Systematic Atavism 649 



to be of rare occurrence, 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, Equisetum Telmateja, which occasion- 

 ally bears cones on its green summer stems. 

 Its whorls are changed on the twisted parts into 

 clearly visible spirals. The ironwood or Cas- 

 uarina quadrivalvis is sometimes 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 



