40 THE CAUSES OP FLUCTUATIONS IN TURGESCENCE 





must be taking place in connection with the varying degree in which the different factors 

 are associated with one another. Every rise in protoplasmic stimulation, so far as it 

 affects the manufacture of osmotic products ; every rise in root-supply ; and every fall in 

 transpiratory loss, must make for increased turgescence: whilst every fall in protoplasmic 

 stimulation or root-supply, and every rise in transpiratory loss, must tell in the opposite 

 direction. In the vast majority of cases, however, no readily appreciable results ensue; 

 and it is only where the fluctuations are considerable, and where the tissues present 

 certain peculiarities of structure and arrangement, that we have conspicuous indices to 

 their occurrence under normal circumstances. 



CHAPTER V. 



tftetts prtrbitceii bg fluctuations m togesrence. 



Fluctuations in the turgescence of masses of vegetable tissue may manifest them- 

 selves in the form of more or less conspicuous changes in bulk, consistence, colour, or 

 position. Changes of the first kind present themselves very conspicuously in cases of 

 artificial plasmolysis in linear series of cells. In these a general diminution in bulk 

 is all that at first presents itself to observation ; and it is not until this has advanced to 

 a certain point, until the elastic recoil of the cell walls has been satisfied ; or, in other 

 words, until turgescence of the cells has ceased to exist, that detachment of the 

 protoplasts occurs. Changes in consistence associated with fluctuations in turgescence 

 cannot be better illustrated than by the temporary drooping and wilting which affects so 

 many leaves during the course of hot, dry days, and which is recovered from when 

 nocturnal conditions put a stop to transpiratory loss. Conspicuous changes of colour 

 are comparatively rare and exceptional phenomena, but some very striking instances 

 of their occurrence may be found. Among these is that which present itself in 

 connection with Selagmella serpens. 



Here the fronds, which under normal conditions are of a brilliant green during 

 the day, become towards, and for some time after, sunset of a chalky whitish colour. This 

 tint persists for some time and then gradually fades off, so that by midnight it has 

 been apparently completely replaced by the diurnal one. The cause of these chants 

 m colour can be readily determined by means of immersing portions of the fronds whilst 

 in the various states of coloration, in 2 per cent, solutions of osmic acid for a minute or 

 two, and then making microscopical preparations from them. This treatment, as Ludwig 

 Klein was the first to point out, not only fixes the protoplasts of the tissue in the form 

 which they had when the specimen was taken, but also acts on the chromatophores in 

 such a fashion as to cause them to retain their normal chlorophyll content and colour 

 even when the specimen is subsequently exposed to the action of alcohol and mounted 

 in balsam, the result being that permanent preparations corresponding to the various 

 changes in colour of the fronds can be obtained. 



Before going on to describe the phenomena presented by any such series of 

 preparations, it may be well to furnish certain details regarding the structure of the 

 fronds. The greater part of their substance is composed of two strata of epidermis, 



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