FERTILIZATION 167 



195. Number of Pollen Grains to Each Ovule. — Only 

 one pollen tube is necessary to fertilize each ovule, but so 

 many pollen grains are lost that plants produce many 

 more of them than of ovules. The ratio, however, varies 

 greatly. In the night-blooming cereus there are about 

 250,000 pollen grains for 30,000 ovules, or rather more 

 than 8 to 1, while in the common garden wisteria there 

 are about 7000 pollen grains to every ovule, and in Indian 

 com, the cone-bearing evergreens, and a multitude of other 

 plants, many times more than 7000 to 1. These differences 

 depend upon the mode in which the pollen is carried from 

 the stamens to the pistil. 



