CliAF. 11] HEAT 37 



extremes, the optimum of each function is characteristic for each species, 

 and stands the higher the more the minimum corresponds to a higher 

 temperature. 



The data indicated above form the only basis for research into the influence 

 of temperature on the distribution and mode of life of plant-species. 



In opposition to the onlj' scientific interpretation of the effects of temperature 

 which J. Sachs established by careful experiments, phaenology'^ assumes much 

 simpler relations between heat and plant-life, as according to its teaching there 

 is a direct connexion between the development of the plant and the degrees of 

 the centigrade thermometer. Thus, in order to ascertain the amount of heat required 

 by an annual plant, the mean temperatures, or also the maximum temperatures, 

 from the sowing to the ripening of the seed on those days on which they exceed 

 zero centigrade, are added together. The sum of these temperatures for one and 

 the same species should be identical for any climate. This theory, as might 

 naturally be expected, is not confirmed by facts. The sum of the temperatures 

 of the individual physiological processes is calculated in a way similar to that of 

 the whole total. For perennial plants the temperatures are usually reckoned from 

 the first of January, and not from the date of germination. 



Further discussion is hardly necessary to show that phaenology cannot lead to 

 exact results. Independently of the arbitrary nature of the choice of the degrees 

 of temperature and the date of commencing the calculation, this theory absolutely 

 ignores the facts that degrees of temperature are not physiologically eqHivaleiit^t\\aX 

 in some cases 35° or 30° are less favourable than 25° or 20", but resemble in their 

 effects 10° or 15° — that different organs and functions require very different amounts 

 of heat, that unfavourable temperatures cause subsequent inhibition, and that other 

 factors besides heat, especially humidity, co-operate and intervene. We need not, 

 then, be surprised if there is very little accord in phaenological observations, and 

 that the utmost one can do is to admit their having a certain importance for purely 

 descriptive geographical botany in the characterization of certain districts. No 

 importance, on the other hand, need be assigned to the theoretical views, nor to the 

 sum total of temperatures. 



Physiological researches regarding the influence of temperature on the 

 vital processes of plants are unfortunately not yet numerous. Particularly 

 necessary is it to determine the extremes of temperature and the optima 

 of plants that are much more dependent on definite conditions of heat than 

 are most of the types already investigated. Information on these points, 

 for instance, does not exist in regard to the tropics, polar zones, or alpine 

 heights. Only on the basis of a great number of exact data can we hope 

 to gain a clearer insight into the connexion between temperature and 

 plant-life in different climates. The few observations relating to the 

 subject, and others which throughout require critical confirmation, are given 

 in the following pages. 



^ See Hoffmann, op. cit. 



