THE MOLECULAR FORCES IN PLANTS. 159 



naturally observed when one of the electrodes is laid on the 

 periphery of the stem, while the other is placed on an artificially 

 made cross-section. 



We further experiment with shoots of Tropaeolum, Vitis, 

 Quercus, etc., etc., which dip with their lower ends in water, or 

 with intact seedlings of Pisum and Vicia. In these latter we lay 

 one electrode on the stem and the other on one of the cotyledons. 

 In seedlings considerable differences of potential are observable. 

 In experiments with leaves (we experiment with young leaves, 

 readily wetted, which are left on the plants) one electrode is 

 placed on the midrib of the leaf near its base, the other on the 

 raesophyll near the middle of the leaf. It is found that in almost 

 all cases the leaf-nerves are positive towards the mesophyll, i.e. 

 positive electricity streams in the outside part of the circuit from 

 nerve to green tissue. 



According to Kurikel, movements of water in the tissues are to 

 be regarded as the chief cause of differences of potential in resting 

 uninjured plant structures. To prove that movements of water 

 can indeed set up differences of potential, we make the following 

 experiment : A fresh clay cylinder, such as is used for a battery 

 cell, is about half filled with water. We place our electrodes 

 on the outside of the cell, one near the base, the other at a point 

 above the level of the water. A current then flows through the 

 galvanometer from the upper 

 to the lower electrode. 



While differences of poten- 

 tial in plant structures may 

 be due to movements of 

 water, etc., they are chiefly 

 clue, as Haake proved, to 



quite different processes, FIG. 54. Apparatus for investigating electric 

 Viz. metabolism and respira- potential in plant structures. Represented in 



section. (After Haake.) 



tion, which are intimately 



associated with the vitality of the protoplasm. 



To prove this interesting fact we require the apparatus indi- 

 cated in section in Fig. 54, which was employed by Haake. It 

 consists of a glass tube, 20 cm. long and 3| cm. internal diameter, 

 on one side of which are fused two glass tubes, 2 cm. in diameter 

 and 1 cm. high, and 2J cm. apart. Over these are slipped pieces 

 of thin rubber tubing, 5 cm. long, which, as is clear from the 

 figure, fit air-tight round the electrodes which pass through them. 



