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 coordination 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 fir.st 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 bj^ 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 



