l62 



HEREDITY AND EVOLUTION IN PLANTS 



rado. According to Rydberg'- there is evidence that it 

 has spread not only in the earUer postglacial period, but 

 also in recent years. The common arctic and sub-arctic 

 grass, Phleum alpinum, occurs as far south as Arizona and 



S"^TO 





4i* 



' •' V- ,'''.^'-> 



i^lfi^ 



Fig. 75. — Diapensia lapponica. Photographed on the summit of Mt. 

 Madison, New Hampshire, June 25, 1917, by Ralph E. Cleland. 



California and the Sierra Madre of Mexico, and also in 

 Patagonia. It is also found throughout the Montane 

 zone, from which it might have spread to the subalpine, 

 following the woods throughout the whole mountain 

 system. Other similar cases might be cited. 

 ^ Letter from Dr. P. A. Rydberg to the author. 



