1 908-1909.] The Sense-Organs of Plants, 171 



In stimulating an organ having a superficial layer of 

 mucilage, such as the head of a Drosera tentacle, unless the 

 touching body be pressed down through the mucilage there 

 is no stimulus. Certain pits in the outer walls of the 

 epidermal cells of Cucurbitacean and other tendrils, first 

 described by PfefiPer, are regarded by Haberlandt as " touch 

 spots" facilitating the transference of the stimulus through 

 the cell wall to the irritable protoplasm. This may be so, 

 but the " touch spots " are not an absolute necessity, as some 

 very sensitive tendrils, such as those of Cobsea and Passiflora, 

 do not possess them; and further, in other cases they are 

 present on the non-irritable parts of tendrils. 



In the majority of cases touch- sensitiveness is localised in 

 an area on the concave under-surface near the tip of the 

 tendril, but some tendrils are sensitive on all sides throughout 

 their length. In tendrils this touch-sensitiveness is developed 

 to a remarkably high degree. Pfeffer (' Unters a. d. hot. Inst. 

 zu Tubingen,' 1885, i. 506) states that the tendrils of Sicyos 

 angulatus react distinctly when stimulated by the contact of 

 a thread weighing .00025 mg. : the minimal stimulus for 

 sensitive skin is about ten times greater than this. 



The tentacles of Drosera leaf furnish a good instance of phy- 

 siological division of labour, inasmuch as the head is the sense- 

 organ for touch whence the stimulus is transmitted to a motor 

 organ — the tentacle stalk. Darwin proved this by his de- 

 capitation experiments ('Insectivorous Plants,' 1888, p. 188). 

 On amputating the tentacle head and stimulating the stalk 

 directly, no response was obtained. He further showed that 

 the tentacles were almost as sensitive to touch as tendrils, 

 — a hair of weight '0008 mg. when rubbed on the head pro- 

 ducing an appreciable reaction (Darwin, I. c, p. 25). 



Touch-sensitiveness is confined to growing organs, and the 

 movements in response to stimuli are therefore growth-move- 

 ments. Seismonic irritability is found, on the other hand, in 

 adult organs, and is correlated with movements of variation. 

 Such movements are exhibited by the leaves of Mimosa and 

 other Leguminous plants, by parts of the flowers of Barberry, 

 Opuntia, and Centaurea. In Mimosa the pulvini perceive 

 and respond to seismonic stimuli, the pulvinic hairs, according 

 to Haberlandt, acting as levers or " stimulators," and when 



