FERTILIZATION. 



159 



future plant is to spring. This kind of union is found to occur 

 in many flowerless plants (Chapter XXIII), resulting in the 

 production of a spore very 

 unlike a seed in most re- 

 spects, but capable of 

 growing into a complete 

 plant like that which 

 produced it. 



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 fig. 142. —Diagrammatic Representation of Fer- 

 1, while in the common tilization of an Ovule. 



garden wistaria there are 

 about 7,000 pollen grains 

 to every ovule, and in 

 Indian corn, the cone- 

 bearing evergreens, and a 

 multitude of other plants, 

 many times more than 

 7,000 to 1. These differ- 

 ences depend, as will be 

 seen presently, upon the mode in which the pollen is carried 

 from the stamens to the pistil. 



inner coating of ovule ; w, outer coating of 

 ovule ; p, pollen tube, proceeding from one of 

 the pollen-grains on the stigma; c, the place 

 where the two coats of the ovule blend. (The 

 kind of ovule here shown is inverted, its open- 

 ing m being at the bottom, and the stalk /ad- 

 hering along one side of the ovule.) a to e, 

 embryo sac, full of protoplasm ; a, so-called 

 antipodal cells of embryo sac ; ??, central nu- 

 cleus of the embryo sac ; e, nucleated cells, one 

 of which receives the essential contents of the 

 pollen tube ; /, funiculus or stalk of ovule ; 

 771, opening into the ovule. 



