IN THE MOTOR ORGANS OF LEAVES. 



45 







In the guard-cells we have to deal not merely with younger elements, and 

 therefore with elements which are for a time structurally weaker than the surround- 

 ing epidermal cells, but with elements which in certain areas remain peimanently 

 weak, owing to the characters of their walls, which, as is well known, remain perma- 

 nently thin, extensile, and elastic on their lateral faces. They are not, however, merely 

 structurally weaker than their neighbours, but they are also physiologically stron_.r 

 on account of their relative excess of protoplasm, and specially of chlorophyll which, 

 under the influence of sunlight, determines an excess of assimilatory activity and with 

 this an excess of turgescence, due to the rise in osmotic capacity which the products 

 of assimilatory activity determine in the cell-sap, and which attains a specially high 

 degree of development in consequence of the complete closure and isolation of the cell 

 cavities. An equal rise in turgescence does not occur in the common epidermal cells, 

 owing to their poorer protoplasmic content and deficiency in chromatophores ; and hence, 

 under the influence of sunlight, the guard-cells become for the time being relatively 

 stronger than their neighbours, and are able to determine a certain amount of dis- 

 placement of them. When assimilatory activity is no longer stimulated, the manu- 

 facture of osmotic products falls, and with this the turgescence of tho guard-tells c< asei 

 to be excessive and tho elastic recoil of their own walls and of tho surrounding dis- 

 placed cells causes the tissue to resume the position proper to its passive condition as 

 determined by the structural strength and arrangement of its constituent elements. Tho 

 common epidermal cells are structurally stronger but functionally weaker than the 

 guard-cells, and hence are relatively weaker during periods when tho tissue is exposed 

 to sunlight than they are when it is in darkness. The guard-cells, on the other 

 hand, are structurally weaker but functionally stronger than their neighbours, and 

 consequently attain their maximum relative strength during periods of insolation. 

 The position of the cells during darkness represents the outcome of the structural 

 features of the tissue along with the degree of turgescence persisting apart from 

 photic stimulation of its protoplasts. The incidence of sunlight determines a greater 

 rise in turgescence in the guard-cells than in the surrounding elements, and conse- 

 quently makes them relatively stronger than the latter are, and in doing so enables 

 them to cause a certain amount of displacement of these. The removal of solar 

 stimulation, on the other hand, gives rise to a general loss in turgescence; but, the loss 

 in the guard-cells being greater than that in the common epidermal cells, the latter 

 now become the stronger and return to the position which they occupy apart from 

 the action of the stimulus. The opening and closure of the stomatic orifices are, 

 therefore, the results of the strnctural and physiological differences existing between 

 the guard-cells and the other epidermal elements ; the relative structural weakness of 

 the former determining closure in tho absence of light, and the relative functional 



weakne 



of solar stimulation 



of the common cells securing that opening shall occur on the incid 



It is clear that these movements, which are really nyctitropic when due to the 



in conditions of 



presence or absence of light, may arise independently of any 

 illumination, so long as the variations in turgescence, which are their immediate causes, 

 are present. As we have already seen, the actual degree of turgescence in any tissue or 

 tissue element at any given time depends not merely on the osmotic capacities of the 

 cell-sap, but on the amount of water available to satisfy these. Accordingly we find 

 that even under conditions of maximum solar stimulation, the stcmatic orifices are more 



