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 1 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. 
