f^ 



120 TEMPERATURE. 



degree of cold. Plants are also fortified against the effects of cold by 

 the amount of carbon and resin they contain, as well as from their 

 clothing of bark, which is readily perceived by its effects on young 

 shoots of fruit trees. Thus also succulent plants, having no distinct 

 bark, are easily destroyed by cold, while the birch, which is covered 

 with numerous layers of dry bark, and pines, the bark of which 

 abounds with resin, are thereby greatly protected. The latter with 

 others in our northern latitudes, sustain unaffected a temperature of 

 44 below zero. The extremes of heat and cold in this country are 

 very trying to many plants, especially fruit trees ; and, but for their 

 hardy constitution, they must perish by the one or the other. 



Temperature, from what has been said, will appear to have very 

 great influence on the geography of plants. It may be considered, in 

 reference to the mean temperature of the ye.ar, the extremes of temper- 

 ature, and the temperature of the months of the year. The 1st is of 

 the least importance generally to vegetable geography, as it affords 

 uncertain data for general conclusions ; while the 2d is more definite 

 though less immediately practical, as almost every locality presents 

 peculiarities of vegetation, arising from local heat and other causes. 

 Widely different temperatures or long intervals of extremes enable us 

 to select and cultivate plants with more certainty in intermediate cli- 

 mates. The vine, olive and orange can thus be cultivated, but if left 

 wild to themselves, even in more genial climes, they would not be 

 productive nor withstand the winter. Hence between wild and culti- 

 vated fruits of the same kind in different latitudes, there is a remark- 

 able difference. This may be termed the acclimation of plants. It is 

 maintained, however, by some that no change in the constitution of 

 plants can be effected by change of climate. The power or property 

 of accommodating themselves to different climates is certainly remark- 

 able. The vine is cultivated in Hindostan and Arabia, between the 

 13th and 15th parallels, also in latitude 51, and at an elevation of 

 10,000 feet in lat. 32. 



The greatest importance of heat in vegetable geography is in its dis- 

 tribution in the several months of the year. Great uniformity is ob- 

 servable in some climates, especially on the sea coast, where it is 

 modified by the sea, which is almost always of an equal temperature, 

 thereby enabling even tropical plants to thrive in northern latitudes. 

 Annual plants, requiring the heat only of summer to ripen their fruit 

 and remaining in the state of grain, apparently torpid in the winter, 

 abound in regions where the extremes are great ; but perennial plants, 

 which can delay or dispense with maturing their fruit, are greatly af- 

 fected by the severity of winter. Of these, such as have deciduous 

 leaves best accomodate themselves to inequalties, while those which 

 retain their foliage, or evergreens, prefer equal temperatures. Seeds 

 having no substances much affected by moisture are less sensible to 



