CHAPTER 3 

 THE POLYPLOID SERIES IN OSMUNDA 



Osmunda regalis, the Royal Fern,* is such a dehghtful cytological object that it may be 

 well to prepare the way for the evolutionary analysis which follows by devoting a pre- 

 liminary chapter to it. This will enable us to amplify the brief introduction to poly- 

 ploidy given in Chapter i and thereby to set up a standard of reference from within 

 the group itself, against which the facts for other plants of unknown origin can be 

 assessed. 



The reasons for the selection of Osmunda are well known. It is easily grown, indeed 

 it is remarkably hard to kill, even in the centre of an industrial city. Both generations 

 can be treated as perennials. The spores are produced in abundance and will germinate 

 at once if scattered on to soil or tap water. The chromosomes are large and few in 

 number {n = 22, one of the lowest known numbers in the ferns) and they are readily 

 accessible, for both roots and sporangia are freely exposed in quantity with a minimum 

 of protective covering. Fixation, moreover, with modern fixatives presents no difficulty, 

 and all the more important special methods of treatment, including that for spiral struc- 

 ture (Manton, 1939), can be applied with success. Finally, in its morphology, the 

 sporophyte at least has shown itself to be remarkably plastic under experimental 

 treatment. 



The origin of the autopolyploid series was apospory induced experimentally in a 

 normal strain of the species. The basic facts of its production were described in some 

 detail by Lang in 1924, and the cytology recorded in a preliminary form by Manton 

 in 1932. Briefly, the process consisted in the persistent depauperation of young plants 

 by mutilation and malnutrition, either deliberate or accidental, enhanced by prolonged 

 neglect during the war of 19 14-18. As a result, the normal adult type of foHage failed 

 to develop, and a variety of abnormalities affected the younger leaves which had other- 

 wise reverted to the juvenile type. Examples of some of these unusual growths are 

 shown in Fig. 1 1 a-d, which are reproduced from Lang's original drawings, and in 

 Fig. 12, which is one of Lang's original photographs. They represent various types of 

 cyhndrical processes, vegetative buds and prothalli which had developed from, or re- 

 placed, the leaf lamina or petiole. Prothalli were somewhat infrequent but were detected 

 more than once (see also Fig. 8a, Chapter 2). Such prothalli could produce rhizoids 

 and some sex organs whilst still attached to the parent plant or, if removed and laid on 

 soil, they grew on as normal-looking gametophytes and gave rise to numerous young 

 plants. Owing to their perennial habit the prothallial cultures could be kept growing 

 indefinitely and they are now over 20 years old and still continue to produce young 

 plants. The prothalli are, however, diploid and the young plants tetraploid, 



* Anyone unfamiliar with the general appearance of this large and characteristic fern will find some 

 leaves of a smaller species illustrated in Chapter 16, Fig. 276a, ^, p. 275. 



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