Questions and Problems 67 



Parthenogenesis 



Eggs normally require fertilization in order to develop into 

 mature organisms, but the eggs of some plants and animals may 

 develop without fertilization. The development of an unfer- 

 tilized egg is known as parthenogenesis. Although this occurs 

 normally in the production of certain insects such as male bees 

 and the parasitic wasp Habrobracon, it can be induced in some 

 eggs by treating them with certain chemicals or other abnormal 

 environmental conditions. 



QUESTIONS AND PROBLEMS 



1. Consult books on general zoology and suggest some animals that 

 have a more complicated life cycle than the vertebrates. What are the 

 chromosome numbers of the various stages of some of these other life 

 cycles ? 



2. Diagram the life cycle in Ulothrix, Fucus, Nemahon, Polysiphonia, 

 black stem rust of wheat, and other lower plants. Consult textbooks on 

 general botany or on the morphology of the Thallophytes. 



3. Discuss the relative importance of the sporophyte and gameto- 

 phyte in various divisions of the Plant Kingdom. 



4. The number of chromosomes in the root tip cells of maize is 20. 

 What is the number in the following cells or tissues: (a) microsporocyte ; 



(b) tube nucleus; (c) nucellus; (d) antipodal cells; (e) cells of anther 

 wall; (/) style; (g) embryo sac mother cell; (h) megaspore; (i) pali- 

 sade cells of leaf; (;') endosperm? 



5. Assume that a plant has two long and two short chromosomes. 

 Diagram cells of that plant in (a) metaphase and anaphase of a somatic 

 mitosis, (6) metaphase and anaphase of the first meiotic division, and 



(c) metaphase and anaphase of the second meiotic division. 



6. Explain what is meant by relational coiling and relic coiling. 



7. What is meant by Darlington's "Precocity Theory"? 



8. If several chiasmata form in one arm of a chromosome and if they 

 all terminahze, why do they all terminalize to the free end of that arm? 

 Why do not some terminalize to the end of the other arm? Would they 

 still all terminalize to the end of that arm if no chiasmata formed in the 

 other arm? 



9. Show by a diagram why a chiasma holds two chromosomes to- 

 gether at diplotene. Use colored crayons to differentiate the two homo- 

 logues or, still better, use colored modeling clay. 



