INTRODUCTION. 



Plants are found, too, in situations in which some peculiar 

 noxious influence might be supposed entirely to prevent their 

 growth ; as for example, in sulphureous springs. In fact, there 

 are scarcely any circumstances, in which there is not some kind 

 of plant adapted to exist. Thus, it is well known that soils, 

 which have any considerable admixture of metallic ores, are not 

 favourable to most kinds of vegetation ; and among such soils, 

 those mixed with the refuse of lead-mines are the most sterile, so 

 that this substance is often mixed with gravel, to prevent weeds 

 from growing on garden-walks. Yet even on heaps of this 

 material, thrown up around the openings of the mines, the 

 Vernal Sand wort thrives, growing perhaps even more luxuriantly 

 than in any other situation. 



The degree in which vitality is sometimes retained by Plants, 

 under conditions apparently the most unfavourable, for a period 

 to which it is difficult to assign a limit, is one of the most 

 interesting and curious circumstances in their economy. In the 

 greater part of those inhabiting temperate climates, an apparently 

 complete cessation of activity takes place every year. The leaves 

 wither and drop off; the stem and branches are reduced to a 

 state of death-like bareness ; and all the changes in which life 

 consists, appear to have entirely ceased. In some instances, the 

 stems also die and decay, the roots only retaining their vitality; 

 yet from these, with the return of the genial warmth and light 

 of spring, a new stem shoots up, and new leaves and flowers are 

 produced, in their turn to wither and decay. The torpor is 

 not, however, so complete as it appears, in those durable and 

 woody stems which defy the winter's blast ; for late experiments 

 have shown that a slight movement of sap takes place even in a 

 frosty atmosphere. In evergreen plants, on the other hand, this 

 cessation of activity is less marked ; but the difference between 

 their summer and winter condition is much greater than is ap- 

 parent. In all these cases, however, the changes are periodical ; 

 and are not altogether dependent on external conditions. For 



