Systematic Atavism 649 



to be of rare occurrences, as I have not suc- 

 ceeded in getting any repetition by prolonged 

 culture. 



Species, which generally bear their leaves in 

 whorls, are also subjected to casual atavisms of 

 this kind, as for instance the tall European 

 horsetail, Equiseteum Telmateja, the same 

 which occasionally bears cones on its green sum- 

 mer stems. Its whorls are changed on the 

 twisted parts into clearly visible spirals. The 

 ironwood or Casuarina quadrivalvis is some- 

 times observed to produce the same anomaly on 

 its smaller lateral-branches. 



Coming now to the discussion of the way in 

 which the twisting is the result of the spiral dis- 

 position of the leaves, we may consider this ar- 

 rangement on stems in the adult state. These 

 at once show the spiral line and it is easy to fol- 

 low this line from the base up to the apex. In 

 the most marked cases it continues without in- 

 terruption, not rarely however, ending in a 

 whorl of three leaves and a subsequent straight 

 internode, of which there may even be two or 

 three. The spiral exhibits the basal parts of the 

 leaves,, with the axillary lateral branches. The 

 direction of the screw is opposed to that of the 

 twisting, and the spiral ribs are seen to cross the 

 line of insertion of the leaves at nearly right 

 angles. On this line the leaves are nearer 



