POLLINATION AND STERILITY 285 



vision (reduction division) , become the microspores or pollen- 

 grains. Usually the anthers are partially developed prior 

 to winter of the season before the blossom opens. If the 

 development has gone sufficiently far in the autumn so' 

 that the first division of the cells in spring is the reduction 

 division, then they are in the pollen mother cell stage at 

 that time. However, they may or may not have reached 

 that point in the autumn. When growth is resumed in the 

 spring, cell division becomes active and pollen-grains are 

 eventually formed. The pollen-grains are unicellular and 

 are known botanically as microspores, each one finally devel- 

 oping two male gametes. The pollen-grain has a covering of 

 two layers, the outer of which in some plants is frequently 

 oily, gelatinous, or possessing minute projections useful in aid- 

 ing in its distribution. This latter adaptation, however, is not 

 encountered with common fruits. When "ripe" or mature, 

 the anthers dehisce or expose the pollen-grains and on trans- 

 fer to a receptive stigma, the latter germinate, as later de- 

 scribed. If conditions are favorable and the two tissues are 

 "congenial," fertiUzation takes place. 



A similar development occurs with grape pollen as for pol- 

 len in general. It has been shown that a fundamental de- 

 fect may occur in its development which accounts for ste- 

 rility in the grape as is described later. 



255. Germination of the pollen. — The condition of the 

 stigma when it becomes receptive or at the time of pollination 

 should next receive attention. The stigmatic surface just 

 prior to the receptive period has a velvety papillose appear- 

 ance which is readily distinguished from the moist often 

 viscid condition when it is receptive. When the pollen- 

 grains reach the receptive stigma, they are surrounded by 

 the stigmatic fluid which has been secreted and in which 

 the pollen gemiinates. 



It has been shown that sunshine had little or no effect on 



