40 GEOGRAPHICAL DISTRIBUTION OP PLANTS, CONSIDERED 



Warsaw and Moscow, though these cities are nearly in the same parallel 

 of latitude as Edinburgh. 



140. Among the physical circumstances which affect the distribution of 

 plants, the temperature of water merits notice. In many parts of the northern 

 regions, water exists during great part of the year in the form of ice ; and 

 hence, as it cannot be imbibed in that state by the roots, no plants can live 

 in such regions, except those lowest in the scale, such as Lichens, &c. ; or 

 such annuals as flower and ripen their seeds during the summer of these 

 regions, though it does not extend longer than two or three months. Hence 

 Barley and other corns can be ripened in the north of Sweden and Russia, 

 where no perennial or ligneous plants equally tender could live throughout 

 the year. In countries which are early in autumn covered with snow, many 

 herbaceous plants will live through the winter that could not exist without 

 this covering, which serves as an excellent non-conductor of heat. The 

 bark of trees is also a bad conductor ; and as the roots of trees penetrate 

 much deeper into the soil than frost, and as a slow circulation is carried on 

 in their trunks and branches throughout the whole winter, the sap they 

 contain is prevented from being frozen by the heat they obtain from the 

 subsoil. " The internal parts of large trees retain a temperature which is 

 about equal to that of the subsoil at one half the depth of their roots." 

 (^Henslow.) Whenever the sap in the vessels of a plant freezes, they become 

 ruptured and the plant dies; and were it not for the supply of heat obtained 

 from the subsoil by the trees, and the protection of herbaceous plants by the 

 covering of snow, there could be neither trees nor perennial herbs in the 

 more northern regions of our hemisphere. 



141. Supposing the temperature of the subsoil and of the trees growing 

 on the surface to be the same, then in high latitudes that temperature will be 

 higher than the atmosphere during winter ; and in low latitudes where the 

 atmosphere is of a high temperature, that of the trees will be lower during 

 summer ; for the bark, which by its non-conducting properties retains heat in 

 high latitudes, excludes it in low latitudes from penetrating into the wood of 

 the tree. Von Buch found that the temperature of the subsoil is principally 

 affected by the infiltration into it of the surface waters ; and hence, in the 

 frigid zones, where the surface is in a state of ice or snow during winter, no 

 infiltration can take place ; and thus the mean heat of the subsoil, in high 

 latitudes, will be higher than the mean heat of the atmosphere. In those 

 latitudes, however, where the surface water seldom freezes, the infiltration 

 will continue daring great part of the winter, and will reduce the mean 

 temperature of the subsoil below the mean temperature of the atmosphere. 

 In those countries in low latitudes where rain falls during the coolest 

 season of the year, the subsoil will be more cooled than in those places where 

 it falls both in hot and cold weather. " Hence the mean temperature of 

 springs throughout the central and northern parts of Europe, as far as Edin- 

 burgh, is much the same as the mean temperature of the air ; whilst from 

 the south of Europe to the tropic of Cancer, the difference is gradually in- 

 creasing in favour of the atmosphere ; but from the latitude of Edinburgh 

 northwards, the difference increases in favour of the subsoil. The conse- 

 quence is, that certain plants which naturally belong to the more temperate 

 parts of our zone are enabled to extend themselves further north and south 

 than they could do if the mean temperature of the soil and air were every- 

 where the same." (Henslow.) 



