THE PLANT AND ITS ENVIRONMENT 155 



scientific attitude as to plant physiology, which demands for 

 every observed change a definite physical cause and not a psy- 

 chological one; and it introduces a deeper problem which is 

 clearly the province of the philosopher rather than of the botan- 

 ist. The latter should content himself with carefully recording 

 all changes induced in the plant by a changing environment, and 

 with analyzing as carefully as p9ssiblc the factors which seem to 

 be responsible for these changes. 



Stimulus and Response. — In such a study it is important to 

 remember that the environmental forces do not act on the plant 

 as they would on a lifeless body — on a stone or a drop of water, 

 for instance — merely raising its temperature, illuminating its 

 surface, pulling it down by gravity or affecting it in other direct 

 ways; but, instead, that each of these forces acts as a stimulus 

 which brings forth on the part of the plant a definite response. 

 This response may be either a change of function or a change of 

 structure. To the same stimulus the response of one plant may 

 be very different from that of another, and the responses of 

 different parts of the same plant may also differ greatly. The 

 stimulus is merely a trigger which releases a response. Just what 

 a given response shall be depends entirely upon the constitution 

 of the living substance of the plant itself. This characteristic 

 trait of protoplasm whereby it is continually reacting or respond- 

 ing to stimuli is known as irritability and is a distinctive quality 

 of all living things. In animals, protoplasmic irritability is 

 extraordinarily developed in nervous tissue, which receives the 

 stimuli and controls the responses of the organism. In plants, 

 however, no nervous system has been differentiated, and although 

 some regions are much more sensitive than others and may 

 evidently transmit the effects of a stimulus for a considerable 

 distance, it is the protoplasm of the ordinary cells which is 

 chiefly concerned in the many responses made by the plant. 



It should be noted that although mature parts of the plant, 

 particularly those which are soft in texture, are able to change 

 their form and position to some extent through regulating the 

 turgidity of their cells, it is the young and growing regions which 

 are most sensitive to stimuli and are thus best able to bring about 

 regulatory changes of structure and position. 



In any discussion of the effect of the environment we should 

 consider not only the reaction of the individual plant but should 

 also look at the problem from the historical viewpoint and study 



