36 PHARMACEUTICAL BOTANY 
surface and so are negatively geotropic. Branches of stems that 
assume a relation parallel to the earth’s surface are diageotropic. 
The Lima Bean, Sarsaparilla, Poison Ivy, and other plants whose 
stems twine about supports exhibit dateral geotropism in their 
horizontal curvatures. 
GALVANOTROPISM is the reaction of protoplasm to electrical 
stimuli. In this connection it may be said that the degree of 
response bears a definite relation to the intensity of the stimulus. 
No visible external electrotropic reactions have been observed in 
higher plants, although when their cells are examined micro- 
scopically, the reaction becomes manifest. Kiihne has shown 
that when an electric current is passed through the hairs of the 
Spiderwort, the cytoplasm becomes gathered into small globular 
masses. : 
THIGMOTROPISM is the response of living matter to mechanical 
stimulation. Examples of this form of irritability appear to be 
far less common among plants than among animals. Certain 
species of Mimosa (Sensitive Plants); Oxalis, Drosera (Sundew), 
Desmodium and Dionea muscipula or the Venus Fly-trap exhibit 
this phenomenon to a marked degree. A few instances only 
will be considered. When the tendrils of climbing plants come 
into contact with the uneven surface of solid bodies they are 
induced to coil. When the tentacles on a modified leaf of the 
Sundew (Drosera) are stimulated mechanically by an insect or 
artificially they are induced to curve over. If a good plant of 
the Venus Fly-trap (Dionaa) is selected, it will be seen to possess 
leaves, the terminal portions of which are modified as traps for 
catching insects (Fig. 17). Hairs will be seen projecting from 
the upper surface of each valve of the hinged blade. If one of 
these hairs is touched with a pencil no reaction will be evident 
but if, after a lapse of twenty seconds, the hair is touched again, 
the 2 valves close. If the stamens of Berberis (Barberry) be 
touched near the base during their pollen shedding stage they 
will be observed to curve toward the stigma. 
The most highly specialized form of thigmotropism observed. 
in plants appears to be found in Mimosa Spegazzini, a member of 
the Bean family. According to Steckbeck “when a mechanical 
stimulus, such as a forceps pinch, is applied to one of the terminal 
