THE GENUS EQUISETUM 



the coast of Wexford {E. trachyodon is a plant of river margins). It has relatively stout 

 erect green shoots more like those of E. hiemale than of E. variegatum, though with char- 

 acteristic details of the surface markings of its own. It cones in August. Fig. 227^ shows 

 metaphase i in E. Moorei. The very irregular pairing, including multivalents and uni- 

 valents, is very striking. This, together with abortive spores, is a clear indication of 

 hybrid origin, and though speculation might be possible to explain the coexistence of 

 both E. Moorei and E. trachyodon in Ireland, it is more profitable to defer this until a 

 hybrid of appropriate kind has been synthesized. Until this has been done it is perhaps 

 best to confine our present conclusions to the statement that E. Moorei is not a good 

 species in the usual sense of the word and to commend it to Irish botanists for closer 

 scrutiny. 



The detection of three species hybrids among little more than a dozen representatives 

 of the genus is a surprisingly large number, especially when the rarity of prothalU is 

 remembered, and it suggests fairly clearly that speciation can still occur. The complete 

 absence of polyploidy is, however, noteworthy, and the extreme uniformity in chromo- 

 some number found throughout the genus is also in striking contrast to the behaviour 

 of ferns. At the same time a haploid number as high as 108 cannot be thought of as 

 primitive, though we are never likely to know by what stages or from what simpler 

 state it was evolved. As we find them now, the Horsetails give the impression of being 

 a very ancient and a very stable group, still able to make new species by genetical means 

 though doing so only slowly, but long out of the habit of giving rise to new generic types. 

 The very stereotyped morphology is perhaps not so much primitive as speciahzed upon 

 an archaic pattern, and the general cytological condition is perhaps also best interpreted 

 as ancient rather than simple. 



SUMMARY 



Mitosis and meiosis in all the European species oi Equisetum and of one American species 

 have been seen. In all, the haploid chromosome number is either exactly or approxi- 

 mately 108, though a slight possibihty remains of a minor numerical difference between 

 species of the two subgenera, Eu-equisetum and Hippochaete. There is a diflference of 

 chromosome size between these subgenera, the latter having the larger chromosomes. 

 Three Irish forms, namely, Equisetum litorale, E. Moorei and E. trachyodon, have been 

 shown to have the meiotic behaviour of hybrids. Spiral structure of chromosomes has 

 been demonstrated in E. trachyodon. The list of species and hybrids examined is : 



Subgenus Eu-equisetum 



231 



