GROWTH AND MOVEMENT 449 



appears from this that a sensitive organism becomes adjusted to a con- 

 stant non-directive stimulus, and then is unresponsive to an intensity of 

 one-sided stimulus of the same sort, to which in the unaccustomed state 

 it reacts. Thus accommodation is really a lowering of irritability toward 

 a particular stimulus. The noteworthy point is that it is a proportional 

 lowering; for, after each adjustment has occurred, it requires a definite 

 increase in intensity (in this particular case a large one — 30 times the 

 constant) to call forth a response. Some ratio of this kind, whether it 

 be an increase of 3 times or 30 times the constantly acting stimulus, 

 has been found to hold good for many forms of response and in many 

 sorts of organisms. In all cases the law is valid for moderate stimuli 

 only; an intensity is soon reached where it ceases to express the facts. 



The law was formulated in 1834, with reference to touch and sight. It has 

 been stated lately thus : " The smallest change in the magnitude of a stimulus which 

 will call forth a response always bears the same proportion to the whole stimulus." 



Aerotaxy. — One form of chemotaxy has received a special name, 

 aerotaxy, which signifies that the air, or more exactly the oxygen of the 

 air, is the excitant. Certain forms of bacteria are motile only when 

 they are in contact with oxygen, and cease to move when they are de- 

 prived of it. In so far, this also might be due merely to respiratory 

 disturbance, just as many functions cease when no oxygen is supplied. 

 But these forms also swim in the direction from which the oxygen is 

 diffusing, and accumulate about its source. Such forms, if evenly dis- 

 tributed under a cover-glass, soon desert the center and gradually accumu- 

 late at the edge, where the 2 is diffusing into the water. These species, 

 motile in oxygen, can be used as indicators of photosynthesis, because 

 2 is a by-product. 



Ionic stimuli. — All chemotactic reactions to substances that dissociate in water 

 probably rest upon the specific action of the various ions and molecules present in 

 the solution, and attempts have been made to correlate the action of the various 

 salts and acids. But the phenomena are too complex to permit satisfactory analy- 

 sis yet; and since undissociable substances also act as stimuli, it is probable that 

 the undissocia ted molecules, as well as the ions, have a stimulating action in many 

 cases. 



Phototaxy. — Phototaxy is particularly characteristic of those organ- 

 isms that have chlorophyll, such as the zoospores of algae and the ciliated 

 colonial algae like Volvox, Eudorina, etc. 1 That they swim towards 

 light of moderate intensity is not to be doubted; but it has been very 



1 Some fungus swarm spores also are sensitive to light. 



