SPECIAL AND GENERAL ACTIONS 141 



and daily movements continue. The latter cease, however, before the power 

 of response to mechanical stimuli is lost, when the air surrounding the plant 

 is rarefied. 



So long as the power of growth is retained, or in general, whenever the 

 motor mechanism remains capable of action, a cessation of the power of 

 response under particular conditions can only be due to their influence upon 

 the power of perception or upon the processes of induction. For instance, 

 when the air is gradually rarefied, first the heliotropic and later the geotropic 

 irritabilities disappear, whereas growth only ceases when a still lower partial 

 pressure of oxygen is reached. The fact that the leaf of Mimosa pttdica 

 returns to its original position when the recovery of the seismonic irritability 

 is prevented by chloroform, cold, or shaking, shows that the return movement 

 is not dependent upon the restoration of seismonic irritability. Since 

 the re-expansion of the active tissues takes place in the chloroformed 

 pulvinus, it is evident the anaesthetization affects some stage of sensation. 



Temperature. The minimum temperature for the photonastic move- 

 ments of the flowers of Crocus, and for those of the flowers and leaves of 

 various indigenous plants, lies between o C. and 4 C. The stamens of 

 Berberis also react to strong mechanical stimuli at comparatively low 

 temperatures, whereas no response is produced in the leaves of Mimosa 

 ptidica when the temperature falls below 15 C. 1 , although weakened 

 sleep-movements and autonomic movements continue. Sachs 2 found that 

 transitory heat-rigor was produced by exposure to 40 C. for an hour, at 

 45 C. in half an hour and at 49 C. to 50 C. in a very short time. In some 

 cases plants which were still irritable at 40 C. became transitorily rigid 

 when brought to a normal temperature, either as the after-effect of the 

 previous exposure, or owing to the shock-effect of the sudden change. 



Light. Organs which are able to develop more or less normally in 

 darkness are also able to curve in response to stimuli in the absence of 

 light. Thus flowers of Crocus and Tulipa which have grown ^in darkness 

 react strongly to changes of temperature, while tendrils as well as the 

 stamens of flowers of Cynara scolymus which have expanded in darkness 3 

 are sensitive to mechanical stimuli. Indeed even the leaves of Mimosa 

 pudica acquire their seismonic and photonastic irritabilities when brought 

 by special treatment to develop strongly in darkness. 



Exposure to light is essential for the continuance of the variation move- 

 ments of adult phototonic leaves. In darkness the pulvini gradually fall into 



1 Sachs, Flora, 1863, p. 451. The older researches of Dutrochet are quoted by Sachs. A few 

 details on the dependence of various aitionastic movements upon the external conditions are given in 

 the previously quoted works of Kabsch and Morren. Cf. also Hansgirg, Physiol. u. phycophytol. 

 Unters., 1893, p. 62. 



2 Sachs, I.e., p. 453. 



3 Pfeffer, Period. Bewegungen, 1875, p. 64. 



