156 THE MECHANISM OF LIFE 



being has as its ultimate cause a stimulus or excitation coming 

 from without. 



The movements of the second category are also conditioned 

 by an excitation, but the stimulus comes from within the 

 organism. These movements consist principally of changes of 

 nutrition, or movements of the circulation and respiration ; 

 they are rhythmic in character and are probably produced by 

 the same chemico-phvsical causes which determine rhythmic 

 movements outside the living body. 



Just in the same wr.y osmotic growths present two sorts of 

 movements, external movements and those which are connected 

 with their nutrition. A free osmotic growth swimming in the 

 mother liquor will alter its position and form under the influence 

 of the slightest exterior excitation or vibration. It responds 

 to every variation of temperature, or to a slight difference 

 of concentration produced by adding a single drop of water, 

 and reacts to every exterior influence by displacement or 

 deformation. 



An osmotic growth also shows indications of movements 

 which are connected with its nutrition, and these movements 

 are rhythmic, like those of respiration or circulation in a living 

 organism. The growth of an osmotic production shows itself 

 not as a continuous process but periodically. The water 

 traverses the membrane, raises the pressure, and distends the 

 cell; at first the cell wall resists by reason of its elasticity, it 

 then suddenly relaxes, yielding to the osmotic pressure and 

 bulging out at a thinner spot on the surface ; the internal 

 pressure falls suddenly, and there is a pause in the growth. 



This rhythmic growth may be best observed by sowing 

 in a solution of a tribasic alkaline phosphate, pellets composed 

 of powdered calcium chloride moistened with glycerine, to 

 which has been added 1 per cent, of monobasic calcium 

 phosphate. The experiment is so arranged as to bend or 

 incline the growing stems which shoot out from these 

 grains. This may be done by carefully pouring above the 

 mother liquor a layer of water, or a less concentrated solution. 

 As the internal osmotic pressure rises, the drooping extremity 

 of the twig will become turgescent and gradually lift itself 



