264 GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



No. III. 



ON THE GEOGRAPHICAL DISTRIBUTION OF PLANTS 

 IN THE COUNTRY NORTH OF THE 49th PARALLEL 

 OF LATITUDE. 



Generic and Specific Forms of Plants' Decrease in Number as the 

 Latitude increases. — Analogy between Altitude and Increase of 

 Latitude. — Culture of the Vine. — Of the Cerealia. — Maize. — 

 Wheat. — Oats. — Barley. — Potatoes. — Botanical Districts. — Their 

 Physiognomy. — Woodland District. — Barren Grounds. — Prairies. 

 — Rocky Mountains. — Sitka. — Polar Plants. — Arctic Zone. — 

 Trees and Shrubs. — Table of Distribution of Species in three 

 several Zones. — Carices. , 



Though the isotharal lines, when the term is restricted 

 to the mean temperatures of the three summer months 

 of June, July, and August, run from Lake Superior 

 northwards to the Mackenzie, yet the short duration of 

 the summer on the banks of that river, and the occasional 

 frosts in June and August, and in some years even in July, 

 render the climate unsuitable for numbers of vegetables 

 which flourish in the northern districts of the United 

 States. Many trees, shrubs, and perennial roots can be 

 frozen without injury if the frost be continuous throughout 

 the winter ; and they acquire so much irritability in their 

 hybernation, that the stimulus of perpetual though less 

 fervid day within the arctic circle causes them to perform 

 the functions of foliation and fructification with a rapidity 

 unknown in more temperate regions. Other plants which 

 need longer time to perfect their fruit or woody fibre, ter- 

 minate in succession according to their several constitutions 

 as the latitude increases. Their place is only partially 



