ON TEMPEEATUEE. 89 



SO that the summer season may be regarded as one 

 long day, and the winter as a night of similar duration. 

 The long days, bright light, and elevated temperature 

 of summer push the powers of vegetation to their 

 limits ; towards the end of the season excitability be- 

 comes impaired, all the vessels andperishable parts are 

 worn out, leaves choke up and can neither breathe nor 

 digest, and the system of a plant, by the incessant ex- 

 halation of aqueous matter, becomes dried up, as it 

 were, and exh austed . At that time, temperature keeps 

 falling, and light diminishing, till at last, upon the 

 arrival of winter, neither the one nor the other is suffi- 

 cient to excite the vital actions, and a plant sinks into 

 comparative repose. At this time, however, its vital 

 actions are not arrested ; if they were, it would be dead 

 or absolutely torpid ; they are only diminished in in- 

 tensity. The roots continue to absorb from the soil 

 food, which is slowly impelled into the system, whence 

 it finds no exit ; it therefore gradually accumulates, 

 and in the course of time refills all those parts which 

 the previous summer's expenditure had emptied. In 

 the meanwhile the excitability of the plant is recovered 

 by rest, and may be even conceived to accumulate with 

 thefood that the absorbent system of the roots^is storing 

 up. At length, when the temperature of the season 

 has reached the requisite amount, excitability is once 

 more aroused, an abundance of liquid food is ready to 

 maintain it, and growth recommences ; rapidly or 

 slowly in proportion to the amount of excitement, to 

 the length of previous repose, and to the quantity of 

 food which had been accumulated. In hot climates, 



