INTRODUCTION TO THE PROBLEM 



This type of life history in which sexual and asexual modes of reproduction are 

 separated on different individuals which succeed each other in regular order is known 

 as alternation of generations. In the animal kingdom such a life history is rare and 

 altogether absent from the higher forms, but in plants alternation of generations is so 

 deeply estabhshed in all the principal land plants except the fungi, that its effects are 

 still easily to be recognized even in the highest seed plants. In these, however, the 

 sexual, haploid generation or gametophyte is so much reduced that for many purposes 

 its existence may be ignored, and it commonly is so by geneticists. The flowering plants 



Fig. 5. The sporophyte generation of a horsetail {Equisetum). 



Willd. in the month of May. Natural size. b. Ripe spores of £. palustre L 

 a young sporangium of £. robustum A.Br, with meiosis in progress, x 100. 



Sterile and fertile shoots of E. limosum 

 X 100. c. Section of 



can then be treated exactly like the higher animals as diploid organisms in which the 

 nuclear reduction (meiosis) takes place as part of the maturation process of the gametes. 

 In the Pteridophyta the complete independence of the two generations for most of their 

 lives makes it necessary to retain the technically correct terminology appropriate to 

 botany or misunderstandings will follow. The conspicuous generation here (as tech- 

 nically also in the flowering plants) is a diploid organism reproducing by asexual 

 means and known in consequence as the sporophyte. The haploid, sexual plant (in the 

 case of the Pteridophyta the inconspicuous but free-living prothallus) is the gameto- 

 phyte. Sporophytes and gametophytes occur in all the higher groups of plants, though 



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