151 



CHAPTER XIV. 



IRRITABILITY OF PLANTS. 



Not only is there a movemeht of plants or parts of plants 

 during growth, but also certain fully formed organs of plants 

 are motile, especially under the influence of an external stimulant. 

 One of the best known examples is the Sensitive Plant {Mimosa 

 pudicd) (fig. 298). The leaves of this remarkable plant are 

 bipinnate, and are articulated to the stem. The four pinnse 

 and the separate leaflets are also articulated. Spontaneously 

 at evening, or under the influence of any irritant, the leaflets 

 fold upwards, whilst the leaf-stalk as a whole bends downwards. 

 The movement is due to the presence at the base of the stalk 

 and of each leaflet of a mass of succulent parenchyma — the 

 pulvinus. The cells are saturated with water, rendering them 

 turgid. On irritation some of the water escapes into the 

 intercellular spaces, and the elastic cell walls contract, producing 

 the movement. When the irritation is removed the sap flows 

 again into the tissue, causing the return of the leaf to its normal 

 position. 



The movement only takes place between the temperatures 

 of 15° C. and 40° C. Oxygen appears to be necessary, as under 

 the exhausted receiver of an air-pump, or in an atmosphere of 

 hydrogen, nitrogen, or carbonic acid, the leaves of Mimosa 

 cease to move. Anesthetics, as chloroform, also cause a 

 cessation of the movements. 



The leaves and leaf-hairs of Venus's Fly-trap (fig. 143) 

 and Sundew are irritable. When a fly alights upon the surface 

 of the leaf it causes a bending over of the sides of the leaf to 

 enclose it. 



The stamens of many flowers are irritable. The Barberry 



