50 THE CAUSES OF FLUCTUATIONS W TURGESCENCE 



displacement which actually occurs will, however, be affected by the conditions of 

 general loss and supply of fluid; for the very tissues which provide for special rise 

 of tumescence from functional causes are also those which present the greater struc- 

 tural facilities for transpiratory loss. In young tissues we have a relative excess of 

 protoplasm providing for the presence of osmotic products in the cell- sap ; but at the 

 same time the facilities for loss of fluid in consequence of pressure or evaporation are 

 greater than they are in older tissues in which the formed materials— the cell-walls 



are more matured. Hence the fullest solar stimulation will be incapable of giving 

 rise to the full assumption of the diurnal position, unless it be associated with certain 

 conditions of general supply and loss of water. During the day, the precise position 

 assumed by nyctitropic leaves will be determined partly by degree of solar stimulation, 

 and partly by conditions of root- supply and evaporative loss. At sunset we have 

 the removal of solar stimulation, but, under ordinary circumstances, no immediate 

 cessation of transpiratory loss; and hence the position assumed by the leaves is not 

 the neutral one determined by persistent protoplasmic activity apart from transpiratory 

 loss; but passes beyond this, so as to reach a nocturnal maximum determined by the 

 fact that the tissues in which the fall of functional activity is greatest are also those 

 allowing most readily of transpiratory loss. Subsequently, as transpiratory loss dimi- 

 nishes and gradually disappears, the tissues will naturally acquire the degree of tumes- 

 cence corresponding to the osmotic capacities of the cell-sap apart from the addition 

 of assimilatory products under the influence of solar stimulation, and the tissues which 

 make for the diurnal position will become relatively stronger than they were when 

 subject to transpiratory loss. There will necessarily be a general rise in turgescence, 

 but the rise will be greater in those masses of tissue which are most affected by 

 transpiratory loss, and hence a reversion towards the diurnal position takes place. The 

 fully developed diurnal position is not, however, attained until solar stimulation comes 

 in to induce an excess of osmotic property in the cell-sap of the masses of tissue of 



greatest functional strength. 



The actual amount of displacement occurring in individual cases is further affected 

 by the arrangement of the opposing masses of tissue in the motor organs. In some 

 cases, we find the masses making for the diurnal position so situated that they are 

 aided by the action of the leverage of other parts of the leaf; in others, we find that, 

 in addition to overcoming the action of their opponents, they have to oppose that of 

 the leverage; and it is clear that, other things being alike, a greater displacement will 



be effected by the same rise in turgescence in the first instance than in the second. 

 In endeavouring to account for the movements occurring in any individual case, there- 

 fore, the precise arrangement of the opposed masses of tissue in the motor organs 

 must be considered not merely locally, but in relation to other parts of the leaf. 



CHAPTER VII. 



9 



<Lhe structural peculiarities uf the mo tar ©rgans 



'eabcB. 



In considering the question of the movements of stomata, it was pointed out that the 

 elements which make for the diurnal position — the guard-cells — differ from the surround- 

 ing epidermal elements in certain features; we have now to endeavour to determine how 



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