THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH 



All these plants agree very closely in their cytology. They show unmistakable signs 

 of hybridity in the irregular appearance of the first meiotic division, which may perhaps 

 be sufficiently demonstrated by Fig. 99 a and b. The first of these shows a section of one 

 of the Italian specimens with numerous unpaired chromosomes; the second (Fig. 99^) 

 is a single mother cell in an acetocarmine preparation of the Welsh specimen in which 

 fuller details of pairs and univalents can be made out. 



In all cases, however, the chromosome number was anomalous. The first detailed 

 count had been made on a root of one of the Italian plants in which about 100, and not 

 the expected 144, chromosomes had been found. This seemed at the time so inexplicable 

 that the record was at first discarded as a possible case of mis-identity since the plant 

 had died during the war without a herbarium record of it having been preserved. 

 The same result, however, was obtained with the British plant, as may be seen from the 

 photographs and diagrams of Figs. 99 and 100. When, as a result of this discovery, 

 the two additional continental specimens (from Sweden and Switzerland) had been 

 examined and an exactly similar result obtained in each, the facts could no longer be 

 doubted; A. germanicum in four European countries and therefore probably always, is 

 not a tetraploid as its putative parentage would suggest but a triploid (3^ = 108). 



The discovery of a triploid hybrid where a tetraploid was expected has already 

 occurred in the case o{ Dryopteris remota (see Chapter 5), but since Asplenium germanicum 

 differs from Dryopteris remota in not being apogamous, the possible explanations of its 

 origin are less numerous. Since the plants are sterile they cannot have been dissemi- 

 nated by stray spores from a common source, and it must be assumed that each has 

 arisen de novo wherever it is found. Since they are all very similar morphologically a 

 complex origin by a, process of segregation from a previous hybrid of different chromo- 

 some number seems impossible. It remains, therefore, to be seen whether a diploid 

 form of one or other of the parent species can be found from which a triploid could be 

 produced directly as a simple hybrid. 



In searching for this it was felt at once that Asplenium Trichomanes was the more prob- 

 able species to be involved, since A. septentrionale had already been examined cytologic- 

 ally several times from A. germanicum localities; moreover, it is so uncommon in Great 

 Britain that the probability of finding more than one form of it in this country at any 

 rate seemed remote. With A. Trichomanes, on the other hand, the position is different. 

 It is so abundant and familiar that it had not previously been felt necessary to take 

 special precautions to collect it from the immediate vicinity of the hybrid, and reliance 

 had been placed on the wealth of wild specimens nearer home. This meant that while 

 chromosome counts, invariably tetraploid, had been made on specimens of /I. Tricho- 

 manes from Devon, Derbyshire, Yorkshire and the south of France, none of these plants 

 had actually been associated with either A. septentrionale or A. germanicum, and it was 

 conceivable that had such an association been insisted upon a different result might 

 have been obtained. Attention was therefore directed towards sampling A. Trichomanes 

 populations from these precise habitats. 



At once what was looked for was found. A form of A. Trichomanes obtained from 

 Snowdon (Wales), and transferred to Kew in 1946 was found to be diploid. The two 

 chromosome numbers are demonstrated in Figs. 1 01-102, and a leaf of each type 



105 



