PROTOPLASMIC MOVEMENTS 117 



complete circle with the thin wall at the outside, is bent 

 back into almost a reverse circle, the whole row being now 

 under high tension. As the evaporation proceeds, further 

 contraction becomes impossible, and the collapsed thin 

 cell walls become dry in spots. These dry spots are per- 

 meable to air, which rushes into them and permits the 

 whole ring to snap back with extraordinary violence, 

 flinging the spores a comparatively long distance. 



171. Protoplasmic Movements. We may distinguish 

 two types of these, the movements of the cytoplasm 

 within the cell and the movement of the cell as a whole, 

 due to the motion of the cytoplasm or special parts of it 

 (cilia or flagella). 



172. The motion of cytoplasm within the cell seems 

 to be a normal phenomenon in all living cells whose 

 protoplasm has imbibed enough water to make it rather 

 liquid, i.e. in all active cells. It is probably 

 entirely absent in so-called dormant cells, such 

 as the cells of dry seeds, etc. In many cells it 

 cannot be distinguished except by special methods. 

 The motion may consist of a rotation of all the 

 cytoplasm of the cell except a thin layer against 

 the cell wall (as in Chara and Nitella), or of 

 large streams in which chloroplasts and cell inclu- fjq. 51. 

 sions are swept along (as in Philotria), or in cur- fng^pSl 

 rents in the parietal cytoplasm and delicate (T?id™ 

 strands crossing the vacuole (as in Tradescantia), '=^°*'^^- 

 or it may consist of rather local disturbances. 



173. Of especial interest are those movements by 

 which the nucleus is carried from one part of the cell to 

 the other. Thus in a cell that is growing rapidly on one 

 side or secreting abundantly at one side, the nucleus 

 is often carried to the point of activity. The chloroplasts, 

 too, change their position with reference to the light. If 



