262 BOTANY. PRINCIPLES AND PROBLEMS 



650. In sexual reproduction, what is the advantage in having the two 

 types of gametes (male and female) so radically different from one 

 another? 



651. Gametes, particularly male gametes, are often motile when all 

 the other cells of the plant are not. How do you think that this has 

 come about? 



652. What various advantages and disadvantages can you think of 

 in a life-cycle which shows an alternation of generations? 



653. Why do you think it is that the alternation of generations, so 

 well marked in lower plants, has practically disappeared in the highest 

 ones? 



654. Do you think that plants or animals were the first organisms to 

 migrate from the sea to the land? Why? 



655. When the first plants invaded the dry land, with what kind of soil 

 did they probably find it covered? What important changes did the 

 presence of plant life make in the soil? 



656. Give a description of the probable appearance of the land 

 surface before the evolution of the Pteridophytes. What regions on 

 earth today do you think it most closely resembled? 



657. What effect did the evolution of the Pteridophytes probably have 

 on the abundance of land animals? Why? 



658. Many fungi now live entirely on land. Do you think that they 

 were the first land plants? Explain. 



659. What are the advantages of the seed over the spore as an agency 

 for reproduction? 



660. Why have seed plants largely superseded pteridophytes? 



661. In what way has the evolution of seed plants probably changed 

 the characteristics of plant-eating animals? 



662. What is the practical use of having a definite system of classify- 

 ing plants into species, genera, families, and other groups? 



663. What organs are chiefly used as a basis for the classification of 

 plants? Why? 



664. What is an "artificial" as opposed to a "natural" system of 

 cla.ssification? 



665. Classify the following objects into a system of "groups within 

 groups," stating briefly the characteristics by which each group may 



