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 O 2 is diffusing into the water. These species, 
motile in oxygen, can be used as indicators of photosynthesis, because 
O 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. 
I 
