76 MOVEMENTS OF CURVATURE 



absent a weakened power of movement is retained l . Since, however, 

 the operation undoubtedly affects the irritability, it is impossible to deter- 

 mine from such experiments the exact part played by the active contraction 

 of the lower half of the pulvinus. Nor is it certain whether all the different 

 cells and tissues of this zone are equally excitable. The parenchyma 

 cells around the vascular bundles appear in fact to be of primary im- 

 portance, but the epidermal cells may also take part in the contraction, 

 although their tangential tension is converted into a tangential pressure, 

 that is, they are compressed instead of being stretched where a strong 

 curvature is produced. The way in which stimuli may be conducted from 

 one pulvinus to another, as well as the fact that the pulvinus may per- 

 ceptibly react after the epidermis has been removed, suffice to show that 

 the cortical cells may be stimulated without the aid of the epidermis. 

 The latter may also receive an excitation 2 , and contact with the hairs 

 alone is able to excite a response in the pulvinus. The hairs probably 

 only act by readily transmitting the pressures to the cells beneath, and 

 hence behave as ' stimulators ' in Haberlandt's sense of the term. The 

 fact that gentle direct contact on the under half of the pulvinus may 

 act as an excitation points to the direct excitability of the epidermal cells, 

 for a much greater pressure must be applied or a more violent blow struck 

 upon the upper epidermis of the pulvinus in order to produce an excitation 

 of the under half. 



The contraction and diminution of volume of the pulvinar cells of 

 Mimosa cannot be directly observed, but they are indicated by the escape 

 of water from the reacting cells, as in the stamens of Cynareae. This 

 water partly fills the intercellular spaces and is partly conducted into 

 the neighbouring tissues of the stem and petiole 3 , and possibly also a little 

 may pass into the vascular bundles. If the leaf-stalk is separated from 

 the pulvinus by a sharp cut, and the still attached pulvinus kept in moist 

 air, on stimulation water escapes from the cut surface, and at first from 

 the inner, but not from the inmost layers of parenchyma in the lower 

 half of the pulvinus. A little later some water also escapes from corre- 

 sponding cells in the upper half of the pulvinus. 



This displacement of air and water causes the under half of a stimulated 

 pulvinus to increase in volume as determined by micrometer measurements, 



1 The observations of Pfeffer and of other observers are given in full in Pfeffer's Physiol. Unters., 



1873- 



2 The researches of Borzi (L'apparato di moto delle sensitive, 1899, p. 17, reprint from Rivista 

 di Scienze Biologiche, Vol. iv) fail to reveal the distribution of sensitivity in the tissues. Cf. Haber- 

 landt, 1. c., 1901 , p. 79. The latter author (p. 88) concludes that in the case of Biophyttim sensitivum 

 the hairs on the pulvini directly perceive stimuli. 



3 Hence arises the fact that Bonnier (Revue generate de bot., 1892, T. iv, p. 512) observed 

 slight variations of the air pressure during a stimulatory movement, when a manometer was inserted 

 in the stem of Mimosa pudica near to the origin of the pulvinus. 



