44 



ELECTRICAL STRUCTURE AND 



mixing a small quantity of ferrous sulphate, in solution, 

 with the soil ; but even where the disease does not exist iron 

 should, in my opinion, be used as an electrolyte and the 

 result noted. 



NOTE FOR GUIDANCE IN TESTING. 



For everything that grows, either in the earth or in a 

 pot, it is only necessary to have flexible wires of low 

 resistance and of a sufficient length to span the space 

 between the galvanometer and the plant. Both wires 

 should terminate in two darning-needles of equal gauge and 

 length. One needle may be inserted in the open ground 

 or in the soil in the pot, and the other carefully placed in 

 between the lignified fibres in the venation of a leaf, i.e., in 

 the interspaces, or areolae, which are filled up with tran- 

 spiratory assimilating tissue. Contact with the venation 

 may introduce error, but if ordinary care is taken there 

 will not be any discordant result. The needles must, of 

 course, be kept scrupulously clean, and should not be 

 insulated for any portion of their length, as such insulation 



a, a, a, a are the areolae. The needle should be inserted as shown. 



whether by india-rubber or gutta-percha, etc. is liable 

 to cause confusion. Plain, clean needles, well -insulated 



