180 Botanical Collections* 



recollected, that in our climate the heat of the deeper parts of the soil, from 

 which trees then largely extract nourishment, is usually greater in March and 

 April than that of the air, and that the surface of the tree itself, so long as it 

 is not shaded by the foliage, may be heated directly by the sun's rays, whilst, 

 in such circumstances, it loses very little heat by exhalation. 



7. The juices of trees are actually frozen during severe cold. In order 

 to be assured that the freezing was not merely the consequence of exposure 

 to the cold air after the tree was felled, the author caused several trees to be 

 cut down at the commencement of a sudden thaw, and when the air was 2° 

 R. above the freezing point ; they were all frozen, to a certain depth, in 

 concentric rings. 



8. The depth to which the frost penetrated varied considerably in different 

 sorts of trees: in a horse-chesnut it was 8.2 lines; a fir tree, 12.5; in a 

 species of maple, 15.2 ; an ash, 16.8 ; and in a willow, 17.3. The mean of 

 several measurements in six different trees gave 14.4 Paris lines as the 

 medium depth of the frozen layer, whilst the thickness of the ice on a small 

 lake in the vicinity was, at the time of these observations, 9.9 Paris inches, 

 or 108.8 lines ; the frost had therefore advanced 7| times slower in the 

 trees than in the water. These phenomena are no doubt to be attributed 

 to the difference between water and vegetable matter in their power of con- 

 ducting heat : and the degree to which the frost penetrates in different trees 

 is found to be proportionate to the quantity of their watery constituents, 

 and the openness of their texture. 



It follows as the general result of these observations, that the temperature 

 of plants is modified in manifold ways by many different circumstances ; that 

 they do not, indeed, like the higher animals, possess the faculty of evolving heat 

 in their ordinary vital processes, but that they have the power of conducting 

 heat from the air, and from the soil, in proportions, varying with the course 

 and impulsion of the sap, and of giving off part of it again by evaporation, so 

 that, being bad conductors of heat, their more equable temperature must 

 necessarily be sometimes either higher or lower than the more variable tem- 

 perature of the atmosphere ; that, with regard to trees in particular, the 

 different depths to which the frost penetrates into their interior, in equal 

 degrees of cold, depend chiefly on the different density and breadth of their 

 concentric annual layers, and on the different proportions of watery elements 

 which enter into their composition. 



It is admitted, however, that the very different degrees in which the plants 

 of different climates withstand the injurious effects of cold, cannot be 

 accounted for by physical causes alone. — Ibid. p. 563. 



Effects of Cold on Plants. — Dr Goppert of Breslavv^ endeavours to shew, 

 by numerous experiments, that the sap in plants freezes in winter when 

 the cold is severe, without injury to life : that the changes which plants 

 undergo when they are killed by cold, do not consist in a bursting of 

 their vessels or cells, but solely in an extinction of vitality, which is 

 followed by changes in the chemical composition of their juices : that the 

 effects of cold on vegetables are not always directly proportionate to its 

 degree, but are modified by the state of development, and numerous other 

 circumstances connected with atmospheric changes : Lastly, that the 

 doctrine demonstrated by Schiibler with respect to trees, viz. that they 

 possess no proper temperature, independent of that of the atmosphere, 

 must be extended to the vegetable kingdom in general. The author has 

 tried to discover an elevation of temperature in the flowers of the Aroidece, 

 and of many other plants ; but all his experiments, though conducted with 

 the greatest care, have given a negative result. — Ibid. p. 497. 



Use of the Cuticular Pores of Plants. — It is well known to botanists, that 

 the cuticle of most plants is furnished, especially on the leaves, with minute 



