46 



THE CAUSES OF FLUCTUATIONS IN TUKGESCENCE 



less closed when such stimu 



While the 



r aporative loss of fluid. 



plasts remain at a maximum, full tumescence is impossible 



i attended by defective root-supply or excessive 



timulation and functional activity of the pro- 



the absence of sufficient 



water-supply 



and 



whe 



the 



pply falls beneath a 



certain limit, the degree of 



turgescence in the guard- cells is so 



lowered that they no longer possess the excess 



of 



functionally 



quired strength sufficient 



to 



come 



surrounding tissue; and, when once this condition 



the structural resistance of the 

 has been reached, closure of the 



fices, or, in other words, an assumpt 



of the nocturnal position, must take 



The guard 



are from their position and structure specially exposed to evaporativ 



and 



therefore specially 



able to lose 



turgescence, in 



pecul 



wl 



the relations between 



pite of their functional 

 pply and transpiratory loss of 



water are such as to imply a relative excess in the latter. They are thus liable to 



lose turg 



diminished 



of water to 



or on excessive 



poratr 



loss determined by depression of atmospheric humidity; and, as a matter of fact, we 

 know that the condition of the stomatic orifices does vary in relation to fluctuations in 



ply, quite apart from any fl 



de 



O 



of 



stimulation, and that 



it is precisely on their 



that one of the main functions of the stomata 



depends. The movements determined by variations in the turgescence of the guard 



cells are sometimes connected with phy 



and sometimes with purely physical 



sometimes with variations in the functional 



ty of the guard 



and at 



others with variations in the relat 



of general supply and loss of fluid dependent 



on 



uric and atmospheric conditions. In the one case we have the cell-sap becomin 



less retentive of fluid, due to diminished manufact 



protoplasts 



the other, we have a deficiency 



3 of osmotic products by the 

 the amount of fluid which is 



arrived at, 



available for retention, and in both cases the same ultimate result is 

 namely diminished turgescence. The closure of the 



stomata during the course of hot 



dry days is thus 



of the nocturnal position in nyctitropic ones under similar conditions. All three 



the parallel of the wilting of common leaves and the assumpt 



phenomena are due to a loss in turg 



which is determined 



diminut 



not by any 



in protoplasmic stimulation, but by deficient water-supply to satisfy the 



osmotic capacities of the products arising in the cell-sap under the infl 



stimulation. 



of solar 



CHAPTER VI. 



Uhc npctitropic movements 



leaks. 





to 



In the previous chapt 

 which the 



er we have found grounds for 

 and closure of the stomata 



certain structural and functional differences between the 



epidermal elements, ai 



open 



believing that 



d secondarily on the relatio 



of water determined by atmospheric and telluric cond 



deal with the movements of entire leaves, whether these be of 



the movements 

 are due depend primarily on 



guard-cells and the other 

 between general loss and supply 



and when wo come to 



or such as occur under 



d it ions 



which they 



a nyctitropic nature 

 Jinarily regarded as the result 



