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Eevieivs and N'otices of Books. 



in France that the refreshing influence of rain and dew upon plants 

 does not arise from the moisture deposited upon their surfaces 

 being absorbed, — for not a particle of water enters into them 

 through the leaves or stem, — but from its supplying the ground 

 with water for the roots to absorb; and likewise, as I should 

 infer, from the check which its presence affords to a too rapid 

 radiation of heat from the parts above ground, by which the pre- 

 judicial effects of a too rapid cooling are provided against." 



The last two lectures are devoted to the discussion of the effect 

 of these influences on vegetation, and the way on which climate 

 determines the vegetation of a country. Palms and most other 

 monocotyledonous trees are ill suited to the climates of temperate 

 regions, because, from the liquid nature of the sap with which they 

 are filled, they are liable to freeze, — an act which, by the expan- 

 sion it occasions, proves of most fatal consequence at all times to 

 the vegetable organisation. On the other hand, dicotyledonous 

 trees, which belong to temperate countries, are protected from frosts 

 either by numerous layers of bark, abounding in air-vessels, or 

 are provided with essential oils and other juices not susceptible of 

 freezing, such as are found in the bark and wood of conifers. 

 The valuable rule on this subject laid down by Decandolle may be 

 briefly stated. The power each plant possesses to resist the 

 extremes of temperature is in direct proportion to the oils con- 

 tained in the juices, the viscidity of the juices, the smallness of the 

 cells and vessels, and the quantity of air entangled between the 

 parts of the vegetable tissues, or the protection afforded by hairs, 

 down, or air-vessels ; whilst the liability to suffer from great heat 

 or from frost is proportional to the quantity of water in the tissues, 

 the mobility of the juices, the size of the cells and vessels, and the 

 absence of air in the external layers. 



But the summer and winter climates of a country are what 

 chiefly determine its vegetation. 



" There are certain plants, like the Vine, which require an in- 

 tense heat in summer to bring their fruit to maturity, but yet are 

 capable of resisting a severe cold during winter ; they thrive, for 

 instance, on the borders of the Rhine, or in Switzerland, where 

 the winters are very severe; but scarcely even ripen their fruit in 

 England, even in Devonshire or Cornwall, where the thermometer 

 rarely falls to the freezing point of water. 



" Other plants, on the contrary, like the Myrtle, cannot resist 

 cold, but do not demand during any part of the year an exalted 

 temperature. 



" Thus they luxuriate near the southern coasts of England, but 

 do not show themselves on the Continent, until we reach a much 

 lower latitude than that of this country. 



The former class of plants, therefore, may be said to be 



