MESOPHYTES. 315 



variations in temperature to which plants arc subjected require 

 special adaptations. 



427. (</) Protection against changes of temperature. - 

 These adaptations are to be found in the presence of special 

 cell-contents, such as oils or resins, which reduce the liability 

 of those cells to freezing ; in the reduction of the amount of 

 water in cells so that less damage results from freezing ; and, 

 finally, in the presence of poor conductors of heat, such as 

 scale-leaves and hairs in profusion, a jacket of old withered 

 leaves, etc., all of which insure slow thawing if the plant is 

 frozen. The winter buds of trees in temperate climates 

 illustrate all of these adaptations. 



428. (6) A dormant period is necessitated by low tem- 

 perature during part of the year in temperate and arctic cli- 

 mates. The period of vegetation in the higher latitudes is 

 often very short. The same conditions prevail at high alti- 

 tudes, with the same effects. In these regions, therefore, the 

 plants are almost all perennials. In many cases the rudiments 

 of flowers are formed in the year preceding that in which 

 they are developed, in order that full opportunity may be 

 given for the ripening of the seeds and fruits in the short 

 growing season. Some plants adapt themselves to short 

 periods of vegetation by the presence of evergreen leaves, 

 which are ready at the first opportunity to resume their work 

 of food manufacture. 



429. (c) The form of plants is modified by the tem- 

 perature of the air and soil. Low temperatures are also 

 likely to bring about the formation of dwarf plants. 



430. (d The rate of development is strikingly influenced 

 by variations in the temperature of the soil. The soil heat is 

 derived from the sun and from the decomposition of organic 

 matter within it. The sun is far the most important source. 

 The amount of heat absorbed varies with the exposure of the 

 soil, its color, porosity, amount of water, and the duration of 



