THE METABOLISM OF PLANTS. 319 



that it is not due to the presence of a substance which, like 

 wax, various oils, quinia sulphate, etc. (Dessaigne), is lumi- 

 nous at a moderate temperature in the presence of free 

 oxygen, but that it is directly connected with the destructive 

 metabolism of the protoplasm. We can only conclude that 

 a portion of the energy set free in the destructive metabolism 

 of the protoplasm is evolved in the form of light. 



Electricity. In view of the changes, both chemical and 

 physical, which, as we have seen, are going on with greater or 

 less activity in the various parts of the plant, it has been not 

 unnaturally inferred that the electrical equilibrium of the 

 plant is being constantly disturbed, and that differences of 

 electrical potential in different parts may be observed by means 

 of appropriate instruments. A considerable number of obser- 

 vations have, in fact, been made, all of which tend to shew that 

 electric currents are constantly traversing the different organs 

 of plants, that is, that different organs and different points of 

 one and the same organ exhibit differences of electric poten- 

 tial. We will not attempt to deal with the extensive literature 

 of the subject, but a few of the more important and trust- 

 worthy observations may be cited in illustration. Buff found, 

 for example, that the exterior subaerial parts of plants are in 

 a state of permanent positive electrification, whereas the roots 

 and the internal tissue are electrically negative ; that is, that 

 when one electrode is placed on the surface of the stem or of 

 a leaf and the other electrode on the root or on the section of 

 the stem, the deflection exhibited by the galvanometer indi- 

 cates the existence of a current passing, through the plant, 

 from the root or the section of the stem to the surface of the 

 stem or leaf, and through the galvanometer, from the surface 

 of the stem or leaf to the root or to the section of the stem. 

 When both the electrodes were placed upon the surface of the 

 stem, currents could be detected passing irregularly sometimes 

 in one direction and sometimes in the other: and Ranke 

 found that when both the electrodes were placed upon points 

 on the surface of the section, a point relatively remote from 

 the centre of the section was always negative to all' points 

 relatively near the centre. Similar results were obtained by 



