180 BOTANY: PRINCIPLES AND PROBLEMS 



521. Why are mesophytes "generally large, thrifty and fast-growing 

 as compared with xerophytes and hydrophytes"? 



522. Which will produce a better crop of fruit, a warm, dry summer or 

 a cool, moist one? Why? 



523. Why should nitrates and potash be applied in large amounts only 

 early in the growth of such a crop as beans or corn, and withheld during 

 later growth? 



524. What different treatment, as to soil richness and moisture, 

 should you give to such crops as celery and lettuce from that which you 

 give to beans and corn? Why? 



525. Why is a Plant Association often a very heterogeneous group, 

 consisting of trees, herbs, saprophytes, climbers and many other types? 



526. What are the advantages and the disadvantages of a parasitic 

 habit of life to a plant? 



527. In the case of a parasitic plant, how are the haustoria able to 

 penetrate the body of the host? 



528. What do you think are the means by which the parasite with- 

 draws food and water from its host plant? 



529. Which type of plant do you think appeared first in evolution, 

 the saprophyte or the parasite? Why? 



530. What important roles do saprophytes play in the economy 

 of nature? 



531. Why are epiphytes more common in dense tropical forests 

 than anywhere else? 



532. Of what value to a pine tree is its pitch? 



533. Of what advantage to a plant may be poisonous substances 

 occurring in its leaves or seeds? 



534. Give at least three reasons why plants which are crowded 

 together do not grow as well as those which have plenty of room. 



535. What barriers hinder the dispersal of marine plants? 



536. What various reasons can you think of to explain the fact that 

 some plant species are much more widely dispersed than others? 



537. There are many more species of plants in the tropics than there 

 arejn temperate regions. Explain. 



