78 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



does not possess the power of generating heat within itself. Now such 

 a complete cessation of activity is quite compatible, in many instances, 

 with the preservation of the organized structure in a condition perfectly 

 unchanged, and, in consequence, with the continuance of its peculiar 

 properties ; so that these properties may be again called into operation, 

 when the temperature shall have risen. But in other cases, the plant 

 may be killed by the intensity of the cold ; that is, the return of warmth 

 will not excite it to activity. We have occasion to notice, in every 

 severe winter, the difference in this respect amongst the plants which 

 are cultivated in our own climate ; some of them being killed by a hard 

 frost, the effects of which are resisted by others, even though their situa- 

 tion be more exposed. In general it will be found, that the cold acts 

 most powerfully (as might be expected) upon plants which are not indi- 

 genous to our country, but which have been introduced and naturalized 

 from some warmer regions. But it is worthy of note, amongst other pe- 

 culiarities in the relation of Heat and Vegetation, that many plants are 

 readily killed by a low temperature, which yet flourish well under a very 

 moderate amount of warmth; so that they will grow in situations where 

 the mean temperature of the year is low and the summers cool, provided 

 the winters are not severe ; whilst they cannot be preserved without 

 special protection, in situations where the winters are colder, even though 

 the summers should be much hotter, and the mean temperature of the 

 whole year should be considerably higher. Thus there are shrubs grow- 

 ing in the Botanic Garden of Edinburgh, which cannot be safely left in 

 the open air in the neighbourhood of London, and which would be most 

 certainly killed by the winter-cold of Central France. 



110. It does not admit of doubt, that the destructive influence of a 

 very low temperature upon the Vitality of Plants, is immediately exerted 

 through its chemical and physical effects upon the tissues and their con- 

 tents. Thus it will produce congelation of their fluids ; and the expan- 

 sion which takes place in freezing will injure the walls of the containing 

 cells, distending, lacerating, or even bursting them. The same cause 

 will probably occasion the expulsion of air from some parts which ought 

 to contain it ; and the introduction of it into other parts which ought to 

 be filled with fluid. And a separation will take place, in the act of 

 freezing, between the constituent parts of the vegetable juices ; which 

 will render them unfit for discharging their functions, when returning 

 warmth would otherwise call them into activity. Hence we are enabled 

 in some degree to account for the differences in the power of resisting 

 cold, which the various species of Plants, and even the various parts of 

 the same individual, are found to possess. For, other things being 

 equal, the power of each plant, and of each part of a plant, to resist a 

 low temperature, will be in the inverse ratio of the quantity of water 

 contained in the tissue ; thus a succulent herbaceous plant suffers more 

 than one with a hard woody stem and dense secretions ; and young 

 shoots are destroyed by a degree of cold, which does not affect old shoots 

 and branches of the same shrub or tree. Again, the viscidity of the 

 fluids of some plants is an obstacle to their congelation, and therefore 

 enables them to resist cold ; thus it is, that the resinous Pines are, of 

 all trees, those which can endure the lowest temperature. The dimen- 



