402 TRANSFORMATION OF ENERGY 



is associated with low temperatures, and that luminous organisms lose their 

 power of emitting light when the temperature is raised, eventually losing 

 it permanently. Beijerinck (1890) showed that, in the case of Bacteria, 

 luminosity was dependent on the presence of certain food-stuffs. Since 

 these, however, differed in the case of different organisms, it was impossible 

 to draw any general conclusions on the subject. As to the uses of lumin- 

 osity in the organisms concerned we know less even than as to its causes ; it 

 is not therefore worth our while to discuss more fully the detailed observations 

 which have been made on the subject but at once turn our attention to 

 the third form of energy which exhibits itself in plant life, viz. electricity. 



It has long been known that it is possible with the aid of an accurate galva- 

 nometer or a capillary electrometer to demonstrate electrical currents in uninjured 

 plant-organs. If we place non-polarizable electrodes on the leaf of a suitable 

 dicotyledonous plant, so that one electrode rests on the mesophyll and the 

 other on the mid-rib, a positive current will, as a rule, be generated which 

 passes from the mid-rib to the blade of the leaf. The mid-rib is positive 

 to the leaf -surface, as also to the weaker lateral veins. If two points, symmetri- 

 cally situated so far as the mid-rib is concerned, be connected, no current is 

 demonstrable, nor does it appear when two corresponding points on a stem are 

 connected. Even if no current at all be observable in the uninjured plant, 

 such a current is at once established if the plant be cut or bruised, the electrode 

 nearest to the wound becoming positive to the one farther away. If the intact 

 epidermis be connected with the transverse section of the leaf a current is set 

 up in the direction of the section. If, however, the epidermis be removed and 

 the exposed surface (or a longitudinal section) be connected with the transverse 

 the current flows from the latter to the former. 



In 1878 KuNKEL attempted to prove experimentally that all electric 

 currents in the plant were traceable to one cause, viz. the movement of water. 

 It is quite true that disturbances of electric equilibrium may be occasioned by 

 streaming of water, and,according to Kunkel' s theory the interesting phenomena 

 just described in the uninjured plant are to be explained by the facts that veins 

 and leaf surfaces are unequally wet, and that when wet electrodes are placed on 

 such regions different water currents are set up. According to this theory the 

 electric phenomena observed would have nothing to do with the plant's vitality 

 but might equally well be manifested by a dead leaf. 



Kunkel' s views have not, however, been able to withstand criticism, and 

 more recent investigations, especially those of 0. Haacke (1892) have demon- 

 strated clearly that the evolution of electric currents in plants is by no means 

 such a simple phenomenon as Kunkel would make out. The movements of water 

 can undoubtedly cause electric disturbances, but they are not the only, or even 

 the chief, agents in the process. It is possible, as Haacke shows, to demonstrate 

 electric currents in leaves of aquatic plants, equally wet all over, and, on the other 

 hand, the very active transpiration currents are unaccompanied by any electrical 

 manifestations. On the other hand, electric currents are inseparably bound 

 up with vital activities, for dead leaves do not show normal electric currents at 

 all. Moreover, the electric phenomena are intimately connected with respira- 

 tion, for the currents at once come to an end when oxygen is excluded, while 

 they are especially prominent in actively respiring organs, such as the inflorescence" 

 of Arum above alluded to. Differences in electric potential are also related to 

 carbon-assimilation. In non-green organs darkness produces no change on the 

 current, whilst in green organs the current ceases at once when these organs are 

 brought into the dark or when carbon-assimilation ceases (compare Klein, 1898). 

 Finally, it may be noted that in plants like Mimosa and Dionaea, which exhibit 

 active movement as a result of stimulus, the movement is accompanied by 

 electric currents which are quite remarkable and regular in their character 

 (MuNK, 1876 ; Burdon-Sanderson, 1888). 



