50 VITAL ACTIONS. 



leaf at the extremity of the branch of a lofty tree 

 must assist in setting in action the absorbing power 

 of roots, at a distance equal, perhaps, to three thou- 

 sand times its own length. If this reciprocal action 

 is not .maintained without interruption, and if any- 

 thing occurs to check it during the period of vegeta- 

 tion, the plant will suffer in proportion to the amount 

 of interruption. For example, if the roots are placed 

 in a warmer medium than the branches, and are 

 thus induced to absorb fluid faster than the slower 

 action of the leaves can consume it, the superfluous 

 sap will burst through the stem and distend its tissue 

 till the excitability is impaired or destroyed. Or if, 

 on the other hand, a branch is caused to grow in 

 a warm medium, while the roots remain in a very 

 cold medium, the former will consume the liquid sap 

 faster than the latter can supply it, and the con- 

 sequence will be, that the leaves will die, or the fruit 

 will fall off, or the flowers be unable to set their 

 fruit, from want of a constant and sufficient supply 

 of food. Not that it is necessary for the temperature 

 of the earth and air to be equal, for this does not 

 happen in nature ; but it is requisite that they should 

 have some near relation to each other. 



74. It is generally, however, believed, that leaves 

 absorb fluid from the air ; and their stomates appear 

 well adapted for that purpose, by their position in 

 most abundance on the under side of leaves ; and 

 the possibility of recovering drooping or sickly plants, 

 by syringing their epidermis copiously, seems to ren- 



