THE GENUS EQUISETUM 



A further illustration of both absolute and relative sizes of the chromosomes at a com- 

 parable stage in one species of each of the two subgenera is given, from sectioned 

 material, in Fig. 217. In the upper photograph, showing diakinesis in E. arvense, the 

 chromosomes are distinctly smaller than in the lower photograph of E. robustum. This 

 demonstration has been included partly for comparison with sectioned preparations 

 illustrated in other chapters but also because Equisetum represents an extreme example 

 of the extent to which chromosome size can be affected by different technical treatments 



1^ 



V t 



i^. arvense n = ca. 108 

 Fig. 219. Explanatory diagram to Fig. 2i8fl. x 1500. 



without the production of any other visible distortion. The fact that acetocarmine 

 almost doubles the apparent dimensions is one of the many reasons for the great useful 

 ness of this reagent, but a very misleading impression could be produced by mixing one's 

 techniques if it were wished to do so. Thus Fig. 218 shows on the same page two cells 

 of the same species, E. arvense, already illustrated in Fig. 217a, photographed at the same 

 magnification ( x 1 000) . The upper figure is from a Feulgen squash mounted in balsam. 

 The lower figure is from a fresh acetocarmine squash photographed in that liquid. The 

 exceptional clarity of the latter specimen was due partly to the fact that it had become 

 spread in close contact with the glass of the cover-slip, and the optical disadvantages of 

 observation in a liquid of low refractive index were therefore absent. The cell was 

 also exceptional in the extreme degree of enlargement which the chromosomes had 



221 



