68 VITAL ACTIONS. 



so as to meet the demand thus .made upon them; then, 

 as light diminishes, and with it the necessity for exces- 

 sive stimulus, temperature fallSj and reaches its mini- 

 mum at night, the time when there is the least demand 

 upon the vital forces of vegetation ; so that plants, like 

 animals, have their diurnal seasons of action and re- 

 pose. During the day, the system of a plant is ex- 

 hausted of fluid by the aqueous exhalations that take 

 place under the influence of sun -light ; at night, when 

 little or no perspiration occurs, the waste of the day is 

 made good by the attraction of the roots, and by morn- 

 ing the system is again filled with liquid matter, ready 

 to meet the demand to be made upon it on the ensuing 

 day.* No plants will remain in a healthy state unless 

 these conditions be observed. 



114. The alternation of seasons seems to be intended 

 to produce the like effects in a more extended manner; 



* The treatment of green-house plants by the majority of garden- 

 ers, is directly opposed to the natural laws here so correctly stated. 

 The gardener raises the temperature in a cold winter night very 

 frequently much higher than it is in the day, and the dry heat 

 stimulates the plants into an unnatural and sickly growth, when 

 they ought to be resting. The heating apparatus of a good green- 

 house should be so arranged as to afford a steady but rather low 

 temperature at night, increasing towards morning, so that with the 

 returning sunlight some ventilation can be allowed. In other words 

 the plant-house should be at its lowest safe temperature about mid- 

 night, and its highest at noon-day. But as usually managed, it is 

 warmest and dryest at midnight, so that the system of the plant is 

 doubly exhausted by the process of growth that takes place at that 

 time. A. J. D. 



t The incessant vegetation of arctic countries during their sum- 

 mer is an exception to this rule; but not such as to affect the general 

 truth of the foregoing propositions. 



