APOGAMOUS FERNS. THE GENERAL PHENOMENON 



appearance, nevertheless produced spores with the same chromosome number as the 

 parent plant and that sporophyte and gametophyte were identical in nuclear content. 



The correct interpretation of Allen's intermediate fusion stages was first given by 

 Steil in 1919 working on ' Nephr odium hirtipes' now known as Dryopteris atrata. Steil's 

 work was favoured by the fact that in Dryopteris atrata these ' intermediates ' are unusually 

 abundant, being, in my experience, more numerous in this species than any other type 

 of sporangium. At first, in a preliminary note, Steil accepted Miss Allen's view that 

 these were signs of nuclear fusion, but in his fuller account (1919) he realized that they 

 were better interpreted as signs of incomplete division, and that the true nature of the 

 compensation process was an incomplete nuclear division immediately preceding 

 meiosis, by means of which the nuclear content of the spore mother cells is momentarily 



doubled. 



Steil's account is incomplete in that he did not fully reahze the extent of the variation 

 in possible sporangial developments within one sorus, which is indeed less obvious in 

 D. atrata than in other species, and this is where Dopp's account of Z). remota (1932) is 

 greatly to be preferred. While authenticating the reality of an incomplete nuclear 

 division immediately preceding meiosis as the basic abnormality without which the 

 continued reproduction of the species would be impossible, Dopp showed clearly that 

 the abnormality is only effectively accomphshed in a proportion of sporangia, the 

 remainder being affected by differences of detail, most of which result in abortive or non- 

 viable spores. In Dopp's account three distinct types of development were recognized, 

 and though in most species the number can be extended to four, for most purposes his 

 account of Z). remota is adequate. In his last paper to appear before the war Dopp (1939) 

 extended his observations to 'D. Filix-mas var. cristata Hort.' and to the two poly dacty las, 

 and showed that they agreed exactly with D. remota in essentials. 



The account which will be given here is in no sense based upon Dopp's work, but is 

 the resuh of an independent investigation begun (cf Manton, 1932) in the early 1930's 

 and continued in the first instance in ignorance of the parallel observations being 

 carried out in Germany. This should mean that something like finality may now be 

 claimed for the straightforward descriptive facts, as far as these go, for Dopp's observa- 

 tions and those to be described below confirm and supplement each other where they 

 relate to similar material, while the present work also extends the description to a 

 number of additional species. Since the actual number of species to be discussed is 

 rather high, it is proposed in the first instance to give a generalized description of the 

 main features in which they all agree, illustrating this fully with reference to a limited 

 number of sample types. This description will occupy the rest of this chapter, after 

 which the separate peculiarities of individual species, together with the evolutionary 

 analyses of all of them, will be added in the chapter which follows. 



The choice oi Cyrtomium falcatum (Fig. 163) as the main illustrative sample type has 

 been dictated partly by historical reasons and partly for convenience. It was a fern 

 grown extensively for the market as an ornamental plant in the neighbourhood of 

 Manchester before the war, and unHmited supplies of material were made available 

 to me by the kindness of Messrs CHbran of Altrincham, who, without charge, gave me 

 free access to their nurseries. Although, as will be seen in the next chapter, I have since 



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