GEOGRAPHY OF PLANTS. 471 



THE DEFINITE AMOUNTS OF HEAT REQUIRED BY PLANTS. 



\V hen considering the coincidences existing between the distribution of 

 plants, according to their respective zones of climate on plains and on zones' 

 of elevation on mountain sides, we have had especial reference to the mean 

 annual temperatures prevailing in the respective regions observed. These, it 

 has been remarked, are in general correct as regards the distribution of forest 

 txees, which brave the extreme cold of winter, and require but a few months 

 of moderate heat for foliation and for maturing their fruit. The discrepancies 

 found to exist between the isotherms in latitude and isotherms in elevation 

 are more especially noted when we come to consider the arrangement of annual 

 plants under these respective relations. Originally the mean annual tempe- 

 rature was alone observed, and the polar limits of plants, it was presumed, 

 could be thereby determined. But opinions on the subject of polar limits, or 

 isothermals, bounding on the north in the northern hemisphere the extension 

 of certain species of plants, have changed with the progress of physical geog- 

 raphy. More recently it was taught that the mean temperature of seasons is 

 of more importance than that of tiie year, and that in general two similar cli- 

 mates may be distributed in fractions very dissimilar, which may neutralize 

 one another in the estimate of their mean annual temperatures. Thus it has 

 been observed that countries in which the summers are short but very warm, 

 and the winters very long and cold, have a vegetation different from that of 

 those which enjoy seasons more equable, and where the march of temperature 

 from month to month is more gradual, or the changes less sudden and violent, 

 although the mean annual temperature of both ma}^ be the same. It is, there- 

 fore, believed that to the rehxtive distribution of heat over the seasons rather 

 than to the absolute amount received through the year that we are to attribute 

 the fitness or unfitness of a region for the growth of certain kinds of vegeta- 

 tion. In other words, that in a knowledge of the mean temperature of seasons, 

 or of monthly temperatures, we may find an explanation of the causes that 

 have influenced certain species of plants in their choice of home. Also that 

 they have advanced from their original centre over a continent to a certain 

 line which passes through points having an equal temperature during some 

 period of the year, which may vary with various plants, and have occupied 

 all the region to the border thus indicated, unless their extension has been 

 arrested by a climate too dry or too humid, or by the impassable ocean. 



In the progress of knowledge, it has become apparent that these lines of 

 equal temperature or isotherms for certain seasons do not answer all the re- 

 quirements of each case ; that they do not limit even the extension of annuals, 

 though theh" vegetation is confined wholly or mostly to the three months of 

 summei". 



It appears that the conditions which define tbc limits of a plant require that 

 we should know the degree of temperature at which its vegetation begins and 

 ends ; that at which it will flower and will mature its seeds or fruit ; and also 

 the sum of the mean daily temperatures during these periods respectively. 

 The hypothesis that a definite amount of heat is required iu order to develop 

 each plant iu its progress from one stage of growth to another was first ad- 

 vanced by Reaumur, better known in America from the thermometer which 

 b^ars his name than through the scientific labors which added largely to the 

 wealth of his native Fra^nce. This philosopher proposed to calculate the 

 amount of heat demanded by a plant, by multiplying the number of days re- 

 quired to pass through its growth by the mean temperature of the period. 



To Michael Adanson, a French naturalist of comprehensive mind, great 

 acuteness and perfect independence of thought, but too far in advance of his 

 contemporaries to be rightly appreciated, we are indebted for the hypothesis 

 that, by adding together the mean temperatures of each day from the com- 



