24 OECOLOGICAL FACTORS AND THEIR ACTION SECT, i 



of many mosses, lichens, and other lowly organized plants. Lignified 

 parts endure cold better than do herbaceous parts, 1 consequently many 

 arctic and alpine species are woody (dwarf -shrubs). Southern shrubs 

 cultivated in North-European gardens, and trees and shrubs on the 

 Faroes, 2 often do not receive sufficient heat to enable them to ripen their 

 wood ; the ends of their branches are killed by the cold in winter ; and 

 they dwindle from shrubs to sub-shrubs. In places with a longer vege- 

 tative season the same species (Broussonetia, Tamarix, and others in 

 Hungary) endure, despite equally severe winter-cold. The forests in 

 arctic Siberia withstand temperatures descending to -7OC. (in Ver- 

 choyansk during January the mean temperature is -51-2 C., the mean 

 minimum -63-9C., and the lowest recorded temperature -69-8^.) 

 When a plant is frozen to death this is usually associated with a formation 

 of ice, and thus with a drying up of the cell-sap. 



3. Bad conductors of heat, for example bud-scales or hairs, often 

 envelop the parts requiring protection ; their cells are mostly filled with 

 air or have between them air-containing intercellular spaces, and contain 

 as little water as possible. Very many protective devices are displayed 

 by young shoots when in foliage. 3 Many arctic and alpine plants have 

 a grey cottony or white woolly coat of hairs (Leontopodium alpinum, 

 which is ' edelweiss ') ; ' Frailejon,' which are Compositae occurring on the 

 paramos of South America and belong to the genera Culcitium and 

 Espeletia, 4 the shoots of these plants are encased by old faded leaves 

 which hang on and envelop them, 5 just as in Central Europe tender 

 garden plants in autumn are artificially clothed with straw, hay, leaves, 

 and the like. It must be pointed out that though these devices hardly 

 exclude extreme cold (for this may reach the interior of the plant), yet 

 they ward off three contingencies rapid change of temperature, rapid 

 thawing, and precarious transpiration. Experience and research have 

 shown that, though cold itself is sometimes responsible for the death of 

 plant-members (potatoes, petals, tropical plants in high stations of Brazil, 

 and so forth) which have been fatally frozen, yet the act of thawing is 

 the critical matter in the case of many plants which can be frozen solid 

 without injury. Hence thawing ought to take place slowly, and in this 

 direction assistance is rendered by the above-mentioned structural 

 features, which are therefore particularly met with in sub-glacial com- 

 munities. Repeated sudden freezing and thawing cannot be endured by 

 the majority of the plants of Central Europe (e.g. beech, oak, and others). 

 In contrast with this, Kihlman 6 asserts that ' the extraordinary power 

 of enduring great and rapid oscillations of temperature, and even of 

 withstanding the recurrence of freezing-point several times within twenty- 

 four hours, is an outstanding peculiarity' of the tundra-vegetation of 

 Lapland. 



Submerged aquatic plants are well protected by the surrounding 

 water ; many of them sink to the bottom or possess buds that become 

 detached and sink in like manner. 



The devices mentioned also serve as a protection against rapid transpi- 

 ration against the desiccating action of dry, cold winds, which are 



1 Mohl. 1848. Borgesen, 1905. 



3 See Griiss, 1892. Gobel, 1892. 



See p. 75 and Chap. LXVI. Kihlman, 1890. 



