VI. The Intluence of Light upon the Growtli of unicellular Organs. 147 
malion in obedience lo the periodic action of internal Stimuli, although 
this influence is not sufficiently strong to prevent contraction in obedience 
to Stimuli acting from without. 
The conditions of growing and of »contractile« cells are in some re- 
spects the same. Turgidity is essential to the proper fulfilmenl of the 
functions of both, and it has been shewu that light has the power of 
inhibiting, more or less completely, the activity of both. The most ge¬ 
neral case of the action of light upon growing cells has been shewn to 
be a diminulion in the rapidity of their growth. The cell with dimi- 
nished or arrested growth may be fairly compared with one of the cells 
of a rigid motile organ. In both, the micellae of the protoplasm are in 
a state of stable equilibrium so that tliey do not yield, in the former 
case to the force which tends to separate them, namely the pressure of 
the cell-conlents, and in the latter to the force which tends to bring them 
nearer together. 
The theory, that the action of light upon growing cells and upon tbose 
of motile Organs is due to such a modification of the relations existing 
between the micellae of the protoplasm that the mobility of the micellae is 
diminished, thus gives a satisfactorv explanation of many phenomena 
which, al first sight, seem not to have much in common. It may be 
that this is only a special case of the general principle, that the influence 
of all physical forces upon vegetable cells is especially exerled upon the 
protoplasm of those cells, and modifies the relations existing between 
the micellae. 
10* 
