50 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



This feature reacts on the duration of life of individuals ; in such spots 

 annual species are rare.^ 



Moreover, roots branch more freely in moist than in dry soil. Upon 

 the production of root-hairs water also exerts an influence.^ 



As regards the forms of roots, many ' water-roots ' are known to 

 assume pecuHar forms, but we are ignorant of the actually operating 

 causes.^ 



CHAPTER XIII. TEMPERATURE OF SOIL 



The temperature of soil is a geographical factor of paramount signifi- 

 cance. In addition to what has already been recorded in Chapter VI 

 regarding the general significance of heat, it may be mentioned that 

 the junctional activity of the root depends upon the temperature of the 

 soil, and that it increases as the temperature rises up to a certain optimum. 

 A plant may wilt in a soil saturated with water if the temperature of the 

 soil sinks below a certain degree, because in such circumstances the roots 

 can absorb no water (the soil is physiologically dry) ; and a plant may 

 be frozen to death by a soil-temperature that is too low, although it be 

 capable of withstanding a far lower air-temperature ; beech, oak, and 

 ash can withstand an air-temperature of 25 C, but their finer roots 

 succumb to cold at from I3C. to i6C.* Many places on high 

 mountains and in Polar countries would be certainly devoid of vegetation 

 were it not for the temperature of the soil, for this may considerably 

 exceed that of the air. The temperature of soil may rise exceedingly 

 high in deserts. Bonnet observed a temperature of 59 C. in desert-sand 

 between low plants, when that of the air was 33 C. Pechuel-Loesche 

 observed a temperature of 75-82 C. in the soil in Loango. The tempera- 

 ture of soil and its fluctuations form the subject-matter of a considerable 

 number of recent papers. 



The effect of the temperature of soil -upon plant-form is but little 

 understood. Vesque ^ has experimentally shown that a high temperature 

 of the soil gives rise to an abundance of sap (short, thick roots, stems, and 

 leaves), possibly because the activity of the roots suffers from the heat. 

 These features may also be regarded as affording protection against 

 increased transpiration. Prillieux also concluded that a high soil- 

 temperature directly induces the production of tubers. In this way it 

 becomes easier to understand why succulent plants often grow on rocks 

 between stones, or on soil that is easily heated. 



Nanism may result from a low soil-temperature, if it causes a diminu- 

 tion in the amount of water absorbed and consequently of mineral 

 nutriment taken in ; this factor probably co-operates in inducing the 

 dwarfed growth generally prevailing in subglacial vegetation. It has 

 already been mentioned on p. 26 that cold soil calls into existence 

 prostrate shoots and rosette-like growths, whereas warm soil brings 

 forth slender, tall plants, as Krasan ^ has proved in Pinus, Juniperus, 



' Hildebrand, 1882. " F. Schwarz, 1888 ; also see Section III. 



^ For further information on the general influence of moisture in the soil, Gain, 

 1893, 189s, should be consulted. 



' Mohl, 1848. ^ Vesque, 1878. * Krasan, 1882-7. 



