INTRODUCTION TO THE PROBLEM 



of these facts may perhaps be helpfully summed up by a glance at Figs. 5 and 6, which 

 show characteristic views of both generations, together with their respective repro- 

 ductive organs for one particular member of the Pteridophyta, the Horsetail [Equisetum) . 

 All this will be so familiar as to be second nature to a botanical reader; nevertheless, 

 even for such, it may perhaps be convenient and save risk of confusion if two other 



Fig. 7. Some examples of apogamy in ferns, a. Obligate (or 'direct') apogamy in i)rvo/;ferw5orrmNewni. 

 ' war . polydactyla Wills ' (for further details see Chapters 5 and 1 1 ) showing a young apogamous outgrowth 

 from the region of the prothallus where the central cushion bearing archegonia should be. x lo. 

 b. Induced apogamy in an abnormal prothallus of Osmunda regalis L. derived from a spore produced 

 by the triploid (see Chapter 3) and too unbalanced genetically to be able to reproduce normally; 

 after several years' sterility a leaf is being produced from the apex of the prothalloid tissue, but so 

 far the specimen has no roots, x 2. c. The same as b in other examples. Natural size. 



terms, without exact zoological equivalents, be referred to at this point. 'Apogamy' 

 and 'apospory' are two aberrations of life history by which the regular sequence of 

 sexual and asexual reproductionjust explained is modified. 'Apogamy' is the production 

 of a sporophyte from a gametophyte without the intervention of a sexual process. A 

 special case of apogamy is 'parthenogenesis', a name used by both botanists and 

 zoologists to denote the development of an egg without the act of fertilization. This is 

 unknown in the Pteridophyta, although it undoubtedly occurs under exceptional 



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