240 BOTANY: PRINCIPLES AND PROBLEMS 



which are found nowhere in the world except on that particular island 

 or island group. Explain. 



611. The potato, tomato, tobacco and various other agricultural 

 plants which are now grown in Europe were introduced there from 

 America after the discovery of the New World. What can you conclude 

 as to their evolutionary history? 



612. Torreya, a genus of coniferous trees, is represented by two species 

 in China, one in California, and another in the southeastern United 

 States, but is found nowhere else in the world. What conclusions can 

 you draw from these facts as to the past history of this genus? 



613. In the Galapagos islands very many of the plant species are 

 confined to this group of islands, but most of the genera are the same as 

 those found on the adjacent coast of South America. In Hawaii, the 

 species are not only distinctive of the islands but many of the genera also 

 are found nowhere else. Which of these two island groups on the 

 basis of these facts, do you believe has been isolated the longer and 

 the more effectively? Why? 



614. Plants living in arid or desert regions usually have small and 

 leathery leaves and deep root systems, in contrast to plants living in 

 regions of more abundant moisture. How would Lamarck explain 

 this? How would Darwin? 



615. Most plant species which are very common belong to genera 

 which are larger than the average in number of species. Explain. 



616. Darwin noted that species belonging to large genera were usually 

 more variable than species belonging to small genera. Explain. 



617. Which species is apt to be more successful, do you think, a 

 relatively variable one or a relatively constant one? Why? 



618. Which do you believe would change more rapidl}^ in evolution, 

 a species which is always cross-fertilized or one which is always self- 

 fertilized? Why? 



619. State at least five advantages which one plant species might have 

 over another which would make it more widespread and successful. 



620. Give an example of a physical barrier to plant distribution; 

 of a "biological" barrier. 



621. Is competition generally keener between two individuals of the 

 same species or between two individuals belonging to different species? 

 Explain. 



622. Is competition apt to be keener between closely related or 

 between distantly related species? Why? 



