76 



REPRODUCTION 



through the style and eventually reaches the ovule. The tube 

 enters the ovule usually through the micropyle (Chapter I), 

 which persists and is plainly visible in many mature seeds. In 

 the meantime, interesting changes are going on within the small 

 ovule, which results in the formation of a minute chamber 

 known as the embryo sac. (Fig. Gl a.) This sac contains 

 eight minute cells, one of which is known as the ovum or egg. 



FIG. 59. Types of pollen grain. 



The tip of the pollen tube finally reaches this egg cell and a 

 certain nucleus from the tube unites with the nucleus of the 

 egg. This is the process of true fertilization, and this fer- 

 tilized egg now develops into an embryo or young plant (Fig. 

 61 b) surrounded by a supply of rich food for its future nour- 

 ishment. This embryo, together with its food supply and cover- 

 ings, constitutes the seed. (Chapter V.) The parts involved 

 in this process are so small that it is impossible to see them 

 without using a compound microscope, and the study of this 

 delicate and complicated process must necessarily be reserved 

 for the more advanced students of botany. 



No two plants in nature are ever exactly alike and, there- 

 fore, if the, pollen of one plant falls on the pistil of a slightly 



