CHAP. VI HEAT 23 



suffering death, be exposed to low temperatures that are more degrees 

 below the minimum than high, actually fatal, temperatures are above 

 the maximum. (Possibly the sole exception is provided by many bacteria.) 

 Moreover, temperatures below the minimum and above the maximum 

 are not always devoid of significance to plant-life, even if they be not of 

 direct utility. 



On the Earth there is scarcely a spot from which plant-life is absolutely 

 excluded by reason of the thermal conditions ; for even in places where 

 the temperature remains for months below the minima of species, or 

 above the maxima (e. g. in parts of Africa), plants thrive at certain seasons 

 of the year. Yet it may be necessary for plants to guard against extreme 

 temperatures and against that which these involve, change of temperature. 

 To the latter many plants (e. g. palms) are much more sensitive than to 

 low temperatures. Sudden thawing is injurious to many plants because 

 the tissue is ruptured ; forests often suffer from night-frosts on the east 

 side, on eastern slopes, and on similar spots where the sun's rays strike 

 them early in the day. 



The following means are adopted as affording protection against 

 extreme temperatures, and particularly against such as are too low'^ : 



1. The cell-contents of some plants have certain (hitherto unexplained) 

 characters in virtue of which they can withstand extreme temperatures 

 for a long time : in phyto-geography, extremes of cold almost alone have 

 to be considered. These resistant characters may be due to the proto- 

 plasm itself, or to the admixture of sugar, oils, or resinous bodies, with 

 the protoplasm or cell-sap. Protection of this kind is apparently exempU- 

 fied by the snow-alga, Sphaerella nivalis, whose thin-walled isolated cells 

 can endure the cold of arctic snow-fields and ice-fields.^ Likewise Coch- 

 learia fenestrata is evidently protected ; for on the north coast of Siberia, 

 in the winter of 1878-9, this plant endured unsheltered a temperature 

 remaining lower than 46 C, and in the following spring it continued its 

 flowering which had been interrupted by winter.^ In a number of trees 

 at autumn time the starch changes to fat ; * this is probably of use, in that 

 fatty oil in the form of emulsion prevents sub-cooling and increases the 

 power of resistance to frost. Fat-storing trees (birch, conifers) are 

 precisely the ones that grow in the coldest lands. The change that takes 

 place during winter of soHd reserve substances into dissolved substances, 

 namely sugar, also prevents the under-cooling of plant-tissue and death 

 of the plant. -^ 



2. Amount of water. The water contained in plant-parts plays the 

 leading role in regard to their power of enduring extreme temperatures ; 

 the power of endurance is inversely proportional to the amount of con- 

 tained water. Consequently the young shoots of North-European trees 

 often suffer from late frosts, while the older shoots are not damaged ; 

 also seeds, which are always poor in water for instance those of the 

 wheat can hibernate uninjured for many years in arctic countries. The 

 smallness of the amount of water possibly also explains the perennation 



' For more recent work on the endurance of plants in winter, and on the eflfcct 

 of freezing, see Mez (1904-5), and Lidforss (1907). 



- Wittrock, 1883 ; Lagerheim, 1892. " Kjellman, 1884. 



* A. Fischer, 1891 ; O. G. Petersen, 1896. 

 ^ Mez, loc. cit. ; Lidforss, loc. cit 



