stigmas are neither too dry nor too green. They 

 usually take on a peculiar glistening appearance and pro- 

 duce a sirupy or sticky fluid on the surface which causes the 

 pollen-grains to stick tightly. The stigmatic secretions 

 seem to favor the germination of pollen-grains of certain 

 varieties and to prohibit the germination of those of other 

 sorts, but the chemical action involved is not at present well 



