450 



to say that none of us have seen seeds in 

 bananas or pineapples. You might well 

 ask whv seeds do not develop in these 

 fruits. To answer this one would have to 

 study the flower of each plant carefully. 

 This has been done for many seedless 

 fruits by botanists, who have found that 

 certain oranges are seedless because the 

 pollen tube that is produced is defective. 

 Other orans^es fail to form seeds because 

 either no embryo sacs are produced or 

 the embryo sacs are imperfect. In some 

 seedless grapes pollination and fertiliza- 

 tion occur, but the embryos die when 

 they are very young and the ovules do 

 not grow into seeds. The hundreds of 

 black specks vou see in bananas are 

 ovules, not seeds; as a rule, neither the 

 pollen nor the embryo sacs are perfect. 

 When seeds fail to develop, the plant is 



How Anhnals and Plants Reproduce unit viii 



propagated vegetatively. You will read 

 in a later unit about a method of vegeta- 

 tive propagation called grafting. 



It is most interesting to know that the 

 growth of the ovary into a fruit does not 

 ordinarily take place unless the stigma 

 is pollinated. Even though the ovules 

 fail to become seeds, the presence of pol- 

 len and growth of the pollen tube is 

 enough to cause the ovary to start grow- 

 ing. Dr. Gustafson of the University of 

 Michigan and others have found that 

 pollen contains a growth-promoting hor- 

 mone. In recent years much experimental 

 work has been done with plant hormones 

 and hormonelike chemicals which, when 

 sprayed on the stigma or ovary, cause the 

 groM'th of the ovary but not the ovules. 

 Seedless tomatoes, t^^ plants, cucum- 

 bers, and squash have been produced. 



Questions 



1. What structures does a seed contain? Define germination. How does 

 a seedlincr difll^er from a seed? 



2. What plant organ must you study to find out how seeds are formed.^ 



3. Can you name a flower-bearing plant that you had not thought of as 

 such? Name in order from the outside the four circles of flower parts 

 found in many flowers. 



4. Which are the essential parts of a flower? Why are they called essen- 

 tial? What names are applied to flowers that have only one of the 

 essential parts? 



5. Describe the three parts of a typical pistil. What structnres make up 

 the ovary? The ovule? The embryo sac? 



6. Describe a stamen. What important cells does it produce? Tell how 

 self-pollination and cross-pollination are alike and how they are 

 difi^erent. 



7. Under what conditions does a pollen tube form? What is the relation- 

 ship between the pollen grain and the male gamete? 



8. Describe the growth of the pollen tube, telling exactly through \\ hat 

 it passes and where it finally ends its growth. State, as specifically as 

 possible, where fertilization in the flower occurs. Which nuclei unite 

 in fertilization? What becomes of the second sperm nucleus? 



