TLANT PHYSIOLOGY 397 



That these influences in directing are very real is testi- 

 fied to by the fact that when the organs are turned aside 

 from their proper direction they will curve toward it and 

 overcome a good deal of resistance to regain it. Although 

 these curvatures are mainly developed in growing parts, 

 even mature parts which have been displaced may be 

 brought back into position. For example, when the stems 

 of certain plants, notably the grasses, have been prostrated 

 by wind, etc., they often can resume the erect position under 

 the influence of negative geotropism, a very strong and even 

 angular curvature being developed at certain joints. 



Hydrotropism. — The influence of moisture is very strong 

 in directing certain organs, notably absorbing systems. 

 Roots often wander widely and in every direction under 

 the guidance of hydrotropism, even against the geotropic 

 influence. Ordinarily geotropism and hydrotropism act in 

 the same direction, but it is interesting to dissociate them 

 so that they may ^^pull" against one another. For such 

 an experiment see Plant Relations, p. 91. 



Other stimuli. — Other outside stimuli which have a 

 directive influence upon organs are chemical substances 

 {chemotro2nsm) , such as direct sperms to the proper female 

 organ ; heat (thermotropism) ; water currents (rheotropisiu) ; 

 mechanical contact, etc. The most noteworthy illus- 

 trations of the effect of contact are furnished by tendril- 

 climbers. AVhen a nutating tendril comes in contact with 

 a support a sharp curvature is developed which grasps it. 

 In many cases the irritable response goes further, the ten- 

 dril between the plant axis and the support developing a 

 spiral coil. 



171. Irritable movements. — The great majority of plants 

 can execute movements only in connection with growth, as 

 described in the preceding section, and when mature their 

 parts are fixed and incapable of further adjustment. Cer- 

 tain plants, however, have developed the power of moving 

 mature parts, the motile part always being a leaf, such as 



