430 PHYSIOLOGY 



resulting in movement, is most likely to be merely the end reaction. 

 Thus if a primary root of a bean be set horizontal, the first reaction 

 occurs instantly and in the very tip of the root, but it is not visible; only 

 after a half an hour or more, at a distance of 2-3 mm. from the tip, does a 

 growth reaction set in that starts to turn the root tip downward. Between 

 the first reaction and the last there must have been a series of changes, 

 each of which was a reaction to a preceding stimulus and a stimulus to 

 a succeeding reaction. By a rough analogy the process may be com- 

 pared to the tumbling of a row of blocks, each falling by reason of the 

 impulse from its predecessor and impelling its successor to fall. The 

 push that displaced the first one is the primary stimulus, and if the last 

 were properly connected mechanically, it might, for the end reaction, 

 ring a bell or fire a gun. Such a series of reactions is often spoken of as 

 the transmission of the stimulus. More properly it is the propagation 

 of the excitation. It is equally the propagation of a reaction. 



None of these phrases nor the above analogy should be understood to require 

 that the reactions in a series are necessarily alike, nor is the end reaction the only 

 one to which the term properly belongs, though it is usually so applied unless the 

 contrary is indicated. 



Perceptive region. The region where the first reaction occurs is often 

 called the receptive or perceptive * region, particularly if a later and ob- 

 vious end reaction occurs at another place. Since in animals a similar 

 localization of sensitiveness for special stimuli marks the peripheral por- 

 tion of sense organs, these regions in plants, especially when very cir- 

 cumscribed, may be looked upon as sensory organs of the simplest sort. 2 

 Regions of this sort, sensitive to gravity and light as stimuli, will be 

 described later (pp. 463, 477). In the great majority of cases, however, 

 perception is not strictly localized, and the condition resembles rather 

 that in the diffuse senses of animals, like those of touch and temperature. 



Transmission. Special tracts, the nerves, exist in almost all animals, 

 along which the excitation is propagated, but nothing at all comparable 

 has been found in plants, though this claim has been made more than 

 once. The most that can be said is that propagation is more rapid 

 lengthwise than crosswise of the cells of a tissue and in some tissues is 

 easier than in others. Presumably the propagation is from protoplast 

 to protoplast by way of the slender threads that connect them, traversing 



1 These words are used in a figurative sense, and the last must not be understood to 

 have its usual psychological implication. 



2 Here again it is necessary to point out that in no sense is consciousness implied. 



