426 PHYSIOLOGY 
variations still mark the rate of growth, indicating clearly that there 
are unknown factors that operate with or against the known factors to 
affect it. The existence of such unknown influences is further shown 
by the fact that growth ceases, sooner or later, in individual cells, and 
often in the whole plant, in spite of all efforts to supply appropriate 
conditions. 
External agents. A study of growth shows that external agents 
produce obvious effects. They do, indeed, affect every function, and 
much investigation is still necessary before the full extent of their influ- 
ence is known. But growth is at once so fundamental and so easy to 
observe, that it affords the best means for showing how extraordinary a 
part external agents play in determining the form and behavior of plants. 
To this phase of plant life attention must now be directed. 
2. IRRITABILITY 
External agents. It is a matter of common observation that the size 
and form of plants is affected by the conditions under which they are 
grown. The luxuriance of weeds in a neglected garden, in contrast with 
their stunted forms on a dry roadside ; the rich green corn of a high 
prairie, in contrast with the yellowish and starved plants on a wet clay 
field ; the thrifty trees of a park, in contrast with the struggling and 
dying ones along a paved street, can hardly fail of notice by the most 
unobservant. These differences show clearly that the complex of con- 
ditions external to the plant profoundly affects its internal processes. 
As all functions center in the living stuff, protoplasm, the conclusion is 
that protoplasm is sensitive to the various agents that act upon it (or 
irritable); that is, that it reacts or responds to these by altering its be- 
havior in some way. In that event the agent producing the reaction is 
a stimulus. These three topics, stimulus, response, and sensitiveness or 
excitability, require consideration. 
Variety of stimuli. The forces that act upon any plant are many, 
and varied in direction and intensity; and their combinations are almost 
infinite. Consider a tree, growing in a Chicago park. Every day the 
light which falls on it varies both in direction and in intensity from 
hour to hour, and is almost lacking at night; furthermore it varies from 
day to day and season to season. The temperature is hardly the same 
from one hour to another, and in this climate occasionally changes 
10 C. within twice as many minutes, while the seasonal changes range 
