234 OUTLIN,ES OF PLANT LIFE. 



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. 



328. (I) 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 rudi- 

 ments 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. 



329. (c) The form of plants is modified by the temr 

 perature of the air and soil. Low temperatures are also 

 likely to bring about the formation of dwarf plants. 



330. [d ) The rate of development is strikingly influenced 

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

 chiefly derived from the sun. The amount of heat absorbed 

 varies with the exposure of the soil, its color, porosity, 

 amount of water, and the duration of illumination. The 

 influence of the temperat.ure of the soil is mainly indirect, 

 acting through its effect on the water supply of the plant. 



331. 4. Moisture and precipitation. — The amount of 

 moisture in the atmosphere largely determines the amount of 

 evaporation from the surface of the plant. The relative 

 amount of moisture in the atmosphere is exceedingly variable, 

 and bears a direct relation to its temperature. Indeed, so 

 closely related are the conditions of temperature, light, and 

 moisture in the air, that the adaptations of shade plants, 

 mentioned above, may be considered as the sum of the 



