THE THALLOPIIYTA 305 



ascocarps are usually either cup-shaped or disc-shaped, as in tlie 

 Discomycetes (Fig. 181). Definite and functional sex organs, 

 somewhat suggestive of those in the red algae, have been demon- 

 strated in certain species. 



Lichens are usually xerophytic and will thrive^ on hare rocks 

 and exposed places where no other vegetation can exist. Their 

 importance in the economy of nature is therefore considerable, 

 but they include only a few species which arc of direct pra(!tical 

 value to man. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



669. The thallophytes are the most varied of all the four main divi- 

 sions of the plant kingdom. Can you suggest why this is so? 



670. In what way is the classification of the thallophytes given in this 

 book an "artificial" one? 



671. All the groups of algae are found in the ocean, Imt few in fresh 

 water. Explain 



672. Of what advantage is the possession of motility to the zoospores 

 of the algae? 



673. Zoospores of algae will generally swim toward the light rather 

 than away from it. Of what advantage is this to the plant? 



674. Unicellular algae are frequently spherical in form, but among 

 larger tj'pes the body is never a solid sphere. Explain. 



675. Algae are in general much more filamentous and finely branched 

 than land plants. Explain. 



676. Algae which are completely and somewhat deeply submersed 

 all the time are much more finely branched and dissected than those 

 which are near the surface or are partially exposed to the air for some 

 of the time. Explain. 



677. Are algae commoner on a rocky coast or on a sand}' one? J']x- 

 plain. 



678. Seaweeds wliich grow between tide marks are often very gela- 

 tinous. Explain. 



679. The "plankton" is more abundant in the cooler parts of the 

 ocean than in tlie warmer ones. Why? 



680. The largest of the algae are usually found in cool northern seas 

 rather than in the warm tropical ones. Why? 



20 



