66 THE MORPHOLOGY OF PTERIDOPHYTES 



ancestry. Such a high number is beheved by some to be 

 characteristic of primitive plants, and in this connection it 

 is interesting to find that Lycopodium selago has a haploid 

 number n=i30, whereas species in other sections of the 

 genus have lower numbers (L. clavatum and L. annotinum 

 n = 34). But such a belief is justified only as a generalization. 

 High chromosome numbers may well point to ancient 

 origins in the majority of cases, but not in all, for polyploidy 

 could have occurred at any stage in the evolution of an 

 organism. Whenever it did occur, further evolution would 

 be retarded because of the masking of subsequent mutations. 

 Thus, if it occurred long ago, the ancient condition would 

 have become 'fixed', as may have happened in L. selago; 

 whereas, if it had happened recently, it would be possible for 

 an advanced morphological condition to be associated with 

 a high chromosome number, as in Phylloglossum perhaps. 



By contrast with the Protolepidodendrales and the 

 Lycopodiales, which are homosporous, the three remaining 

 orders of the Lycopsida (Lepidodendrales, Selaginellales 

 and Isoetales) are heterosporous. Another feature that they 

 share is the possession of a ligule, on the basis of which they 

 are sometimes grouped together as the Ligulatae. The hgule 

 is a minute tongue-like membranous process, attached by a 

 sunken 'glossopodium' to the adaxial surface of the leaves 

 and the sporophylls. A study of hving heterosporous lyco- 

 pods shows that it reaches its maximum development while 

 the associated primordium of the leaf or the sporophyll is 

 still quite small. The mucilaginous nature of the cells and 

 the lack of a cuticle have led to the suggestion that the hgule 

 may keep the growing point of young leaves and young 

 sporangia moist, but the fact is that no-one knows its true 

 function. It may even be a vestigial organ whose function 

 has been lost. 



