EVERGREENS. 281 



to that of a twig of the first or second year than to 

 that of a deciduous leaf. The cold acts in it in nearly 

 the same way, and it becomes rigid ; so that there 

 is little or no action of any kind in it during the 

 winter. And, as even the evergreen leaf is but a 

 temporary organ, which accomplishes one purpose 

 and then decays, the leaf of the evergreen does not 

 revive in the revival of spring. 



If, after the winter is over, the leaves of a com- 

 mon evergreen those of the laurel, for instance be 

 tried by a proper test, it will be found that their 

 vitality is gone, although their colour remains. The 

 very best test of life in vegetables, as well as of 

 health in animals, is the thermometer. The living 

 thing, be it what it may, has always one tempera- 

 ture which is the most favourable to healthy action, 

 and it struggles to preserve that; but when the 

 principle of life fades, the mere matter obeys the 

 laws of matter ; and thus, when the evergreen leaf 

 has ceased to perform its functions as a leaf, it has 

 no longer the uniform temperature of the growing 

 plant, but gets heated in the sun and cooled in the 

 shade, in the same manner as though it were a por- 

 tion of inorganic matter. 



If the experiment is made after the young leaves 

 are expanded, but before the old ones show any dis- 

 coloration or other symptom of decay, and made 

 when the sun shines brightly, the contrast is very 

 striking; so much so, that an instrument is quite 

 unnecessary ; for the young leaf feels cold, and the 

 old one warm. This shows that all action in the 

 old leaf has ceased ; because it is the action, the 

 evaporation of moisture at the surface of the leaf, 

 which preserves its coolness. 



In all cases where there is an annual leaf, there 

 is also an annual plant. In trees which last for 

 years that is a very beautiful study ; and if we could 

 separate the oak which has stood a century into the 

 hundred oaks that have been produced in the sue 

 Aa 2 



