THE LYCOPODS (CLUBMOSSES) 



being radially symmetrical, with uniform leaves, and no rhizophores, the other two 

 species being of the dorsiventral type with rhizophores. Their very close cytological 

 similarity must mean that considerable uniformity is likely to prevail throughout the 

 genus, and yet in spite of this, or even perhaps because of the low chromosome number, 

 far more active speciation seems to have been at work here than in most other living 

 members of the group. Why such a low chromosome number should have persisted here 

 and not elsewhere in the Pteridophyta is a question which we are not likely to answer, 

 but the fact that it has done so is of importance to this inquiry as a clear indication that 

 the high chromosome numbers found elsewhere and increasingly so in the most ancient 

 groups are, in a sense, secondary. The Pteridophyta seem almost certainly to have pos- 

 sessed in the past, and to have retained in these two genera, the simple nuclear state that 

 the modern Flowering Plants still for the most part display. In this one particular of 

 chromosome number the heterosporous members of the Lycopodiales seem to have re- 

 tained a primitive condition, though little else about them would suggest that they are 

 simple. 



SUMMARY 



In a review of all the British species of Lycopodium and Isoetes and of all the European 

 species oi Selaginella special morphological interest attaches perhaps to the new observa- 

 tions on the prothalli and growth forms of the terrestrial species of Isoetes [I. hystrix) . 

 Special points of cytological interest are the wide diversity and peculiar shapes of the 

 chromosomes of the species of Lycopodium together with the very remarkable display of 

 unpaired chromosomes suggesting hybridity in L. Selago. Selaginella and Isoetes hystrix 

 are exceptional among all known members of the Pteridophyta for the small size and low 

 number of their chromosomes. The other cytological facts provided in the chapter are 

 contained in the following list: 



261 



