CONDITIONS FOR THE GROWTH OF PLANTS. 31 



is naturally that of temperature ; nevertheless, we must not 

 think, as many botanists did till very recently, that the limits 

 of the zone of vegetation of each plant are marked on the 

 continents by the insinuosities of the isothermal lines. In fact, 

 as Charles Martins and Alphonse de Candolle remark, each 

 plant requires for its germination and development a certain 

 amount of temperature, differing according to the species. 

 With some, life resumes its activity after the sleep of winter, 

 when the thermometer marks three or five degrees above the 

 freezing-point ; others need a heat of eighteen, twenty, and 

 even twenty-five and thirty-five degrees, before taking the first 

 step in their career of the year. Each species has, so to say, 

 its particular thermometer, the zero of which corresponds to 

 the degrees of temperature when the vegetating force awakens 

 its germs. It is, therefore, impossible to indicate by such 

 general climatal lines the limits of habitation for such or such 

 species, since each one of them has for the commencement of 

 its vital period a different starting-point." 



