56 
reduced; that is, the under surface tends to increase, and Vie 
upper to decrease, hence the leaf closes. 
16. — Summary. 
It has been proved by Loeb and others that proteid takes 
up ions to form a loose compound, which they call ion- 
prcteid. 
Since these ion-proteid molecules must always be breaJK- 
ing down, tnere must be, for this reason, if not for others, a 
number of free ions in any protoplasmic body, and therefore, 
in general, a difference of potential between it and the 
medium in which it lives. 
It is acknowledged by many physiologists"^ that the 
movements of unicellular organisms are due to chan2:es in 
surface tension, while others, as Schafer,t consider it pro- 
bable that the movements of muscles may be due to the same 
cause. It is, indeed, obvious from the structure of amoeba, 
cilia, muscle, etc., that, if changes in surface tension take 
place, movements must follow. 
But since, for obvious reasons, the number of free ions 
in a protoplasmic body must always be chanp-ing or subject 
to change, it follows from known physical laws that the sur- 
face tension must also change. 
We have shown that this mode of accounting for the 
movements of organisms enables us to explain the galvano- 
taxis and chemotaxis ot unicellular organisms — the contrac- 
tion of muscle — the electro-motive and other phenomena ac- 
companying muscular contraction and the nervous impulse — 
the rhythmicity of certain muscles and nerves and 
the variations in their rhythm — the action of inhibitory and 
augmentor nerves, and the movements and electro-motive phe- 
nomena of plants. 
It seems, therefore, certain that this explanation of the 
genesis of movement in living bodies is, in the main, true, 
and that it is probably capable of explaining the whole of that 
vast complex of facts which have been gathered together 
under the head of phenomena of contractility and irritability. 
* Vide Biitschli : Protoplasm and AIicro8oopic Foams : Trans. 
by E. A. Minchin, 1894, page 289: and Verwom : Gen-era! 
Physiology: Trans, by Frederic S. Tvee, poge oOl. 
t Schiifer : E.ssentials of Hi.«tology, sixth edition, page 50. 
