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 wistaria there 
are about 7000 pollen grains to every ovule, and in Indian 
corn, 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. 
