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.” 
(d) Seismonastie 
Illustrated by movement of stamens of Centaurea, Berberis, and 
Mimosa leaves. 
(e) Thigmotaxis 
Reaction of motile organisms (Chlamydomonas) resulting from 
contact of cilia with different substances. 
(f) Wound Stimuli 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 
21 Stark (3) has observed that many plants and some roots react to contact 
stimulation similar to tendrils when rubbed with a stick or cork. 
