222 RESEARCHES ON IRRITABILITY OF PLANTS 



Leaflets of Biophytum 



No specimen appears better suited for experiments 

 on polar excitation than the leaflets of Biophytum. There is 

 but little difficulty here in securing good electrical con- 

 nections ; the conductivity is moderate, the velocity of 

 transmission being about 3 mm. per second. The suscepti- 

 bility of Biophytum to polar excitation is considerable, an 

 extremely feeble current being sufficient to induce excitation. 

 Its greatest advantage, however, lies in the rapidity of its 

 recovery from excitation, which is practically effected in 

 the course of about 3 minutes. Thus employing the same 

 plant, experiments can be repeated under different conditions 

 without undue waste of time. 



Young leaflets are highly excitable and the older speci- 

 mens less so. Thus we have a wide range of selection accord- 

 ing to the particular characteristics which we wish to bring into 

 prominence. When the plant is highly excitable the bi-polar 

 connections are made, on the middle points of opposite 

 leaves, at a distance of 7 cm. from each other. The electrical 

 resistance is high, being in the case described about 15 million 

 ohms. With older and less sensitive specimens the two con- 

 nections may be made on the same leaf, about 35 cm. apart. 

 There will then be about five pairs of leaflets in the intra- 

 polar and about two pairs in each of the extra-polar regions. 



When the electrical connections are made with two 

 leaves of fairly equal sensitiveness the excitations induced 

 in the two will be equal. But if one leaf be old and the other 

 young,then a given excitatory effect will occur earlier,or under 

 feebler stimulus, in the younger. The minimally effective 

 e.m.f. willthus be lower in this case than in the older. 



Effect of feeble current. — In order to study the effect of 

 feeble current, I took a pair of leaves of Biophytum whose 

 sensitiveness was approximately equal. The minimally 

 effective e.m.f. was here found to be 4 volts, the current 

 being of the extremely feeble intensity of "5 micro-ampere. 

 The excitatory effect was induced at the right contact, 

 when that point was made kathode, six pairs of leaflets 



