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56 THE CAUSES OF FLUCTUATIONS IN TUMESCENCE 



so long as the tissues retain their vitality, but this is by no means the rule with 

 other nyctitropic movement. In certain cases we do find the latter retaining a 

 uniform character throughout, but in most cases they undergo considerable modi- 

 fication in regard to their precise nature, and especially in regard to their extent 

 as time goes on, and in many cases we find them almost or entirely absent 

 during considerable periods in the latter part of the life of the leaves in which 

 they once were conspicuously present. As we have already seen, they only begin to 

 manifest themselves after the evolution of the leaves has reached a certain point, and 

 subsequently we find them, as a rule, rapidly increasing in amount, attaining a maximum, 

 and then declining from this to a greater or less extent. The various parts of the leaf 

 are primarily in a condition of stable equilibrium; this is succeeded by one of unstable 

 equilibrium which gradually attains a maximum and then declines from tins towards, 

 or absolutely to, renewed stability. The position of the various parts of any leaf 

 is necessaiily determined by the structural peculiarities and degree of turgescence which 

 they possess. Structural peculiarities are necessarily liable to modification with increasing 

 age, but at any particular time they may be regarded as fixed factors. This is not, 

 however, the case in regard to conditions of turgescence which are liable to undergo 

 constant fluctuations in connection with variations in protoplasmic stimulation and acti- 

 vity, and fluctuations in the relations between root-supply and transpiratory loss. Where 

 all the tissues are subject to like fluctuations in turgessence, mere alterations in their 

 consistenc3 culminating in one direction in free excretion of water, and in the other 

 in general wilting will occur, but no definite movements will present themselves. It 

 is only when special masses of tissue, differing from one another in their capacity for, 

 and liability to, fluctuation in turgescence are present that orderly, definite movements 

 will occur, and then only in association with certain structural peculiarities. Nyctitropic 

 movements, therefore, only present themselves after leaves have attained a certain 

 stage of development, and in connection with the evolution of specially situated masses 

 of tissue, so disposed as to provide an active opposition between structural and functional 

 factors, and it is only so long and so far as the differences in structural and functional 

 properties of these masses persist that movements will occur. The relative youth of the 

 masses of tissue making for the diurnal position at first provides that the difference 

 shall be very considerable, and movements are accordingly very conspicuous for a time, 

 but as in most cases, at all events, this difference naturally tends to diminish witl 



the increase in structural strength and expenditure of active protoplasm of the younger 

 tissues, a condition of stable equilibrium is once more approached or actually attained 

 with a corresponding diminution or abolition of movement. 



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Great differences occur in regard to the rate at which the movements, whether 

 of a normal periodic character or arising as the result of extrinsic agencies affect- 

 ing filtration or water-supply, are carried out in different leaves, the rate hearing no 

 definite relation to the magnitude of movement, but being related to structural features 

 affecting the redistribution of fluid throughout the tissues The movements are 

 primarily due to fluctuations in the osmotic properties of the cell-sap, to fluctuations 

 m external pressure, or to fluctuations in water-supply, giving; rise to corresponding 

 alterations m the degree of turgescence of the various parts of the motor organs ; but 

 he rate at which these alterations can be effected must necessarily be influenced by 

 the structural features of the walls of the cells entering into the constitution of the 



