Chap. II] HEAT 49 



4. ACCLIMATIZATION. 



The absolute extremes of temperature of plant-life are constant for 

 individual species, but the cardinal points vary for the separate functions 

 according to climatic conditions ; they are susceptible of a limited displace- 

 ment, either upwards or downwards, so that a plant when transferred from 

 one climate to another with a different temperature may, provided the 

 latter climate is not too dissimilar, often adapt itself to its new environment 

 and become acclimatized. The possibility of acclimatization always varies 

 with the species ; in some it appears to be unlimited under natural 

 conditions, while in others it takes place only within very narrow limits. 

 Complete acclimatization is therefore only possible when all the cardinal 

 points change in harmony with the new temperatures. If this is not the 

 case, or not sufficiently so for certain functions, acclimatization is confined 

 to definite processes, and the plant either is not capable of existing or does 

 not develop completely. It has already been mentioned that many 

 temperate cultivated plants exhibit vigorous vegetative growth in the 

 tropics, whereas they blossom but seldom ; in such cases, the cardinal 

 points for the growth of flowers, and perhaps also for their inception, 

 are either not raised or not raised sufficiently. On the other hand, many 

 plants from the warm zones do not produce flower or fruit in colder 

 countries, because the minimum temperature necessary for their production 

 is not attained. 



A plant may be cultivated and, as observations in the polar zones and 

 in the Alps show, in cases of rich vegetative propagation, even exist in 

 the wild state, without producing seed. If, however, the cardinal points 

 for such indispensable functions as germination, growth, nutrition, and 

 so forth in a new climate have not been correspondingly displaced down- 

 wards or upwards, then the continued existence of the plant is obviously 

 impossible. 



If we compare individuals of the same species in climates of different 

 temperatures, we soon become convinced that certain functions in hot 

 climates are associated with higher temperatures than in cold climates. 

 The cardinal points of temperature are therefore not the same everywhere. 

 The difference is first of all hereditary, so that, for instance, seeds from 

 a cold climate germinate in a warmer one for some years at lower tempera- 

 tures than do seeds of the same species that have been produced in this 

 warmer zone, and the plants developing from them grow more quickly. 

 But soon, however, owing to a gradual upward displacement of the cardinal 

 points, this difference disappears. The reverse takes place when the 

 transference is from a warmer to a colder zone. 



'In the year 1852, maize for poultry (from Hohenheim near Stuttgart) was 

 harvested on September 22. 120 days after the sowing. . . . This maize year 



SCHIMFER t- 



