THE GENUS EQUISETUM 



could be similarly interpreted ; this cell could therefore possess a maximum number of 

 I lo. On the other hand, with small chromosomes of rather unequal sizes it is perhaps 

 more likely that a tiny pair lying near another might be falsely regarded as part of it. 

 Examples of doubt of this kind are shown by the arrows in Fig. 219, which refers to the 

 Feulgen squash oi E. arvense in which the maximum number is 108 though it could be 

 107 or 106. That the last alternative is incorrect is, however, proved by Fig. 218^. In 

 this there are also two doubtful places, but the range of possible numbers is 1 07-1 10. 

 The correct interpretation of this cell is probably that the upper doubtful place is 

 actually one pair with the halves pushed somewhat asunder, while the lower doubtful 



' ♦ ' ^ f , / 





/ 



Fig. 221. Drawing of the cell of Fig. 220 made whilst it was still fresh, x 1000. In addition to the 107 

 bodies shown there were four small granules which could have been the arms of a dismembered pair. 



place is composed of two rather more compact and smaller pairs held deceptively near 

 one another by their attachment to the surface of the nucleolus. It must be recognized, 

 however, that a genuine difference of one or two chromosomes between Eu-equisetum and 

 Hippochaete would be very difficult to demonstrate and is therefore almost equally diffi- 

 cult to disprove. Should it exist, all these estimates will need slight revision, but if it 

 does not the only number which will fit the facts for every species is n = 108. 



The general uniformity of these cytological results coupled with the signs of antiquity 

 to be found among the living species might, at first sight, lead one to suspect that 

 evolutionary activity within the genus Equisetum is perhaps becoming exhausted. Direct 

 evidence that some at least of the raw materials of evolution are nevertheless present is, 

 however, provided by the surprising number of interspecific hybrids which have been 

 discovered in Ireland. That this small country is specially favoured in this respect is 

 probable from the known rarity of Horsetail prothalli in most other countries. No 

 actual records of gametophytes exist for Ireland, but it may safely be assumed that its 

 mild, moist, oceanic climate is likely to favour their production. 



223 



