

ELEMENTARY VITAL PHENOMENA 233 



a secretion. Recently, Schewiakoff has shown the same also for 

 the Gregarince (Fig. 22, I, p. 80), which are parasitic unicellular 

 organisms that likewise perform very slow, gliding movements 

 without special locomotor organs. 



/. Movements ly Growth 



Movements that are associated with the growth of cells need 

 only be mentioned briefly; their principle needs no elucidation. 

 All growth is accompanied by movement, for, as a cell increases in 

 volume, it becomes expanded. Hence growth-movements are 

 common to all living substance, but they take place so slowly that 

 they can scarcely be followed with the eye. If, however, growing 

 objects be compared with their earlier stages after considerable 

 spaces of time, if the sprouting seed be first considered and then the 

 plant that has developed from it with all its branches, leaves and 

 flowers, it is evident that extensive movements have taken place, 

 by which the building material has been transported to the places 

 where it is laid down. Growth-movements are recognised also 

 especially clearly in long plant-stalks or tendrils, when the cells 

 grow or multiply more rapidly upon one side than upon the other, 

 so that the part becomes curved. But the most apparent move- 

 ments caused by growth are in those cases in which the mechanical 

 energy developed by growth is not continually set free, but is 

 accumulated in the form of tension, and finally by some stimulus 

 is suddenly transformed into kinetic energy; this appears most 

 beautifully in the seeds and fruits of certain plants, e.g., Impatiens, 

 which, upon being touched, suddenly burst with a jerking motion 

 and throw out their contents. It is not necessary to go further 

 into the mode of growth-movements, since their principle is plain 

 and they are met with at every step in living nature. That the 

 phenomena of growth are powerful sources of energy is clear when 

 it is recalled that trees growing between rocks are able to force 

 apart huge masses of stone by their roots. 



g. Movements by Contraction and Expansion 



Finally, movements that are produced by the contraction and 

 expansion of the cell-body, and which are usually termed, in brief, 

 contraction-phenomena, are distinguished from all other organic 

 modes of motion by the fact that they consist of changes in the 

 form of the surface of the living substance itself, which changes 

 are associated with an alternate shifting of position of its particles. 

 All contraction-phenomena comprise two phases of move- 

 ment, that of contraction and that of expansion. The particles 

 of living substance arrange themselves with reference to one 

 another in contraction, so that the mass presents a smaller surface, 



