LOCOMOTION OF CELLS 119 
distributing various food products as well as other sub- 
stances throughout the cell. 
176. In motile cells there is observable a response 
in direction of the movements to various external stimuli. 
Thus many cells swim toward the light, or away from it 
(positive and negative phototaxy). Others swim to- 
ward or away from various chemical substances (e.g. 
food matters, acids, etc.) diffusing through the water, 
this being called chemotaxy. In many cases a degree 
of light or of concentration of a chemical that causes 
positive reaction, when increased beyond a certain point 
repels the cell. It is not always the case that harmful 
chemical substances (poisons) repel the cell, although 
usually this is the case. 
Laboratory Studies. (a) Insert the point of the fruit of 
porcupine grass (Stipa) into a cork or fasten the fruit of cranes- 
bill (Erodium) to a cork with a drop of sealing wax, with the 
main shaft of the fruit upright, and place a marker opposite 
the tip of the bent portion. Place a bell jar partially lined with 
-wet filter paper over it and note how it changes its position and 
the direction of the motion. Remove the bell jar and note the 
change in the direction of motion. By spraying a fine mist on 
the specimen a lively movement will be obtained. 
(6) Mount several ripe sporangia of a fern in a very little 
water without a cover glass and watch the motion as the water 
dries out. 
(c) Examine some of the end cells of Chara or Nitella for 
rotatory movement of cytoplasm, the leaf of Philotria for large 
streams of cytoplasm carrying the chloroplasts with them, the 
stamen hairs of Tradescantia or the stem hairs of petunia, 
tomato or watermelon for more delicate strands of streaming 
cytoplasm. 
(d) With some of the foregoing test the effect on the move- 
ment of cold (laying on a block of ice) and heat (up to 40° or 
45° C.), examining again at room temperature. 
(e) Place some green felt (Vaucheria) that has been growing 
on the surface of the ground in a dish of water. Often this will 
