THE ANCIENT FERNS 



three species and the rather more diffuse outhne which seems to be characteristic of 

 0. javanica. 



The available species of Todea and Leptopteris are grouped together in Fig. 278. Todea 

 barbara (L.) Moore is in Fig. 278^. The other three figures are of three species of 

 Leptopteris, namely, L. Frazeri (Hk. et Grev.) Presl, L. hymenophylloides (A. Rich.) Presl, 

 and L. superba (Col.) Presl. The close agreement of all these species also, both with each 

 other and with Osmunda, is very evident. Again, they all have n = 22 as regards 

 number, and the only detectable difference in this case is a slightly larger size of the 

 chromosomes in Leptopteris. 



That the resemblance between the Osmundaceous genera concerns not only their 

 chromosome number but also some features of their chromosome structure is indicated 

 by the few observations on spiral structure with which this chapter may conclude. In 



Fig. 279. Spiral structure in three genera of Osmundaceae. x 3000. In each case two chromosomes 

 from one species are shown, a. Todea barbara (L.) Moore, b. Osmunda gracilis Hort. c. Lepto- 

 pteris superba (Col.) Presl. 



Fig. 279 a there is Todea barbara, in Fig. 279^ Osmunda gracilis and in Fig. 279c we have 

 Leptopteris superba, all differing characteristically in chromosome size according to the 

 specific differences already noted but alike to a striking degree in the pitch of coil, the 

 number of gyres per chromosome and so on. 



One is left with the impression that the Osmundaceae alone, of all the ferns con- 

 sidered, are a genuinely primitive group which has remained primitive because it 

 has changed very little since a remote geological period. It is not merely a case of a 

 resemblance to an archaic ancestor shown by a form which has become specialized. The 

 primitive character is expressed in morphology, in anatomy and in the relatively simple 

 chromosome complement. This may perhaps suggest that the longevity and stabihty of 

 the family may in part be the expression of the stabihty of its nuclear construction. 



This idea will be touched on again in the next chapter. In the meanwhile we may 

 offer one last comment on the possible significance of the chromosome number itself In 

 the Flowering Plants, as explained in Chapter 2, the commonest haploid numbers are of 

 the order 7, 8, 9 and 1 1, though unexpected jumps to 22 occur in several places in the 



279 



