884 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



downwards and a portion of the pollen thus to escape above it, which is then carried away 

 by insects to other flowers and capitula where the stigmas are already unfolded^. 



Among irritable female reproductive organs are the lobes of the stigmas of MlmuIuSf 

 Martynia, Goldfussia anisophylla, &c., which close when their inner side is touched, 

 evidently in order to retain the pollen brought to them by insects. More striking are 

 the movements which follow a light touch on the gynostemium of Stylidium, a genus 

 ■ almost peculiar to Australia (e.g. S. adnatum and graminifoUum). The cylindrical 

 gynostemium which bears the stigma and close beside it two anthers is, when at rest, 

 turned sharply downwards ; irritation causes a sudden elevation and even reversal to 

 the other side of the flower. 



A more detailed description of these and other contractile organs will be foun d in 

 Morren's treatise named below '^. 



(4) Mobile and immobile condition of the motile parts of plants^. The parts of plants 

 endowed with periodic motion and irritability may present alternately two different 

 conditions according to the external influences to which the plants are subjected. 

 Their properties may be suspended for a shorter or longer time, and may give place 

 to a condition of immobility which again disappears if the external influences are 

 favourable, provided the organ is not in the meantime killed. This immobile condition 

 differs from that caused by death in the fact that it is transitory, and that the internal 

 changes which cause it are reparable. 



The following particulars are taken from the detailed illustrations in my work already 

 quoted. 



(i) Transitory rigidity from cold occurs in the leaves of Mimosa pudica when the 

 influences are otherwise favourable if the temperature of the surrounding air remains 

 . for some hours below 15° C; the lower the temperature falls below this point, the 

 more quickly does the rigidity set in ; the irritability to touch and concussion disappears 

 first, then that to the action of light, and finally also the spontaneous periodic movement. 

 The lateral leaflets of Desmodium gyrans are, according to Kabsch, immotile when the 

 temperature of the air is below 22° G. 



(2) Transitory rigidity from heat occurs in Mimosa within an hour in damp air at 

 40° C, within half an hour in air at 45° C, in a few minutes in air at 49° to 50° C. ; the 

 sensitiveness returns after exposure for some hours to air at a favourable temperature. 

 In water the rigidity from cold of Mimosa sets in at a higher temperature, 'vi%. in a 

 quarter of an hour between 16° and 17° C, and the rigidity from heat at a lower 

 temperature than in air, nji^. in a quarter of an hour between id" and 40° C* During 

 the rigidity from heat, whether in air or water, the leaflets are closed, as after irritation, 

 but the petiole is erect, while when irritated it is directed downwards. 



* These phenomena were discovered as long ago as 1 764 by Count Battista dal Covolo, and 

 are well described by Kolreuter in his preliminary Nachrichten von einigen das Geschlecht der 

 Pflanzen betreffenden Versuchen; 3rd Appendix, 1766, pp. 125, 126. 



^ C. Monen, On Stylidium, Mem. de I'Acad, roy. des sci. de Bruxelles, 1836; on Goldfussia, 

 ditto, 1839; on Spar ma7inia africana, ditto, 1 841 ; on Megaclinium, ditto, 1862. Also on Oxalis, 

 Bull, de I'Acad. roy. des sci. de Bruxelles, vol. II. No. 7 ; on Cereus, ditto, vols. V and VI. [On the 

 irritability of the stamens o( Ruta, see Carlet, Comp. rend., August 25, 1873, and May 18, 1874; 

 Heckel in Comp. rend., July 6, 1874. On Sparmannia, Cistus, and Helianthevium, see Heckel, in 

 Comp. rend., March 23 and April 6 and 20, 1874.] 



^ Sachs, Die voriibergehende Starrezustande periodisch bev/eglicher und reizbarer Pflanzen- 

 Organe, Flora, 1863, No. 29 et seq. — Dutrochet, Mem. pour servir, vol. I. p. 562. — Kabsch, Bot. 

 Zeit. 1862, p. 342 et seq. 



* A plant oi Mimosa immersed in water of from 19 to 2i-5°C. remains sensitive to impact 

 and light for eighteen hours or more. Bert's statement (Recherches sur le mouvement de la 

 sensitive, Paris 1867, p. 20) that Mimosa remains irritable up to 56 or even 60° C. is not suffi- 

 ciently confirmed, and is opposed to all that we know about the superior limits of temperature 

 for vegetation. 



