360 PLANT PHYSIOLOGY 



Free-swimming movements by means of cilia or flagella are 

 seen in the swarm spores of many of the Thallophytes, in bacteria, 

 and in the sperms of plants as advanced as the Gymnosperms. 

 The cilia are protoplasmic structures which extend from the cell 

 and produce movement by lashing the surrounding water. The 

 swarm spores of Fuligo may move a millimeter in a second. This 

 is sixty times their length and, when it is recalled that a good 

 runner goes only 100 yards in ten seconds or five times his length 

 per second, the speed of Fuligo swarm spores is not to be lightly 

 regarded. The cholera Vibrio moves a millimeter in twenty-two 

 seconds, which is also very rapid considering the size of the or- 

 ganism. 



The most puzzling of the free-swimming movements are the 

 excretory movements as seen in Desmids and Diatoms. These 

 forms possess no visible cilia and yet they are able to move at a 

 very high rate. Although their motion is not completely under- 

 stood, it seems to be due to the excretion of some sort of muci- 

 laginous material against the water through which they are gliding. 

 Others (Miiller) have thought that protoplasmic protrusions were 

 thrust against the surrounding medium or substrate to produce 

 the mysterious, majestic motion which these forms exhibit. The 

 forward movement of Oscillatoria seems to be due to some similar 

 push exerted by the secretion of something against the substrate, 

 connected with high osmotic pressure (Krenner, 1925) in the 

 motile filaments. 



Taxies. — Only a comparatively few plants show the independent 

 locomotions thus enumerated, but this does not mean that such 

 motions are unimportant to the plants which possess them. While 

 the intensity or rate of movement is affected by the general condi- 

 tions which determine optimum growth and activity, the direction 

 is also controlled and regulated by external agents. Such directed 

 movements are called tactic motions or taxies. Among the external 

 factors which influence these movements, several of the more 

 important and the responses evoked, will be briefly described. 



Phototaxy is the response of free-swimming organisms to light. 

 If Volvox or the zoospores of some species of Chlamydomonas, Eu- 

 glena, or other Algae are illuminated from one side with a light of 

 moderate intensity, they swim toward the light and get as near 

 to it as possible, i. e., they are positively phototactic. If the light 

 becomes too strong, they become negatively phototactic and swim 



