74 PLANT PHYSIOLOGY 



which external forces may act upon Protoplasm. Two of these 

 have already been considered. 



Third of the ways is this: the forces so act that the Proto- 

 plasm moves (or bends) towards, from, or across their line of 

 action. A familiar case of this is the bending of stems towards 

 light, of leaves across it, and of roots from it, and there are in- 

 numerable others, some of which the student will later study 

 in detail. Here the force acts like a signal, and there is an 

 active response determined by internal factors of a nature at 

 present quite unknown, though it follows from the existence 

 .of positive, negative, and lateral responses to the same force 

 that the relation between force and response cannot be either 

 direct or tonic. This relation is only very remotely quantita- 

 tive, as will be shown later in this course (indeed, a notable fea- 

 ture of this relation is the smallness of the stimulus necessary 

 to produce a great result), and it usually, or always, involves 

 advantage to the organism. The Protoplasm is said to be irri- 

 table to the force, which is said to act as a stimulus, though the 

 latter word is also used, unfortunately as I think, for forces 

 acting tonically, and even sometimes, though certainly errone- 

 ously, for forces acting directly. In a general way the proto- 

 plasmic response is direct to a considerable intensity of a force, 

 tonic to an amount too small to produce a direct response, and 

 irritable to an amount too small to produce a tonic response; 

 and through this curious relationship the real nature of irritable 

 responses may yet be elucidated. 



The action of stimuli (of this third relation) does not pro- 

 duce any effect upon the streaming of Protoplasm or upon any 

 other visible feature, aside from certain movements of loco- 

 motion of chlorophyl grains, motile bacteria, myxomycetes, or 

 zoospores. Hence it is nearly always much easier to infer the 

 processes from the actions of entire plants than to observe them 

 in the Protoplasm. Accordingly the student may here best 

 leave these matters until he takes up the subject of Irritability, 

 where they will all be fully considered. 



