788 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



almost peculiar to Australia [e.g. S. adyiatum and gram'mifolium). 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 of 

 the flower. 



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

 Morren's treatise named below \ 



Sect. 28. Mobile and immobile condition of the motile parts of 

 plants^ The parts of plants endowed with periodic motion and irritabihty may 

 present alternately two different conditions according to the external influences to 

 which the plants are subjected. These properties may be suspended for a shorter 

 or longer time, and may give place to a condition of immobility which again dis- 

 appears 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. It is very important, in order to understand the phenomena of move- 

 ment, to make a clear distinction between the terms 'movement' and ' motihty' ; the 

 causes which produce any particular movement must not be confounded with those 

 on which motility or the power of moving depends. This distinction has, however, 

 been neglected by more than one writer, and great obscurity has resulted. The follow- 

 ing illustration may serve to explain the distinction. The theory of walking presup- 

 poses a condition of the muscles and sinews in which they are capable of motion — 

 a proper arrangement of the bones, the activity of the nerves, and the nutrition of all 

 the parts of the body by the blood. The question is a purely mechanical one when 

 all the parts necessary to the act of walking are known to be present and in their 

 normal position. But in attempting to show why the organs necessary for walking 

 sometimes refuse their work — as after severe fatigue, when the extremities are 

 paralysed, &c. — we have to do with altogether different questions. When once the 

 mechanical laws which regulate the act of walking under normal conditions are known, 

 it is only necessary to show why the power of motion is lost in the abnormal 

 condition ; but this may result from purely mechanical causes, from change in the 

 molecular structure of the substance of the muscles or nerves, &c.' — questions which 

 have nothing to do with the mechanical phenomena of walking. It is easy to apply 

 these observations to the motile parts of plants. Their anatomical and true experi- 

 mental investigation in the normal motile condition lays the foundation of the 

 mechanical explanation of every single movement of a leaf. On the other hand the 

 question why leaves under certain circumstances are immobile, although it no doubt 



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

 ditto, 1839; on Sparmannia africana, ditto, 1841 ; on Megaclinium, ditto, 1862. Also on Oxalis, 

 P3ull. 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 of Ruta, see Carlet, Comp. rend., August 25, 1873, and May, 18, 1874 ; 

 Heckel in Comp. rend. July 6, 1874. On Sparmannia, Cistus, und Helianthemum, see Heckel, in 

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



' Sachs, Die voriibergehende Starrezustande periodisch beweglicher und reizbarer Pflanzen- 

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

 Zeit. 1862, p. 342 et seq. 



I 



