CHAP. VI HEAT 25 



dangerous to plant-life when the soil is cold and physiologically dry and 

 the activity of the roots is arrested. 



To species both in respect of their conditions of life and their 

 distribution it is by no means without import which of the efftcient 

 temperatures (those between the maximum and minimum) prevail. The 

 life of the individual is influenced not only by the height of the 

 temperatures to which it is exposed, but also by the amount of efficient 

 heat received or the duration of efficient temperatures. Annual mean 

 temperatures are devoid of significance to plant-life. Only the season 

 during which useful temperatures prevail is of import.^ Thus in 

 Northern Siberia, where the mean annual temperature is below 15 C, 

 forests occur, yet on Kerguelen Island, where the mean temperature 

 ven of the coldest month is above freezing-point, the vegetation is 

 arctic. 



In most regions of the Earth change of season causes plant-life to 

 undergo a period of rest. The cause in north-temperate climates is change 

 of temperature, and particularly lowness of temperature ; in the tropics 

 it is lack of water. The time during which efficient temperatures are 

 available may be so short, sometimes only a few weeks in length, that 

 many species are excluded because they cannot obtain sufficient heat. 

 This certainly explains why annual species are so rare in arctic latitudes 

 and at alpine altitudes ; they require for the completion of their life-cycle 

 more time than is available. 



Perennial herbs in arctic countries and on high mountains display 

 much variety in their adaptation to climate. For example, they may 

 have perennating foliage-leaves, which sometimes contain reserve food, 

 and with the aid of these they can utilize each passing moment that is 

 favourable to assimilation, and 'lose no fraction of the vegetative season 

 in producing new assimilating organs.- They display another adaptation 

 in that they initiate their flowers in the year before these open, so that 

 they can burst into blossom at the immediate commencement of the 

 succeeding spring, and thus have as long as possible a period of blossoming 

 and fertilization, and can utilize the warmest season for maturing the 

 seed.^ 



The temperature and length of the vegetative season affect the 

 physiognomy of the individual plant and of the whole vegetation. At 

 one extreme are equatorial countries, where resting seasons are all but 

 imperceptible, and where high temperature is linked with humidity ; 

 here is developed the evergreen tropical plant-life whose luxuriantly 

 growing species clothe the soil with the densest of vegetation. At the 

 other extreme are arctic countries and regions on high mountains, where 

 Nature doles out her gifts with niggardly hand perhaps for only three 

 months in the year ; here the plants developed are, in places, not sufficient 

 to cover the soil ; here, too, dwarf-forms present themselves, because, 

 among other reasons, the vegetative season is too short and the efficient 

 heat too feeble. With increasing heat the rate of growth accelerates until 

 an optimum is attained. But in the two last-named sites, low vegetation, 

 condensed shoots, rosette-plants, small leaves, and caespitose habit, 

 inevitably result. In the tropics, dwarfed growth may result when a high 

 temperature is combined with drought. 



* Koppen, 1884. ' Kcrner, 1896. * See Warming, 19080, 



