HYDRONASTIC MOVEMENTS H 7 



It is possibly owing to a hydronastic response that the position of many 

 foliage and floral leaves alters when the plant is freely watered or is brought 

 from dry air into a moist chamber. The changed position is maintained 

 under the new conditions, and ultimately becomes permanent when the 

 adult leaf ceases to grow. Evidently we are dealing with a physiological 

 growth-reaction, and not with a movement due to a temporary change ot 

 turgor. Similarly, the changes in the position of the foliage and floral 

 leaves observed by Kraus, Wiesner, and Hansgirg \ as the result of 

 alterations of turgidity, appear largely to be hydronastic in character. 

 Unfortunately, the other external conditions were frequently not kept 

 constant during these observations, and, in addition, insufficient attention 

 has been paid to the physical movements resulting from the changes of 

 turgor and to the influence of the latter upon the power of physiological 

 response. Hence the observations are not altogether satisfactory, and fail 

 to indicate the extent to which hydronastic reactions are responsible for the 

 result observed. From Kraus's researches it does, however, appear as 

 though the hydronastic equilibrium of the foliage-leaves of a variety of 

 plants was considerably disturbed by pronounced rises or falls of turgidity, 

 and the experiments of Wiesner and Hansgirg seem to indicate the same 

 for floral leaves. Thus the flowers of Anagallis coerulea and Gentiana 

 amarella close or remain closed in air saturated with moisture, according 

 to Wiesner, even when exposed to optimal illumination ; and the same 

 applies to the flowers of Stellaria media and Holosteum meditim, according 

 to Hansgirg, when submerged under water 2 . 



, Since a variety of factors come into play under natural conditions, it is 

 not possible to say in what degree hydronastic actions may be responsible 

 for the assumption of different fixed positions by leaves on dry and moist 

 habitats 3 . Hydronastic responses take little or no part in the daily move- 

 ments of leaves and of periodic flowers, for these are primarily induced by 



term ' hydronasty ' for this phenomenon seems preferable to that of turgonasty employed by 

 Hansgirg (Physiol. und Phycophytol. Unters., 1893, p. 1 1). [No additional terms are likely to become 

 essential even when the subject is further studied, and there seems to be no valid reason for retaining 

 the term turgonasty to represent those instances in which changes of turgor act as the stimulus. In 

 any case the terminations f -nasty ' and ' -tropism ' must be restricted to physiological responses, and 

 no special terms are needed for physical movements induced by turgor, by hygroscopicity, or by 

 imbibition and swelling. To invent unnecessary special terms is merely to strew the path of know- 

 ledge with useless lumber which tends to acquire a fictitious value in the eyes of those forced 

 subsequently to struggle over these obstacles.] 



1 C. Kraus, Flora, 1879, p. n ; Wiesner, Sitzungsb. d. Wiener Akad., 1882, Bd. LXXXVI, 

 Abth. i, p. 212 ; Hansgirg, Physiol. u. Phycophytol. Unters., 1893, pp. 32, 42, 48. 



2 According to the authors named (cf. also Planchon, Bull, de la Soc. bot. de France, 1896, 

 T. XLIII) there are also flowers which close when their turgidity decreases, and it appears that 

 certain flowers which are expanded when the turgidity is normal perform a hydronastic closing 

 movement when the turgidity either rises or falls. 



3 Cf. Stenstrb'm, Flora, 1895, p. 132. 



