VEGETABLE IREITABILITY. 



387 



P%> t>y means of whicli the pollen is forcibly scattered (p. 283). In 

 Euellia anisophylla the style has a curved stigmatic apex, which 

 gradually becomes straightened, so as to come into contact with the 

 hairs of the corolla, upon which the pollen has been scattered ; and in 

 Mimulus and Bignonia (fig. 441, p. 249) the stigma has two expanded 

 lobes which close when touched, a movement apparently in some way 

 connected with fertilisation. In the Passion-flower, and some Cacti, the 

 styles move towards the stamens. The species of Stylidium (fig. 662) 

 have the filaments and styles united in . 

 a common column, at the upper part of 

 which the anther-lobes and stigma are 

 placed. The column often projects be- 

 yond the flower, and is jointed. At the 

 articulation an irritable swelling occurs, 

 which, when touched, or acted upon by 

 heat and light, causes a sudden incur- 

 vation by which the column is thrown 

 to the opposite side of the flower, burst- 

 ing the anthers and scattering the pollen 

 on the central stigma. After a time the 

 column recovers its position. These 

 movements take place in the flower for 

 some time after it has been removed 

 from the plant and kept in water (p. 

 284). Certain petals in some flowers, as in Orchidacese, are said to 

 move. Morren notices this in the case of species of Megaclinium 

 and Pterostylis. Draksea elastica, a Swan Eiver terrestrial Orchid, 

 shows irritability in the stalk of the labellum. This stalk has a 

 movable joint like an elbow. Bolbophyllum barbigerum has a movable 

 bearded labellum. Gentiana sedifolia closes its flowers when touched. 

 Chemical agents have an effect on the movements of plants. 

 Some act by causing irritation, others by destroying irritability. Nar- 

 cotic poisons, as opium, belladonna, and hydrocyanic acid, either 

 taken up by the roots or applied externally, destroy the irritability of 

 plants. They cause closure of the leaves of the sensitive plant, and 

 render it insensible to the action of stimuli. Their prolonged action 

 causes death, but if they are applied in moderate quantity, the plant 

 may recover, and again unfold its leaflets. It frequently happens, 

 however, that the irritability continues for some time much impaired ; 

 so that mechanical stimuli do not act in the same rapid and energetic 

 manner as at first. Similar efiects (as already noticed) are produced by 

 ether and chloroform when sensitive plants are introduced into an atmo- 

 sphere through which these substances are diffused. The effects may be 



Fig. 662. Stylidium tenuifoliiim. a and &, Separate flowers, showing the irritahle column 

 which bears anthers and stigma at its summit. 



Fig. 662. 



