NATURE AND OFFICE OF THE FLOWER 225 



each ovule, and where more than one seed is produced to 

 a carpel, as is commonly the case, at least as many pollen 

 tubes must find their way to each cell of the ovary as 

 there are ovules provided all are fertilized. 



327. Formation of the Seed. When a pollen tube has 

 penetrated to the ovary it next enters one of the ovules, 

 usually through the micropyle (Fig. 443, m). There it 

 penetrates the wall of a baglike inclosure called the em- 

 bryo sac (Fig. 443, u, t, z], where a series of changes takes 

 place too intricate to be described here, by which a fusion 

 is brought about between a portion of the contents of cer- 

 tain cells emitted by the pollen tube and a large cell con- 

 tained in the embryo sac, known as the germ cell, or egg 

 cell (Fig. 443, *.). | The fusion of these two bodies is what 

 constitutes fertilization. ) The cell formed by their union 

 finally develops into the embryo. and the other contents of 

 the sac into the endosperm, and the ripened ovules become 

 the seeds. 



328. Stability of the Process of Fertilization. The pro- 

 cesses of fertilization and reproduction are very obscure 

 and difficult to understand without a degree of skill in the 

 manipulation of the microscope and a knowledge of tech- 

 nical details that the ordinary observer can seldom acquire. 

 The phenomena that characterize them, however, are the 

 most uniform and stable of all the life processes, varying 

 little not only in different species and orders, but through- 

 out the whole vegetable kingdom. For this reason they 

 furnish a more reliable standard for judging of the real 

 affinities of plants than mere external resemblances, which 

 are more liable to variation and may often be accidental, 

 and so they have been chosen by botanists as the ultimate 

 basis for the classification of plants. 



329. Embryology. The study of the developing ovule, 

 known as embryology, is a comparatively recent branch of 

 science, and has resulted in overturning many of the ideas 

 of the older botanists and the abandonment of many of the 



ANDREWS'S EOT. I 5 



