FUNCTIONS OF LIVING THINGS 



55 



ence thus attracts the pollen tube. When it reaches the ovaiy, 

 the sperm cell penetrates an ovule by making its way through a 

 little hole called the micropyle. It then grows toward a clear 

 bit of protoplasm known as the embryo sac. The embryo sac is 

 an ovoid space, microscopic in size, filled with semifluid protoplasm 

 containing several nuclei. (See Figure.) One of the nuclei, with 

 the protoplasm immediately surrounding it, is called the egg cell. It 

 is this cell that the sperm nucleus of the pollen tube grows to- 

 ward ; ultimately the sperm nucleus reaches the egg nucleus and 

 unites with it. The two nuclei, 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. After fertilization the ovide grows into a seed. 

 The first beginning of the growth of the seed takes place at the 

 moment of fertilization. From that time on there is a growth 

 of the fertilized egg within the ovule which makes a baby plant 

 called the embryo. The embryo will give rise to the adult plant. 



A Typical Fruit, — the Pea or Bean Pod. — 

 If a withered flower of any one of the pea or 

 bean family is examined carefully, it will be 

 found that the pistil of the flower continues to 

 grow after the rest of the flower withers. If 

 we remove the pistil from such a flower and 

 examine it carefully, we find that it is the 

 ovary that has enlarged. The space within 

 the ovary has become nearly filled with a 

 number of nearly ovoid bodies, attached 

 along one edge of the inner wall. These we 

 recognize as the young seeds. 



The pod of a bean, pea, or locust illustrates 

 well the growth from the flower. The pod, 

 which is in reality a ripened ovary with other 

 parts of the pistil attached to it, is considered 

 as a fruit. By definition, a fruit is a ripened 



s — 



The fruit of the locust, 

 a bean-like fruit. 

 p, the attachment 

 to the placenta ; s, 

 the stigma. 



