STONE: CONTACT STIMULATION 475 



tropism, geotropism, etc., which are characterized by both positive 

 and negative reactions, but for our purpose it is necessary to consider 

 only a few which are somewhat similar in character, and which have 

 not unlikely originated from a simple fundamental type. 



(a) Contact Stimulation 



Probably a universal type and one of the most fundamental forms 

 of response in plants, which is characterized by a stimulated growth of 

 various organs due to contact of one organ with another, or with 

 various substances. The extent of the reaction is determined by the 

 degree of contact, and probably influenced by the nature of the 

 contact substance. 



(b) Haptotropism 



Illustrated by various tendrils reactions; tentacles of Drosera. 



(c) Haptonastie 



Illustrated by certain types of contact of some tendrils and tentacles 

 of Drosera}^ 



id) Seismonastie 



Illustrated by movement of stamens of Centaiirea, Berberis, and 

 Mimosa leaves. 



(e) Thigmotaxis 



Reaction of motile organisms (Chlamydomonas) resulting from 

 contact of cilia with different substances. 



(/) Wound Stimidi and Reactions 



Wound stimuli in general caused by mutilation, presence of organ- 

 isms, and direct contact of plant members with one another induce 

 various reactions, the nature of which depends upon the host plant, 

 and nature of the cause responsible for the same. The general 

 response to wound stimulation is manifested by the production of 

 abnormal growth due to active cell division, and in some cases enlarge- 

 ment of the cells. These reactions are characterized by local effects 

 and the reactions are often disproportionate to the stimulus. 



In some types of reactions there would seem to be involved more 

 than one class of phenomena. For example, the haptotropistic reac- 

 tions of tendrils, or at least the subsequent effects involved in the 

 formation of the spiral, etc., by the stimulus, and which is manifested 



2^ Stark (3) has observed that many plants and some roots react to contact 

 stimulation similar to tendrils when rubbed with a stick or cork. 



