250 GROWTH 



4. Induced movements of growth. — In addition to the vital 

 movements previously discussed which arise in consequence 

 of internal inherited causes operating within the plant organs 

 themselves, other movements are observable in various organs 

 of plants, which are induced by some external provocation or 

 stifnulus. 



The protoplasm of living plants is irritable and sensitive like 

 that of animals only in a somewhat different manner, and is 

 capable of responding to the action of various external influences. 



The chief exciting causes which induce movements in the 

 different plant members are (i) contact with a foreign body ; (ii) 

 alterations in temperature and the periodic alternation of day 

 and night ; (iii) lateral or one-sided illumination ; (iv) the force 

 of gravitation ; and (v) variations in moistness of the surrounding 

 soil and atmosphere. 



(i) Movements induced by contact with a foreign body. — 

 The best examples of movements of this class are met with in 

 tendrils and roots of plants. 



The tendrils of peas, vetches, vines, passion-flowers and other 

 plants are susceptible to slight contact. 



If a tendril while nutating round and round touches a foreign 

 body, such as the stem or twig of a neighbouring plant, it begins 

 to curve towards the irritating structure. If the latter is not too 

 thick and contact with it is prolonged the tendril becomes more 

 turgid on the side not irritated and also grows more rapidly on 

 the same side, so that the tendrils soon coil completely round 

 the structure. 



The particular part of the tendril which is sensitive varies in 

 different plants : sometimes a considerable portion all round the 

 tip is irritable, while in other cases the sensitive region is limited 

 to a short part on one side only. The curvature of the tendril 

 is not confined to the portion actually irritated, but the stimulus 

 is usually transmitted backwards along the tendril, and coiling 

 takes place in the parts which have not been touched. 



