34 BOTANY 



chemical influence thus attracts the pollen tube. In flowers in 

 which the style is short, the tube reaches the ovule in the 

 course of a few hours. In plants with a long style, from one 

 to several days may elapse before the pollen tube reaches the 

 locule of the ovary. Once it reaches the ovary, the tube pene- 

 trates an ovule by making its way through a little hole known as 

 the micropyle. It then grows toward a clear area of protoplasm 

 known as the embryo sac. The embryo sac is an ovoid area, 

 microscopic in size, filled with semifluid protoplasm containing 

 several nuclei. (See figure.) One of the nuclei, with the proto- 

 plasm immediately surrounding it, is called the egg cell. It is this 

 cell that the sperm cell of the pollen tube grows toward ; ultimately 

 the sperm cell reaches the egg cell and unites with it. The two 

 cells, after coming together, unite to form a single cell. This 

 process is known as fertilization. This single cell formed by the 

 union of the pollen tube cell or sperm and the egg cell is now called 

 a fertilized egg. 



Development of Ovule into Seed. — The primary reason for the 

 existence of a flower is that it may produce seeds from which 

 future plants will grow. The first beginning of the growth of the 

 seed takes place at the moment of fertilization. From that time 

 on there is a growth, within the ovule, of a little structure called 

 the embryo. The embryo will give rise to the future plant. After 

 fertilization the ovule is called a seed. 



History of the Discoveries regarding Fertilization. — Although 

 the ancient Greek and Roman naturalists had some vague ideas 

 on the subject of fertilization, it was not until the latter part of 

 the eighteenth century that it was demonstrated that pollen was 

 necessary for the growth of the embryo within a seed. In the 

 latter part of the eighteenth century a book appeared in which a 

 German named Conrad Sprengel worked out the facts that the 

 structure of certain flowers seemed to be adapted to the visits of 

 insects. Certain facilities were offered to an insect in the way of 

 easy foothold, sweet odor, and especially food in the shape 

 of pollen and nectar, the latter a sweet-tasting substance manu- 

 factured by certain parts of the flower known as the nectar glands. 

 Sprengel further discovered the fact that pollen could be and was 



