32 



THE AMERICAN BOTANIST. 



sels (latitude 50°) fifty clays, and at Cliristiania (latitude 

 60°) eighty-six days. In continental Russia the retarda- 

 tion is much greater, and while at Paris many spring 

 plants are in full bloom, vegetation on the Russian steppes, 

 at the same latitude, is still deep in winter sleep. Again, 

 we may ascertain the places where spring plants blossom 

 simultaneously, and a comparison of the latitudes will 

 reveal the influence of climate. Thus, at New York, which 

 is at the same latitude as Naples, plants blossom at the 

 same time as at Marburg, Germany, 10° latitude farther 

 north. 



Comprehensive statistics of this kind are available and 

 would repay detailed investigation. I think, however, in 

 phenol ogical observations too exclusive attention is given 

 to the effect of temperature on growth. The condition of 

 the soil, which is the seat of complicated physical and 

 chemical changes, is of the highest importance to the 

 growth of vegetation, and it may be that frequently the 

 permanent warmth of the soil has more to do with start- 

 ing the flow of sap in spring than the temperature of the 

 air, since in countries without a winter covering of snow 

 the temperature of the soil is higher than that of the air 

 above it throughout the year. The influence of moisture 

 is also largely neglected. In tropical countries many 

 plants undergo a period of rest during the dry season, 

 which corresponds with our winter, and an increase in the 

 amount of moisture at the beginning of the rainy season 

 calls forth all the phenomena of a luxuriant spring. In 

 addition, while many plants possess elaborate means of 

 protection from excessive loss of moisture by transpira- 

 tion, it is still doubtful whether any means of protection 

 from changes of temperature have been discovered. A bot- 

 anist can tell at a glance whether a plant occupied an arid 

 or a humid region, but he can not say that it belonged to 

 th€ flora of a cold or warm climate. 



The determination of spring by the successive advance 

 of an adopted isothermal line is the method which natur- 

 ally commends itself to the meteorologist, and is the only 



