FUNCTIONS OF VEGETAL NERVE 555 



stimulus is essential to the maintenance of the proper excitatory 

 condition of a tissue. 



It is known that in the animal, when the conduc- 

 tivity of a nerve is abolished by nerve degeneration, the 

 connected muscles also rapidly waste away. Thus the 

 maintenance of the proper excitability of various tissues is 

 dependent on their constant reception of stimulus or energy 

 through the mediation of the attached nerve. It is therefore 

 highly probable that the excitability of the indifferent vegetable 

 tissues is kept at its normal level by the reception of energy 

 of stimulus through the conducting nerve. 



I shall next briefly refer to a fact which I have 

 demonstrated fully elsewhere, that all the principal physio- 

 logical activities of the plant, such as autonomous movement, 

 ascent of sap, and growth are fundamentally excitatory 

 phenomena. Thus, for example, the autonomous rhythmic 

 movements of the lateral leaflets of Desmodium gyrans come 

 to a standstill when their store of latent energy is exhausted. 

 And it is only by the accession of fresh stimulus from 

 outside that these multiple responses can be renewed. We 

 describe the state of the plant, when its internal energy is 

 below par, by saying that it is sub-tonic, the normal tonic 

 condition, or health of the plant, being dependent on the 

 sum total of stimulation previously absorbed by it. Turning 

 next to the question of the ascent of sap, I have shown that 

 the most important factor in bringing this about is the 

 multiple rhythmic activity of certain interior tissues of the 

 plant. Under such circumstances as cause the tonic condi- 

 tion of the tissues to fall below par, the rate of the ascent of 

 sap will be lowered, or it may possibly even be brought to a 

 standstill, owing to the depression of excitability induced. 

 On now supplying fresh stimulus, we find the excitability to 

 be renewed and the normal rate of ascent restored (p. 383). 

 Another instance of this is seen in the pileus of Coprinus 

 which droops when kept too long in the dark, but recovers 

 its normal turgidity on exposure to light. 



Growth, again, I have shown to be a result of multiple 



