92 HEAT. 



and it is not improbable, as Dr. Lindley remarks, that 

 the cells forming the upper stratum of their paren- 

 chyma perform a function analogous to that of the sto- 

 mach in animals, digesting the crude matter they re- 

 ceive from the stem, the lower stratum taking up the 

 matter so altered, and submitting it to the action of the 

 atmosphere, which must enter the leaf purely by means 

 of the stomata. That it is by the stomata that the 

 exhalation of watery matter takes place, seems to be 

 evident also ; but whether they inhale it likewise, or 

 whether the absorption of fluid takes place by means of 

 the hairs on leaves, or in consequence of a general per- 

 meability of tissue, is more doubtful. According to 

 Saussure, the petals of plants consume more oxygen in 

 proportion to the green leaves, the anthers more than 

 the pistils, and single flowers more than double ones 

 in lieu of whose generative organs petals have been 

 developed. 



GENERATION" OF HEAT AND LIGHT. 



85. Heat. John Hunter made some experiments to 

 show that plants must be first killed by the cold before it 

 could freeze their sap, (but which is now, however, known 

 to be by no means the case); that the natural heat of plants 

 varied according to the species and its native climate ; 

 that the temperature of the interior of a plant was often 

 six degrees higher than that of the surrounding air, and 

 that the lowering of the he.at of the external air had but 

 little influence upon the temperature of the interior of 

 the plant. The next experiments after Hunter's, of any 

 use, were those of Salome, who observed the tempera- 

 ture of a tree by placing a thermometer nine inches 

 deep in its stem, which was eighteen inches in diameter 

 and eight feet above the earth ; and at the same time 

 noticed the temperature at the same depth in a dead 

 trunk. The temperature of the living stem showed itself 



