THE FLOWER 293 
coats of the pollen grain forming a pollen tube. The pollen 
tube, carrying within its walls two sperm nuclei and one tube 
nucleus, penetrates through the loose cells of the style until it 
reaches the micropyle of the ovule, then piercing the nucellus, 
it enters the embryo sac. The tip of the tube breaks and one of 
the sperm nuclei unites with the egg nucleus to form the odspore. 
The second sperm nucleus is absorbed as food. ‘The oéspore 
develops at once into an embryo or plantlet which lies passive 
Fic. 220.—Stages in the germination of the pollen grain and development of the 
pollen tube. (From Robbins, after Holman and Robbins, after Bonnier and Sablon.) 
‘until the seed undergoes germination. In a number of plants 
including corn, some lilies, etc., the second sperm nucleus unites 
with the previously fused polar nuclei to form the endosperm 
nucleus. The union of one sperm nucleus with the egg nucleus 
and the other sperm nucleus with the two polar nuclei is called 
double fertilization. 
FoRMATION OF ENDOSPERM.—The endosperm nucleus within 
the embryo sac soon undergoes rapid division into alarge number 
of nuclei that become scattered about through the protoplasm 
of the embryo sac. These accumulate protoplasm about them, 
cell walls are laid down, ENDosPERM resulting. 
