I. -Plants 
One of the common properties of living things is irritability. 
All living substance reacts, responds to stimuli, whether they come 
from the outside or from within. Transmission of stimuli is also 
a common property of living matter. 
Plants are sensitive to many sorts of stimuli without much 
indication of organs of special sense. Only in certain cases are 
there tissues for the transmission of the effects of stimulation and 
central organs for coérdination and control seem to be entirely 
lacking. 
In unspecialized organisms, both plants and animals, the sur- 
faces are sensitive to many sorts of stimuli without special organs 
for their perception. The whole surface or the whole body may in 
a general way be sensitive. If there are special parts associated 
with special stimuli, there are no histological features to indicate 
them. This diffuse perceptive capacity is more characteristic of 
plants than animals, yet some animals are of this type, and many 
plants have structures which are truly organs of sense, and in some 
cases special tissues for the transmission of the effects of stimu- 
lation. 
In certain parts of most plants there are areas of surface 
where the perception of stimuli takes precedence over the pro- 
tective or other functions; such surfaces may be called sensory. 
Certain cells or cell groups in plants which have perception as their 
chief or only function may be called sense-organs, even though 
they may not be responsible for sensation in the psychological 
sense. So far as we know, plants have developed sense organs 
only in relation to a few forms of external stimulation, such as 
those of contact, shock or jar, gravity or static and photic or light 
stimuli. So far as we can tell, the real act of perception, so-called, 
always takes place within the living substance, mainly or entirely 
in the solid portions, or in the ectoplast. 
Tactile pits occur in the outer walls of some surface cells. The 
cell walls are thin at these points, which are just over the sensitive 
protoplasm within the cells. These pits are usually confined to the 
sides of tendrils which may come into contact with surfaces. 
Darwin first determined that tendrils can be stimulated only by 
contact with, or friction against, solid objects, not by the impact 
of water. 
Tactile papillae, knobs and hairs occur on various parts of 
plants, such as staminal filaments. Parts of flowers which exhibit 
movements are often stimulated by means of hairs or knobs. 
Movements of parts of insectivorous plants are initiated by means 
of special sensory structures, such as hair, knobs, or spines. 
Plants respond to light in general without special organs of 
sense, but it is probable that the epidermal cells of many leaves are 
