Botanical Collec€iom, 179 



"the formation of the bark. The requisite materials for the increase of the 

 plant, and the formation of new organs, being drawn from the formative 

 juice, substances, in many cases, remain, which, through the influence of 

 .light and chemical affinities, combine anew into a juice, differing much in 

 qualities in different vegetables. This juice, which either exudes from the 

 plant by glands and hairs, or is stored up in proper receptacles, is to be 

 •looked on as excrementitious, and has hence got the name of excreted juice, 

 ^uccus excrementitius. The more watery portion of the sap, as is well 

 known, escapes by transpiration from the surface of the plant. 



The author ascribes these different motions of the sap to a sort of polarity, 

 induced in that fluid by thermo-electric agency, by which it is forced te 

 assume opposite directions in the different parts of the plant. The direction 

 of thermo-electric currents has been proved to be from the warmer to the 

 colder part ; and the author applies this principle in explanation of the course 

 of the sap by endeavouring to px-ove, flrst, that the heat is greatest in the nodus 

 indifferentialis, from which it gradually diminishes towards the extremities of 

 the root and branches : secondly, that in the root, stem, and branches, the 

 temperature diminishes in proceeding from the centre or axis to the circum* 

 ference. With all this, however, he does not mean to exclude the influence 

 of vitality, which he makes use of to account for the anomalies which cannot 

 Jbe explained on the supposition of polarity alone. 



The whole of this doctrine is very important, if correct ; but before we 

 <?an be satisfied on this point, it appears to us requisite for the author to 

 bring to the consideration of the subject a larger share of appropriate and 

 well ascertained facts, and less of vague statement, and illogical deduction, 

 ^Ibid. p. 502, 



Temperature of Plants. — Professor Schiibler of Tubingen, has made an 

 extensive series of experiments on the variation of temperature in plants, 

 and particularly with a view to determine whether plants, as has been sup- 

 posed by many eminent naturalists, have the power of generating heat, and 

 maintaining a fixed temperature, independent of that of the atmosphere. 

 The following are the results : — 



1. No perceptible heat is evolved by trees during the process of vegetation : 

 their mean temperature is very nearly that of the atmosphere, or even a 

 little (0.2° to 0,4° R.) lower; which last circumstance is probably owing to 

 the constant evaporation which takes place from their surface. 



2. During severe cold, the temperature in the interior of most indigenous 

 trees often sinks several degrees below the freezing point. 



3. The temperature of trees sinks less rapidly after it has once got below 

 freezing. This is most probably to be attributed to the heat evolved during 

 the congelation of their juices, which, in a certain degree, counteracts the 

 effects of cold till the tree is frozen to the centre. 



4. When a thaw sets in, a phenomenon, in some respects the converse of the 

 foregoing, takes place : the temperature of trees which have been frozen and 

 ■cooled considerably below the freezing point, rises with less rapidity than 

 when the heat is above that point. 



5. On comparing the mean of entire seasons, it appears that in summer 

 trees have a lower temperature, in proportion to that of the air, than in 

 other seasons : the mean differences amounted to 1.27° — 0.74° R. In 

 individual observations this difference was often much greater, even after 

 continued warm weather. It is very probable that this is owing partly to 

 the copious evaporation from the surface of the tree in warm weather, and 

 partly to the low temperature of the watery fluid absorbed by the roots 

 from the deeper parts of the soil, which absorption is most abundant during 

 the heat of summer. 



6. The temperature of trees in spring is somewhat higher than that of 

 the surroimding air : this apparent exception may be explained when it is 



