DEVELOPMENT OF LIVING THINGS 



231 



The stamens and pistils are necessary for produc- 

 tion of seeds and are therefore the essential organs of 

 the flower. The stamens produce the sperm cells and 

 the pistils produce the eggs. The sepals and petals 

 protect the stamens and pistils and add to the attrac- 

 tiveness of the flower. 



How are seeds formed? A seed contains a baby 

 plant with a food supply to nourish it until it is able 

 to make its own food. Let us learn now how seeds are 

 developed by flowers. 



"Petal 



Pistil 



FIG. 368. STRUCTURE OF A TYPICAL FLOWER 



In order that a seed may be formed from an egg cell, 

 the egg must first be fertilized by a sperm. How can 

 this happen when the egg cells are deep within the 

 ovary at the base of the pistil ? See Figure 369. On the 

 top of the pistil is a structure called the stigma, which 

 has a sticky surface. Pollen grains from stamens of 

 the same flower or other flowers get upon the stigma. 

 They burst open and grow down through the style 

 into the ovary at the base of the pistil where the egg 

 cells are located. Here the sperm unites with the egg. 

 This is known as the process of fertilization. 



ANTHER 

 -T,- o POLLEN GRAIN 







-4- POLLEN TUBE 



OVARY 

 , OVULE 



FIG. 369. POLLINATION OF A FLOWER 



From the union of the nucleus of the egg cell with 

 the nucleus of the sperm cell, a single cell is formed. 

 Very soon after the union the new cell divides into 



two, then these two into four, and so on until the 

 embryo or baby plant is formed within a seed. The 

 seed, as we know from experience, shows no signs of 

 life unless it is given certain conditions of water and 

 temperature. Under these conditions the baby plant 

 starts to grow and eventually produces flowers. These 

 flowers may produce more seeds that in turn give rise 

 to new plants. 



What is pollination? The transfer of pollen from the 

 stamens to the pistils is called pollination. In many 

 flowers it is necessary that pollen come from a differ- 

 ent flower. Many insects are attracted to flowers by 

 their colors and odors. Some flowers produce nectar, 

 a sweet fluid which insects use for food. Have you 

 ever watched bees crawling around on flowers? They 

 crawl down into the flower and become covered with 

 the pollen. On their visit to another flower some of 

 the pollen is brushed on to the stigma. Some birds 

 are agents of pollination. Have you ever watched a 

 humming bird in its visits to flowers? What is it af- 

 ter? Wind is also a carrier of pollen. Pollen grains are 

 light and are picked up by wind, some of them falling 

 on the stigmas of other flowers. 



Do higher animals reproduce in a manner similar 

 to flowering plants? They develop from female egg 

 cells that have been fertilized by male sperm cells. 



One of the surest signs of spring is the loud singing 

 of frogs and toads, calling to their mates. The mother 

 frog lays her eggs in the water, the tiny eggs being 

 separated and surrounded by a large mass of trans- 

 parent, jelly-like substance. The male frog deposits 

 sperms over the eggs as the female lays them. The 

 sperms swim in the water, penetrate the "jelly," and 

 each unites with an egg, forming a new cell. 



The formation of an adult frog from a fertilized egg 

 is a wonderful process (Fig. 370). The fertilized egg 

 cell divides into two cells, then into four, eight, six- 

 teen, and so on, until a young tadpole is formed. The 

 little tadpole wriggles out of the jelly surrounding it 

 and lives on tiny plants in the water. During this stage 

 its body is somewhat like that of a fish. Eventually 

 the tadpole grows hind legs and then front legs and at 

 the same time its tail is absorbed and disappears. 

 Lungs develop in the chest cavity of the body, and 

 it comes to the surface to breathe, and grows into an 

 adult frog. 



All the higher animals develop from fertilized eggs 

 in a manner similar to the development of a frog. In 

 birds and some reptiles the egg is protected by a hard 

 shell, and the development of the young takes place 

 within it. In mammals (man, dogs, horses, etc.) the 

 fertilized eggs are retained in the body of the mother, 

 where they grow until the young are ready to be 

 born. 



