PROTOPLASMIC MOVEMENTS 117 
complete circle with the thin wall at the outside, is bent 
back into almost 4 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 cellinclu- yy. 51. 
sions are swept along (as in Philotria), or in cur- BU". 
rents in the parietal cytoplasm and delicate (f2isee 
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 
