Books and Current Literature. 203 



when distilled water is employed, and that the reaction is "par- 

 alyzed" when a 0.1% solution of a neutral, acid, or alkaline salt 

 is added, while with a weaker concentration, say 0.01%, a dis- 

 tinct positive reaction follows. From these and numerous other 

 experiments the author concludes that the curvature of roots 

 in different media may be classified, at least in part, under the 

 phenomena of chemotropism. 



Junge, in a painstaking monograph of the "Cyperaceae of 

 Schleswig-Holstein," classifies the 52 species there found under 

 four heads as follows: (1) Species (45 in number) for which 

 Schleswig-Holstein is considered the center of distribution. 

 (2) Three species which have entered this region from the south- 

 west or west. (3) One species which has entered from the north. 

 (4) Three species from the east or southeast. One reason for the 

 limited area of distribution of the species of groups 2 to 4, as 

 compared with those of group 1 , seems to be that their movements 

 have taken place largely in a later period. Peculiarities of soil, 

 especially of temperature and moisture, have also been potent 

 factors. 



Wolf's "Monograph of the Genus Potentilla" (Stuttgart, 

 1908) shows that the species of this genus occur chiefly from the 

 Tropic of Cancer northward. In America its representatives 

 are not found beyond Mexico, except two in Guatemala, another 

 on the Andes in the northern part of South America, and P. 

 anserina, which is capable of enduring transportation in salt 

 water, and occurs as far south as the coasts of Chile and New 

 Zealand. In Asia species of Potentilla are seldom found beyond 

 the Sahara, and in Australia the genus is unknown. It is as- 

 sumed from these and other facts that the genus originated in 

 the temperate and colder regions of the northern hemisphere 

 and has there attained its present development. 



Wettstein records the occurrence in the Tirol of an annual 

 form of Ranunculus alpestris which came true in cultures and 

 appears to be a plain instance of an annual arising as a mutant 

 from a perennial. Observations of the same author on Solda- 

 nella pusilla indicate the occurrence of mutation in nature of 

 this species. 



