98 



THE CAUSES OF FLUCTUATIONS VSt TURGESCENCE 



pulvini of Mimosa pudica, very considerable alterations in relative strength must thus 

 be rapidly established and, the equilibrium of position being correspondingly disturbed, 

 movements must occur with proportionate rapidity. 



But if all the movements of the leaves of Mimosa pudica, apart from those which 



attend the presence or 



absence of solar stimulation or 



other agencies affecting assimi- 



latory 



activity, owe their origin to 



such purely physical causes, the readiness and the 

 themselves ought certainly to bear some evident relation 



degree in which they manifest 



of conditions favouring the occurrence of rapid alterations 



to the presence or absence 



in 



the relations existing between general loss and supply of fluid, or affecting the 



activity of processes of local filtration; and there is abundant evidence that such a 

 relation really exists. Its existence is demonstrated by the fact that the occurrence, rate, 

 and extent of movement following the incidence of various disturbing agencies are alike 

 manifestly affected by conditions of telluric and atmospheric humidity 

 I laberlandt's tbeory, the extent of propagation of 



According to 



movement ought to bear 



* 



a direct 



* 



but 



relation to conditions securing excessive root-supply and depressed transpiratory loss ; 

 in fact the relation, in place of being a direct, is an inverse one. When the soil is 

 saturated and atmospheric humidity excessive, the capacity for movements and the rate 

 and extension with which they occur are alike at a minimum, and it is when the soil is 

 relatively dry and atmospheric humidity low that they attain their maximal development. 

 It would be strange indeed were this not so, seeing that conditions of atmospheric and 

 telluric saturation are calculated to increase the stability of turgescence, whilst aridity 



80 long as air and soil are 

 alike saturated, there can be nothing to interfere with the tissues fully satisfying any 



of soil and air must inevitably tend to render it unstable. 



osmotic capacities which they may possess ; and consequently, apart from variations in 

 the degree with which they exercise their assimilatory functions, there is nothing to 

 interfere with the continuous maintenance of turgescence at a given standard. All the 

 tissues may not possess osmotic capacity in like degree, but the conditions are such as to 

 allow them all fully to satisfy what capacity they have. There is an entire absence of 

 any 



struggle 



between osmotic capacity and any agencies tending to give rise to general 



loss of water. The only loss of water which is going on is that connected with assimi- 

 latory decomposition, and this is amply covered by the excessive root-supply. But when 

 the soil and air are not saturated, the equilibrium of turgescence can only be maintained 

 so long as general supply and loss of water continue to bear a uniform relation to one 

 another. Any rise in loss without corresponding increase in supply must make for de- 

 creased turgescence, and any fall in loss without corresponding depression of supply for 



may in certain cases coincide with 



increased turgescence. 



A high degree 



of turgescence 



a great capacity for movement, but only when it is maintained in spite of rapid 

 general loss of water ; only when it exists as the result of an accurate balance between 

 supply and loss; and therefore we do not find the capacity for movement high in cases 



where conditions are such as to give 



assimilatory activity and therefore osmotic property. 



rise to diminished transpiration without affecting 



c 



It would be easy to multiply examples of experimental results showing the truth of 



the statements contained in the previous paragraph ; but in order to avoid breaking the 

 continuity of the text to an inconvenient extent, one or two selected examples alone are 



troduced he 



and the mass of experimental evidence is transferred to the tables which 



* Op. cit. 



