426 PHYSIOLOGY 



\arialions 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 com 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 cHmate occasionally changes 

 io° C. within twice as many minutes, while the seasonal changes range 



