IN THE MOTOE ORGANS OF LEAVES. 



; 



filtration ; but we have no positive evidence to show that any loss of turgescence ever 

 arises owing to the development of special filtrative facilities in the protoplasm apart 

 from alterations in osmotic property of the cell-sap. The elastic recoil of the cell-walls 

 is the essential factor in maintaining the pressure which makes for nitration and is 

 constantly struggling with the osmotic capacities of the cell-sap. When a condition of 

 equilibrium has been established between these opposed agencies, it will remain stable so 

 long as they remain unaltei'ed, but will be disturbed by anything affecting their mutual 

 relations. Any rise in osmotic capacity of the sap will thus at once tend to increase 

 turgescence, and any rise in external pressure will tend to diminish it; and it is 

 only where fluctuations of equal value occur in both simultaneously that tumescence 

 will remain unaltered. 



CHAPTER IV. 



^he extrinsic agents affecting turgescence. 



In the previous chapter we have seen reason to believe that turgescence is essentially 

 dependent on certain osmotic properties in the oell-sap which cause tho cell caviti- $ to 

 include more fluid than their passive structural peculiarities will account for. Wo Jiavo 

 now to consider the influence of certain extrinsic factors in affecting the actual dcreo of 

 turgescence present at any particular time. Tho functional activities of the protoplasts 

 of the tissues provide that the cell-sap shall have certain osmotic capacities, but tho 

 degree of turgescence at any time must be affected by the extent to which the latter are 

 able to satisfy themselves. Protoplasmic activity secures the presence of osmotic products 

 but external conditions must determine the extent to which these will be ablo to give 

 rise to turgescence. The nature and quantity of osmotic materials in a mass <>f tissue 

 may remain unaltered; and yet, in spite of this, its degree of turgescence may vary 

 greatly, owing to variations in the amount of water available for them to act upon. 

 The total mass of water contained within any terrestrial plant is subject to constant 

 variations which are due to fluctuations in root-supply and evaporative loss; and where 

 the former of these is defective or the latter excessive, the highest degree of proto- 

 plasmic stimulation and activity will be unable to maintain the turgescence of tho 

 tissues to its normal extent. Solar stimulation certainly tends to establish the maximum 

 of turgescence in chlorophyll-containing tissues where sufficient water is available to 

 allow them to satisfy their osmotic properties; but the strongest solar stimulation will 

 not suffice to prevent loss of turgescence under conditions of defective supply or 

 excessive loss of water. Every one, and specially every one who has lived in the 

 tropics, must be familiar with the phenomena of temporary wilting in certain common 

 leaves, and the assumption of the nocturnal position by nyctitropic ones, which so 

 constantly manifests itself in the early part of the afternoon during hot dry weather. 

 The two phenomena have been by some observers regarded as the results of two 

 distinct causes, wilting of common leaves being allowed to be the result of deficient 

 water-supply,* whilst the movement in nyctitropic leaves is ascribed to tho direct 

 action of excessive solar stimulation,! but the following experimental data clearly 



* Sachs, Vorlesung, XIV. s. 274. 

 t Sachs, Vorlesung, XXXVI. s. 767. 



