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 lke 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 
1, while in the common 
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 to1. These differ- 
ences depend, as will be 
> 
0 weirs faze ott 2 
*FiG. 142. — Diagrammatic Representation of Fer- 
tilization of an Ovule. 
i, inner coating of ovule; o, 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 fad- 
hering along one side of the ovule.) a toe, 
embryo sac, full of protoplasm; a, so-called 
antipodal cells of embryo sac; 7, central nu- 
cleus of the embryo sac ; e, nucleated cells, one 
of which receives the essential contents of the 
pollen tube; f, funiculus or stalk of ovule ; 
m, opening into the ovule. 
seen presently, upon the mode in which the pollen is carried 
from the stamens to the pistil. 
