30 



diately abates it. When the sun was clouded du- 

 ring the rising of the sap, it would visibly subside 

 with great rapidity ; but when the sun-beams again 

 broke out, it would immediately return to its rising 

 state, just like the fluid in a thermometer *. A plant 

 of spearmint, placed in a syphon one-fourth of an 

 inch in diameter, absorbed, during the day, so much 

 water, that it fell into the opposite end to the depth 

 of an inch and a half; in the night only to one- 

 fourth of an inch ; during a rainy day still less ; 

 and, when the thermometer stood at the freezing 

 point, the plant absorbed no moisture at all f. That 

 it is to the influence of heat alone, and not tc? any 

 other peculiarity in the season, or in the plant itself, 

 that, the other requisites being duly supplied, its ve- 

 getation is to be ascribed, we learn from the fact, 

 that trees in a hot-house continue to grow during 

 the winter, while others of the same species in the 

 open air cease to do so. Even the branch of a tree, 

 planted in the open air, will, during winter, when 

 led into the hot-house, undergo the usual vegetative 

 changes, provided its stem be guarded from severe 

 frost ; but, on the contrary, a branch brought into 

 the open air from a tree that is flourishing in the hot- 

 house, will at the same season cease to grow J ; 

 affording sufficient evidence of the necessity and the 

 influence of heat. 



* Statical Essays, vol. i. p. 122. f Ibid. p. 27. 

 J Philosophical Transactions, vol. Ixiii. part 1. 



