u 



PORTIONS OF SOUTH AUSTRALIA. 33 



or on the otfeer to that of material capable cjf imbibmg water in large 

 amounts. ThuSva condition of succulence may be induced, and 

 possibly also theNOTmation of mucilage/ cells frequently found in 

 xerojihytes. The presmce of heavy cell-walls, and possibly also the 

 condition of spininess cm«;acteristic of maiiy plants of dry regions, may 

 thus, at least in part, finoXrational ejqjlanation. ^ 



The temperatures of the air and of the soil are of very great im- 

 portance in many physiological processes of xerophytes as well as 

 other types of plants. Certain of 'the temperature relations may be 

 here mentioned. For example, ^e critical temperatures for growth 

 are to a degree specific, and on this fact may depend in part the charac- 

 teristic distribution of the species, its time of vegetative and repro- 

 ductive activity, and, in certain instances, the type of root-system 

 developed (Cannon, 1914:81, 1914:83, 1915:62, 1915:87, 1915^:211, 

 1916 : 75, 1916^* : 435, 1917 : 82). In evaluating the temperature of the 

 soil as an environmental factor the critical temperatures for growth of 

 any given species must be known, as well as the soil-temperatures at 

 the depth normally attained by the roots. The total expected growth 

 during the growing season with the aid of these data can be easily de- 

 termined. In this manner also we may learn the relative efficiency 

 of two stations as regards any species, so far as the soil-temperature is 

 concerned; also, the biological significance of a summation of soil- 

 temperatures may be foimd by the same means (Cannon, 1917: 91). 



As to the imme2^ate effects of temperature, only a few especially 

 applicable referencesNneed be given. The.o§Kioiic.ri]cegi.sure§ morease 

 with an increase in temperature and the rate at wl»©h.4issolved sub- 

 stances diffuse through -FKotcailasnLalso depends o!^ temperature. The 

 h^diQl^sis-of starch isJaaamedJay ,highfic,iie«>^atui to 45° to 



60°. C. The acid-content isSlowered with ^jpiertempeEatures. The 

 ra^joi gaseous -exchange) in respiration, ig' nearly propcoxtionaL to the 

 tempei^ture. The maximum mte occ^s at about 40° C, and the 

 minimum at 10° to 15° C. (Palladuy49l7). The carbohydrate equi- 

 librium of Opuntia sp. depends ^s^part on the water-content, and 

 j5L_pait- on the -temperature lif Jihe4>l§3at . An .increase, in. Jthe , tem- 

 perature results in...the- more rapid jusmg^ up of the availahle.,si3nple 

 carhoiiydratesT.the-jnonQ8accharid£S,.(^^o^> 1917:73). The rate of 

 water absorption in agar and in biocolloids Wreas^Jji,,.ggnjgjcail-'Hdth a 

 rSe ili'Femperature up to'ma:simjamjwe^BgJg^ihfi pla^t^^ 

 near 40° C. in agar and somewhat higher in the biocolloids (Mac- 

 Dougal, 1918:68). 



.The position4;ak©n in the^-ound-by Jihe roots of certaio-species has a 

 very definite relation tp_thg.>«reri!ioircondition&'€^> the -soil (Cannon, 

 1918: 81) and the distrilsution of cultivated plants (Howard and How- 

 ard, 1915), as well as certain species native to a semi-arid region (Can- 

 non and Free, 1917: 178), may also be directly related to the root re- 



