■14 



ON THE IRRITABILITY OF PLANTS. 



occupied the attention of many distinguished vegetable physiologists, particularly 

 Desfontaines, Smith, Sprengel, Marcet, Macaire, and Dutrochet. The obvious 

 movements in plants, resulting from what is usually termed their irritability, may 

 be excited by atmospheric causes, such as light and heat ; or by the actual con- 

 tact of other bodies. The expansion of the flowers of the common crocus under 

 the influence of the sun, and their speedy closure when that influence is withdrawn; 

 the shutting up, at the approach of night, of many flowers, which re-open on the 

 return of morning ; these and many other similar facts may be regarded as illus- 

 trations of the effect of atmospheric or natural stimuli. A striking and well- 

 known example of the excitability of plants by artificial stimuli, or actual contact, 

 is afforded by the sensitive plant, Mimosa pudica, which shrinks and folds up its 

 leaflets on the slightest touch. Another and no less curious instance presents 

 itself in the Dionma muscipula, (Venus's Fly-trap,) the leaves of which are so sensi- 

 tive, that, should the centre of one of them be touched by a fly or other insect, its 

 sides instantly collapse like a steel trap, and detain the little animal a prisoner. 

 But besides these two kinds of irritability, there are other movements observed 

 in plants, which are apparently independent of any direct stimulus, and which, 

 being actually spontaneous, are in their character still more analogous to animal 

 excitability. Many instances of this spontaneous irritability might be mentioned, 

 but the more immediate object of the present paper is to notice especially the 

 movements of the sexual organs. 



The stamens of a great number of plants are found to manifest this power of 

 movement about the period of fecundation : it may be observed in Liliaceous 

 plants, in the different species of Saxifraga, and Parnassia, the stamens of which 

 at this time approach the pistil. In the Geranium and the Kalmia, the filaments 

 bend down for the purpose of laying the anthers upon the stigma. In the Pink 

 and in the Sue, the stamens approach the stigma successively ; commencing with 

 those which are opposite- to the petals, and terminating with those which are 

 alternate. In the Nasturtium the eight stamens incline towards the stigma, each 

 in its turn, with a sort of regularity, during eight hours. In the Tobacco plant 

 the action is different : all the stamens approach the stigma at the same time. 

 The stamens of Amaryllis aurea, at the period of fecundation, are oftentimes so 

 much agitated as to resemble an automaton movement. In some instances a 

 movement of these organs may be readily excited by mechanical stimuli, as in 

 Berberis vulgaris, the common Barberry, the stamens of which, when touched 

 near their base by the point of a needle, spring up elastically and strike the 

 stigma. The same fact may be observed in some species of Chenopodium ; in the 

 tube of the anthers of many thistles ; and in the filaments of the opuntice. The 

 sexual apparatus of many orchidaceous plants also offer striking instances of irri- 

 tability, as in Cirrhcea, Myanthus, and Catasetum. 



This movement or irritability is less apparent in the stigma than in the 

 stamens of flowers, although it may be observed in many instances. The stigma 



