RESPONSE OF NERVE 721 



was due to the molecular transformation caused by the 

 stimulus of the mechanical or thermal section. The effect 

 of such stimulus on neighbouring points is to induce moderate 

 excitation, raising them to the higher excitability of the 

 condition B (fig. 311). At or very near the section point 

 itself, on account of over-stimulation, the transformation is 

 to condition D or E, and the result should be one of loss of 

 excitability. In accordance with this, it is found that at 

 such points there is depression of excitability (fig. 3 1 2). 



As regards the place of the vegetal nerve in the plant 

 economy, it may be said that the normal excitability of 

 a tissue, by which its proper functions are discharged, can 

 only be maintained fully by a supply of energy, which must 

 be received from the environment. Both animal and vegetal 

 nerves have been shown, when isolated, to lose their normal 

 conductivity and excitability, their response becoming ab- 

 normal or being abolished. It is only by the accession of 

 fresh energy of stimulus that the normal conductivity and 

 excitability are restored. It is known, further, that when the 

 nerve loses its excitability, undergoing consequent degenera- 

 tion, the attached muscle also exhibits rapid decay. It will 

 thus be seen that the various tissues of the organism are 

 maintained in their normal functional activities by means of 

 energy conveyed to them through the nerves. 



One of the principal forms of energy in maintaining the 

 tonic condition of a green plant is sunlight ; when deprived 

 of this, its various normal activities come gradually to a stop, 

 and the plant ultimately dies. But if any portion of the 

 plant be exposed to light even its shaded parts will be found 

 to continue in natural vigour. This is exemplified by the 

 experiment of Sachs, in which an undetached branch of 



ICucurbita was kept in a dark box, and was found to grow, 

 and produce flowers and fruits, as if under normal conditions. 

 The fact that a plant, when totally deprived of sunlight, dies, 

 shows how essential to its tonic condition is energy of light. 

 The fact that so long as a portion of it is kept in light the 

 whole flourishes, proves the transmission of energy from one 

 3i 



