POLTPODIUM VULGARE 



is not always detectable when the leaf has been artificially flattened by herbarium 

 treatment, though it can sometimes be inferred by the crushed condition of the lowest 

 pinnae. Another detail which seems very constant is the shape of the sorus, which is 

 characteristically oval and not round. This character is more clearly visible in a young 

 leaf at or before the stage at which sporangia are ready for fixing than it is when the 

 spores are ripe. A very important character, since it directly affects the ecological 

 requirements, is the seasonal periodicity of the leaves. In the tetraploid, new fertile 

 leaves are put up in early summer (May or June in Britain, early July in Scandinavia) 

 and remain fresh unless battered to pieces at the end of the winter; the spores are ripe 

 in July or August. In 'var. serratum\ however, the principal dormant season is the 

 summer during which, in times of drought, the leaves may die away. New fertile fronds 

 only appear in autumn (August or September), and the spores are therefore ripe so 

 late that it may be suspected that in bad years in the more northerly habitats they may 

 perhaps fail to ripen at all. Lastly, the microscopic character of the number of cells in 

 the annulus is found to be extremely helpful; in all the localities listed above, the average 

 number of indurated cells is 5 with a total range from 4 to 6. A sporangium is 

 photographed in Fig. 133. 



It may be suspected that the claims of var. serratum to be regarded as a separate 

 species would have been generally recognized long ago but for the existence of the 

 hexaploid which is almost certainly in origin a hybrid between diploid and tetraploid 

 and which therefore not unnaturally combines characters of both. Some leaves are 

 photographed in Fig. 130^?, and a silhouette appears in Fig. 135^. The hexaploid 

 is always a coarse plant and often very large. The leaves are thicker and fleshier 

 than either of the others, but their shape, size being discounted, is almost exactly 

 intermediate between diploid and tetraploid. In some details, however, the characters 

 of the diploid seem to show simple dominance. Thus the projecting lower pinnae 

 otherwise characteristic of the diploid are often also present in the hexaploid. The 

 sori likewise are oval, as in the diploid, but the number of indurated cells in the 

 annulus is exactly intermediate, being on an average 9 (with a range of 8-10), although 

 all the cells are distinctly larger than in either diploid or tetraploid. The shape of the 

 sori and the nature of the annulus are illustrated in Figs. 131c and 134. With regard to 

 seasonal periodicity the hexaploid differs from both the others in having an extended 

 season from summer to autumn, and it was therefore probably no coincidence that 

 when fixings were first attempted in September 1944 at Windermere in the Lake District, 

 a region in which tetraploids abound, it was only on a hexaploid that a young fertile 

 frond was found. 



Ecologically and geographically the hexaploid seems to prefer a moister climate than 

 do either of the others. It is the commonest type in Ireland, Wales, south-west England 

 and the Channel Islands ; indeed in Jersey, where it is abundant all over the island, no 

 other form has so far been found. On the mainland of Europe its distribution is less 

 well known, though it is certainly present in coastal districts from Portugal to Holland 

 and inland it reaches the lower slopes of the Alps. 



That the hexaploid has indeed originated from a hybrid between diploid and tetra- 

 ploid which has attained stability by doubling its chromosomes is suggested not only by 



134 



