22 OECOLOGICAL FACTORS AND THEIR ACTION SECT, i 



essential reason for the structural differences in question. Transpiration 

 increases with an increase of insolation. Thus light is one of the most 

 important factors influencing transpiration, and the plant regulates the 

 latter according to the intensity of the former. But for further infor- 

 mation in this matter we must look to the future. 1 



CHAPTER VI. HEAT 



HEAT is to a far higher degree than is light an oecological and geo- 

 graphical factor, not only in general, but also in detail. 



Each of the various vital phenomena of plant-life takes place only 

 within definite (minimum and maximum) limits of temperature, and 

 most actively at a certain (optimum) temperature ; these temperatures 

 may even differ in respect to the different functions of one species. 

 Heat is of import in the manufacture of chlorophyll, in the processes 

 of assimilation, of respiration, and of transpiration, the functional 

 activity of the root, germination, the production of foliage and blossom, 

 growth, movement, and so forth. It is therefore clear that conditions 

 as regards heat determine the boundaries of the distribution of species 

 on the Earth. 



As the lower and upper critical temperatures vary greatly in different 

 species, we can only say generally that the lower critical point (' specific 

 zero' of the species) descends to oC., or slightly lower in certain rare 

 cases, which include many arctic and alpine species, mostly of low organ- 

 ization. Algae in the Arctic Ocean, off the coast of Spitzbergen, at about 

 80 N., grow and fructify vigorously during winter, in darkness at a tem- 

 perature of 1-8 C. to oC. ; of twenty-seven species Kjellman observed 

 twenty-two with reproductive organs. Usually the functions do not com- 

 mence activity until a temperature several degrees above o C. is reached, 

 in some cases (especially in tropical plants) not before 10 C. or 15 C. The 

 upper critical temperature does not attain 50 C., and generally not 

 even 45 C. 



The different organs of a plant usually have different capabilities of 

 enduring extreme temperatures. A species may therefore thrive in 

 a country and produce blossom, but its seeds may not ripen, or if they 

 do so may not be able to resist frost, or the seedlings may suffer from 

 the cold. Such a species would be dependent upon vegetative propagation 

 for its permanent existence in such a country. 



Heat has also an indirect significance, in that the relative humidity 

 of the air and transpiration depend upon temperature. 



Temperatures outside those that are critical to species are not necessarily 

 equally lethal ; in this respect there is a certain amount of latitude, which 

 is greatest below the specific zero that is to say, plants can, without 



li F further information reference should be made to the works of Areschoue 



Stahl Pick, Dufour, Haberlandt, Heinricher, Vesque, Viet, Mer, Lothelier Tohow 



AV ii n ' Eb ? rdt Schimper, Grabner, Wiesner, Hesselman, Woodhead, Stebler' 



and Volkart. In regard to photometry the works by Wiesner, K. J. V. Steenstrup 



(i 90 1), and Hesselman (1904) should be consulted. 



