116 ALTERNATION OF SEASONS. 



"Prom the preceding tables it will be seen, that the average daily 

 range is nearly one-half less in. a tropical climate than, it is in the 

 neighbourhood of London, and that it does not exceed 11°, the average 

 being a little more than 9^°; whilst ia the higher latitude it is 

 nearly 18°." 



The alternation of seasons seems to be intended to produce 

 the like effects in a more extended manner, so that the summer 

 season may be regarded as one long day, and the wiuter 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 excitabihty 

 becomes impaired, all the vessels and perishable parts are worn 

 out, leaves choke up, and can neither breathe nor digest, and 

 the system of a plant, by the incessant exhalation of aqueous 

 matter, becomes dried up, as it were, and exhausted^ At that 

 time temperature keeps falling, and light diminishing, till at 

 last, upon the arrival of winter, neither the one nor the other 

 is suf&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 intensity. 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 the food 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, where winter is unknown, 

 the requisite periodicity of stimulus and rest is provided for by 

 what are called the dry and the rainy seasons, the former 

 being equivalent to the winter, the latter to the summer, of 

 northern latitudes. 



