226 REPRODUCTION IN PLANTS AND ANIMALS 



the sperm nucleus with the nucleus of the egg to form a single cell is 

 known di,^ fertilization. This single cell formed by the union of the 

 pollen tube sperm cell and the egg cell is now called a fertilized egg 

 or zygote and is the beginning of a baby plant, or embryo. 



The primary reason for the existence of a flower is that it may 

 produce seeds from which future plants will grow. After fertili- 

 zation the ovule grows into a seed which contains the embryo and will 

 develop into an adult "plant. 



The second sperm nucleus unites with another nucleus in the 

 embryo sac and grows into a food substance called the endosperm, 



which in some plants is a very 



r. 



The remains of the 

 flower sfalk (peduncle) 



..Oldcalyx 



Placenta , the place seeds 

 are attached by the 

 Funiculus or 

 swollen stalk 



A pea,seed,is a developed 

 ovule 



important part of the seed since 

 it supplies the growing embryo 

 with nourishment. 



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 

 continues to grow after the 

 rest of the flower withers. If 

 we examine the pistil from such 

 a flower, we flnd that it is the 

 ovary that has enlarged. The 

 space within the ovary has be- 

 come nearly filled with a number 

 of ovoid bodies, attached along 

 one edge of the inner wall. 

 These we recognize as the 

 young seeds. The pod, which 

 is in reality a ripened ovary with other parts of the flower at- 

 tached to it, is considered as a fruit. By definition, a fruit is a 

 ripened ovary together with any parts of the flower that may be attached 

 to it. The chief use of the fruit is to hold and to protect the 

 seeds ; it may ultimately assist in distributing them where they 

 can reproduce young plants. 



Development of a Simple Animal. — Many-celled animals are 

 formed in much the same way as are many-celled seed plants. A 



.The dried style 



The pea pod is a developed ovary with 

 other parts of the flower. 



