Origin and Development of the Composites. 2 71 
discovered the localisation of the irritability in Berberis and 
Goeppert (12) and others followed with physiological studies of 
that genus. Kabsch (29) seems to have been the first to make a 
proper physiological investigation of the movement in Centaurea. 
He figures and describes the vascular supply and the tactile hairs. 
He suggests that the spiral vessels are perhaps responsible for the 
well-known elasticity of the filaments, and considers that the tactile 
hairs occur so frequently in all irritable floral organs that they 
appear to be the chief factor in irritability. 
Heckel (22) distinguishes between nutritive irritability with an 
automatic mechanism as in Ruta and functional irritability as in the 
many other cases where a touch is required to set the mechanism 
in motion. The latter class can be conveniently sub-divided into 
those cases where the action is mechanical, as in the explosive 
stamens of Urtica, and those where the response is protoplasmic in 
origin, as in Berberis and the Compositse. 
Pfeffer (45) made the next important contribution, and there 
is an easily accessible account of this work in his Physiology of 
Plants (46). He shows that in Berberis the mechanism is similar to 
that of the pulvinus in Mimosa, and that the structure and distribu¬ 
tion of the irritable tissue in Heliauthemum, etc., cause the stamens 
to move always in the same direction wherever they are touched. 
He also shows that the contraction of the filaments in the Cynareee 
is due to a loss of turgor, that the filaments contract with a decrease 
in volume and the exudation of water into the inter-cellular spaces, 
which are largely developed in the tissue of the filament. The 
contraction varies from 8% to 30% of the length of the stamens, 
and as the injection of a ’5—1’0% solution of potassium nitrate 
causes the same amount of contraction Pfeffer concluded that the 
energy of contraction amounts to from 1 to 3 atmospheres pressure. 
On these grounds he controverts Cohn (9) and Unger (54-55), who 
maintained that the filaments broadened as they contracted and 
that there was no diminution in volume. Cohn also considered 
that the active contraction of the protoplasm was responsible for 
the movement, but Pfeffer considered this improbable on account 
of the high energy of contraction. He also contradicts Hofmeister 
(24), who suggested that the cell-wall was the responsive part of the 
cell, although he agrees that the power of contraction lies in the 
cell-wall, and shows that the filaments after stimulation and con¬ 
traction when put in boiling water contract 10%—40% of their 
length more on account of the elasticity of the cell-wall. 
Oliver (44) showed that in the case of the stigmas of Mimulus 
