IL] A BUTTERCUP. 13 



Place one pair of tumblers in the sunshine, the other pair 

 in a shady place. In five or ten minutes examine the 

 inverted tumblers. That exposed to the sun you will find 

 already lined with dew on its cool side, while that kept 

 out of the sun is still nearly or quite clear. It is manifest, 

 therefore, that evaporation from the leaves must be not 

 only rapid, but considerable in amount, when plants are 

 exposed to the sun, especially in a dry atmosphere. 



How far this increased evaporation in sunlight may be 

 due to the specific action of light apart from increase of 

 temperature and modification of the condition of the sur- 

 rounding atmosphere in respect of its humidity, is a ques- 

 tion which does not at present admit of a clear answer. 



This exhalation of vapour from the surface of plants is 

 technically termed transpiration. A correct understand- 

 ing of the process explains how it is that plants growing 

 in parlours are apt to become faded even when watered, 

 because the taking up of water (termed absorption} by the 

 roots cannot always keep pace with the transpiration from 

 the leaves, owing to the rapid evaporation excited by dry- 

 ness of the air. Since the specimen No. 3, experimented 

 upon at the beginning of the present lesson, faded, not- 

 withstanding the immersion of some of its leaves, it is 

 clear that the root is the part which performs the office 

 of absorption principally. 



Every part of a plant or animal appropriated to a dis- 

 tinct purpose or function is termed an organ. Hence the 

 root may be called the organ of absorption of the plant. 



4. Now, not only is water absorbed by the root, but 

 also various substances which are dissolved in the water. 

 Hence we find, if we burn a plant carefully, that an ash 

 remains, consisting of such of these substances as are not 

 dissipated by heat, which were absorbed in this way, and 

 which had been made use of by the plant, or stored away 

 in its tissues. Of the simple elements known to chemists 

 about twenty occur in the ash of plants ; many of these, 

 however, in very minute quantities, and never all in the 

 same plant. Sulphur, phosphorus, potash or soda, lime, 

 and silex, are those most generally found. 



5. But if we analyse an entire plant, and not the ash 

 only, we shall find constantly present, besides some of the 

 above, the elements carbon, oxygen, hydrogen, and nitro- 



