FLOWERS 5,3 



sexual process. It is seen to differ fundamentally from 

 pollination which is simply a mechanical process. The 

 fertilized embryo nucleus now develops into a young plant 

 (embryo). The synergids and antipodals are usually dis- 

 organized. In grasses and lilies and some other plants, so- 

 called double fertilization has been observed. One sperm 

 nucleus has been accounted for, as uniting with the embryo 

 nucleus. The other unites with the two polar nuclei of the 

 embryo sac. The body resulting from this triple fusion also 

 carries both maternal and paternal characters. It grows 

 and develops into the endosperm of the seed. Immediately 

 following fertilization, there is a series of changes not only 

 in the ovule which results in a seed, but in the ovary wall as 

 well. 



Just one pollen tube penetrates the embryo sac to bring 

 about fertilization. Many pollen tubes, even hundreds, 

 may penetrate the style, although comparatively few may 

 function normally. Those which do not, wither and die. 

 We may be sure that every ovule that develops into a seed 

 has been visited by one, and only one, pollen tube. 



Placentation. — We said that the placenta is the tissue in 

 the ovary to which the one or more ovules are attached. It 

 is traversed by vascular bundles from which branches are 

 given off to the ovules. In currants and gooseberries (Fig. 

 129) the placentae are on the ovary wall. Such placentation 

 is said to be parietal. In lilies (Fig. 31^!), the placentation 

 is axial, or central, that is, the placentae are on the ovary 

 axis. A third kind of placentation is the free central, in 

 which the ovules are attached to an up-growth of the floral 

 axis in the center of the ovary, which is not connected to 

 the ovary wall by radial partitions. 



Symmetry of Flower. — A flower such as the apple, cherry 

 or asparagus can be divided into two approximately sym- 



