306 BOTANY: PRINCIPLES AND PROBLEMS 



681. There are no living algae or other green plants at depths greater 

 than a few hundred feet below the surface of the ocean. Why? 



682. What factors may affect the dcptli t(j which algae will grow in 

 the ocean? 



683. Since no green plants live in the deeper portions of the ocean, 

 how is it possible for fish and other animals to live there? 



684. What is the ultimate source of the food of fishes which live 

 near the surface of the ocean, in the open sea? 



685. Why are very shallow parts of the ocean, like the Grand Banks 

 of Newfoundland and the North Sea, such great fishing grounds? 



686. The algal flora near shore, and especially near mouths of large 

 rivers, is apt to be very rich. Explain. 



687. What great element of the food supply of man has its source in 

 the algae? 



688. Blue-green algae can thrive in relatively dry places as compared 

 with other algae. What structural peculiarity of theirs makes this 

 possible? 



689. Blue-green algae are generally found in waters which are rich 

 in organic matter. What does this suggest as to differences between 

 their methods of nutrition and those of ordinary green plants? 



690. Blue-green algae can thrive in hot springs and in warmer waters 

 generally than can most of the other algae. Can you suggest a reason 

 for this? 



691. What do we mean by saying that one group of plants is "on 

 the direct line of ascent" to another? 



692. Why is it believed that the green algae rather than the other 

 algal gr()ui)s are on the direct line of ascent to the bryophytes? 



693. What arc the advantages and the disadvantages of the coeno- 

 cytic condition? 



694. Brown seaweeds in general have a much thicker and tougher 

 plant body than the rest of the algae. Explain. 



695. What are the advantages conferred by the possession of bladder- 

 like floats in the brown algae? 



696. In what important particular is fertilization in Fucus different 

 from that in any other alga described in the text? 



