216 PROPER HEAT OF PLANTS. [BOOK n. 



The glass was put into another vessel, containing water of a 

 higher temperature, in order in this manner to increase the 

 activity of the roots. The needles of the thermo-electrical 

 apparatus were then inserted into the external parts of the 

 flower-stalks, and instead of an increase of temperature, they 

 observed a fall; the apparatus indicated 17*5 C., while that 

 of the water was 28' 5. The experiment was repeated 

 several times with like success, as also with the flower-stalk 

 of Entelea arborescens. This was owing to the powerful 

 perspiration caused by increased activity promoted by 

 warm water. When the needles were inserted into the 

 middle of the flower-stalk of the hyacinth, the temperature 

 of the interior was found to be 1 higher than that of the 

 surrounding air. 



At a later period M. Dutrochet published some further 

 observations, and stated generally that plants possess a 

 peculiar heat, which is principally located in the green parts. 

 This heat exhibits a daily periodicity ; it reaches its maximum 

 towards mid-day, and its minimum during the night. The 

 hour at which plants reach their maximum temperature is 

 the same for each species, but different in different species ; 

 thus, Rosa canina at 10 a.m.; Allium Porrum at 11 a.m.; 

 Borago officinalis at mid-day ; Euphorbia Lathyris at 1 p.m. ; 

 Sambucus nigra at 2 p.m. ; and Asparagus and Lactuca 

 sativa at 3 p.m. The greatest heat is in the neighbourhood 

 of the principal bud, and in woody plants often only in the 

 green extremities. Other experiments confirm the fact, 

 that plants growing in the dark lose their vegetable heat ; 

 but experiments on different fungi showed that these also 

 possess a daily periodicity. Boletus seneus exhibited a heat 



