94 , THE CAUSES OF FLUCTUATIONS IN TITKGESCENCE 



affecting the presence or absence of solar stimulation have constantly been regarded as 

 of an entirely peculiar and specific nature. The ordinary periodic movements are re- 

 cognised as being connected with alterations in tumescence dependent on the presence 

 or° absence of solar stimulation ; but it has apparently never been questioned that the 

 other class of movements are owing to a certain inherent and peculiar irritability and 

 contractility in the protoplasts of . certain areas in the motor organs, which enable them 

 to effect alterations in turgescence, when they are called into activity by various 

 stimulant agencies which are quite incapable of producing any similar effect on the 

 protoplasts of the motor organs of common nyctitropic leaves. 



The motor organs of Mimosa padlca are, in fact, commonly regarded as being pro- 



vided with a sort of undifferentiated muscular apparatus, and the faith in the existence 

 of this has been so implicit that within recent years an attempt was ^ven made to 

 ascribe the phenomenon of propagation of movement from one part of the plant to 

 another, to the presence of an undifferentiated nervous system provided by the continuity 

 of the protoplasts of the various tissue- elements.* This latter theory hardly merits 

 discussion, as it was very soon demonstrated that protoplasmic continuity is a pheno- 

 menon of almost universal occurrence in vegetable tissues, and that the phenomena of 

 propagation of movement in Mimosa pudica are in many instances quite inexplicable as 

 the result of any changes occurring along the course of any continuous tracts of living 

 protoplasm. The theory affords, however, an evidence of the fixed belief in the presence 

 of certain specific functional peculiarities in the motor organs of the plant, and its 



abandonment only led to a renewed search for other possible agencies which might 



be regarded as capable of calling these into play.t 



The belief in the presence of certain specific irritable and contractile apparatus 

 within the motor organs of Mimosa pudica appears to have arisen mainly in consequence 

 of the exceptional ease with which movements may, under certain circumstances, be 

 induced, the exceptional rapidity with which they may be carried out, and the extent 



hich they may be propagated from one part of the plant to another: for, as h 



been pointed out, their mere magnitude is by no means exceptional, and they 

 agree with those which are conducted by ordinary nyctitropic leaves in leading, as a rule, 

 to the resumption of a position which, at an earlier period in the histoiy of the 

 leaves, was the permanent one. In addition to this, it has to be borne in mind that 

 the investigations in regard to the nature and origin of the movements have in greater 

 part been carried out in plants exposed to unnatural conditions, and consequently liable to 

 exhibit phenomena of more or less abnormal character and limitation, and that they are 

 consequently very imperfectly representative of those proper to the plant when exposed to 

 its normal environment. The phenomena which are ordinarily described as characterising 

 plants cultivated in pots in European conservatories and laboratories, as has been already 

 pointed out in connection with the question of the initial nocturnal position of the primary 

 petioles, certainly do not correspond in detail to those occurring in plants grown under 

 natural conditions. The artificially-grown plants are subject to the influence of 

 limited root-supply and to abnormal conditions of atmosphere, and it is, therefore, no 

 wonder that the phenomena which they exhibit fail to afford an exact index to those 



• Lectures on the Physiology of Plants. By Sydney Howard Vines, M.A., D.Sc, F.E.S. Cambridge, at the Uni- 

 versity Press, 1886. a 



f Das Eeizleitende Gcwebesystem der Sinnpdanze. Yon Dr. G. Haberlandt. Leipzig, 1890. 











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