S. WALKER 



197 



produce large numbers of antheridia. Prothalli required as females were pricked out 

 into pots at an early stage so that each prothallus was separated from its neighbour by 

 about f in. ; this almost precludes fertilisation within the cultures before hybridisation. 



Somatic chromosome counts have been made mainly by embedding and sectioning 

 root tips and using iron haematoxylin stain, though some aceto-carmine root-tip squashes 

 have also been made after hydroxyquinoline pretreatment (Tjio and Levan, 1950; Meyer, 

 1952). Hybrids became fertile approximately 12-18 months after fertilisation and detailed 

 analyses of meiotic chromosome pairing have been made, using the aceto-carmine squash 

 technique (Manton, 1950). Photographic evidence is supported by explanatory diagrams 

 where necessary. In most cases the analyses have been made from several plants of the 

 same hybrid combination. 



(4) Survey of the previous work 



The taxonomic history has been outlined very briefly above. The three recognised 

 species of the complex, D. cristata, D. spinulosa and D. dilatata, have a circumpolar distri- 

 bution in the Northern Hemisphere, particularly within the Temperate Zone, and can 

 be separated by their morphology and type of habitat (Warburg, 1952 ; Hyde & Wade, 

 1948). D. cristata is distinguished more readily from the other two species than these 

 two are from one another. Consequently the exact distribution of D. spinulosa and 

 D. dilatata is incompletely known as the two species have not always been treated as 

 distinct. 



The hybrids D. X uliginosa and D. dilatata X spinulosa can be found in habitats 

 which will support both parents ; D. X uliginosa is therefore restricted to very wet situations 

 whereas D. dilatata X spinulosa abounds in marshy woods which are gradually drying 

 out. Both hybrids exhibit morphological characters intermediate between those of their 

 parents and produce abortive spores. The differences in frond shape and degree of 

 pinnation in the species and species hybrids are illustrated by Fig. 1. 



D. cristata, D. spinulosa and D. dilatata occur as tetraploids (Manton, 1950), each 

 with regular chromosome pairing at meiosis and a gametic count of n = 82 (see Plate 15, 

 fig. (a)). Manton also gave cytological evidence of hybridity for D. X uliginosa and the 

 suspected hybrid between D. spinulosa and D. dilatata; both are tetraploid and show a 

 number of paired and unpaired chromosomes during meiosis. 



In 1948 a diploid form of D. dilatata was found on the continent and collected from 

 Trondheim in Norway, Storlien in Sweden and AroUa in Switzerland (Manton, 1950). 

 All show typical D. dilatata features but possess a more finely cut pinnation. This 

 character was mentioned by Moore (1863) in his description of D. dilatata var. alpina. In 

 1949 a specimen agreeing with the characters of var. alpina was collected from the upper 

 regions of Ben Lawers in Perthshire, Scotland (this was the location of the type specimen 

 in Moore's herbarium) by Mr. A. H. G. Alston of the British Museum. This specimen 

 is diploid and Ben Lawers is, as yet, the only confirmed locality in the British Isles where 

 it can be found. 



Another form of D. dilatata was collected from the island of Madeira in 1948 and 

 is also diploid (Manton, unpublished). This is a very elegant form with an even more 

 finely cut lamina and is quite distinct from the European diploids as illustrated by 

 Figs. 2 (a) and 3 (a). All these diploids exhibit 41 bivalents during meiosis as seen in 

 Plate 16, figs, (a) and (b). 



(5) Experimental 



(A) Hybrids of wild origin 



The extent to which hybridisation occurs in the wild is relatively unknown in 



