276 COMPENDIUM OF GENERAL BOTANY. 



function of an equally illumined stem as well as to that of the 

 root, which serves to fasten the plant to the soil and to take 

 up soluble food-substances from all sides by means of the 

 numerous secondary roots and rootlets. 



D. Electricity. Moisture. Water-currents. Kadiating Heat. 



Electrical Currents Produced by Plants. — Electrical currents 

 have been demonstrated in the plant-body ; also variation in 

 these currents. Movements of water-currents in the plant are 

 supposed to be the cause of the variations in the electrical cur- 

 rents (Eanke, Velten, Munk, Kunkel). The significance of these 

 currents is unknown. 



The following are some of the effects of the electrical currents 

 upon the processes of plant-life^ though no important significance 

 has yet been ascribed to this knowledge. Electrical discharges 

 cause cessation of the movements of the swarm-spores of 

 Vaucheria (Unger) ; they cause cessation of the motion of the 

 granules in streaming protoplasm (Kuhne, Nageli, and ScmvEN- 

 dener) ; they also cause closing of the stoma (N. J. C. Muller), 

 perhaps due to changes in turgor. Strong currents may kill the 

 cells. At this point it is well to mention Brunchhorst's galvan- 

 otropism. Roots growing in water incline toward the negative 

 electrode with weaker currents. Stronger currents cause the 

 roots to incline toward the positive pole, dwe to pathological influr- 

 ences (Elvfing). 



The observation made by Sachs that growing root-tips will 

 turn toward a moist body {positive hydrotropism) is of physiolog- 

 ical importance. The same author observed negative hydrot- 

 ropism in stems of seedlings and in the spore-bearing hj^j^^* 

 of the Phycomyces. 



Growing roots of Zea Mays turn toward the current of run- 

 ning water (JOnsson, rheotropism). According to Wortmann, some 

 growing plant-portions turn toward a source of warmth {positive 

 thermotropism), while others turn away (negative thermotropism) : 

 examples for both are found among young stems of various 

 plants. In one and the same root a temperature below 27.5 ° C. 

 produced positive thermotropism ; a higher temperature pro- 

 duced negative thermotropism. 



