I 



48 THE CAUSES OP FLUCTUATIONS IN TUMESCENCE 



that co-existing with the fullest solar stimulation and excessive transpiratory loss. A 

 nyctitropic leaf at any time of night is normally in a very different condition from one 





! 



which has been immersed in boiling water or exposed to the continued action of 

 the vapour of chloroform. Maximum turgescence is only attained where full solar 

 stimulation co-exists with free supply and abolished loss of water; but all degrees of 

 turgescence short of this are constantly present in all ordinary tissues in relation to 

 the varying conditions of protoplasmic activity and general loss and supply of water 

 present in them under different circumstances. A greater degree of turgescence will 

 naturally tend to be present where depression of functional activity coincides with 

 an absence of transpiratory loss than where the latter is still actively going on, and 



hence we find the maximum departure from the diurnal state taking place at and 



shortly after sunset. With the arrest of transpiratory loss the osmotic capacities of 

 the cell-sap are able fully to satisfy themselves. These, although not so great as 

 they are under the influence of solar stimulation, are yet sufficient to cause the tur- 

 gescence of the tissue to attain a higher degree than it attains under the fullest solar 

 stimulation where the latter co-exists with excessive transpiratory loss and insufficient 

 root-supply. In the study of movements dependent on alterations in turgescence, 

 whatever their nature be, it must always be borne in mind that we are dealing with 

 phenomena arising not as the result of the action of a single ultimate factor, but of 

 the action ot various factors which may be associated with one another in very different 

 combinations. Fluctuations in turgescence are capable of arising not only as the result 

 of fluctuations in osmotic property of the cell-sap, but also in consequence of fluctuations 

 in the relations between general supply and loss of water, or in the conditions of 

 pressure, and therefore of filtrative loss to which the tissues are exposed. 



All nyctitropic movements are in their origin essentially similar to the 

 ments determining the opening and closure of the stomatic orifices. They are due to 

 the presence of masses of tissue differing from one another in structural and functional 

 property and so related to one another that they tend to give rise to displacements in 

 opposite directions. Their structural and functional differences give rise to the occur- 

 rence of unlike fluctuations in their turgescence under the influence of like conditions 

 and therefore under the influence of varying conditions we find their relative streno-ths 



mov 



varying, and movement taking place in one or other direction accordingly. In the 

 case of the stomatic movements, the motor apparatus— the guard-cells and the common 

 epidermal ceils— are in immediate relation to one another ; but, in cases where move- 

 ments of entire leaves or of large" parts of leaves occur, the motor agents are generally 

 so disposed as to tell on opposite aspects of masses of passive tissue whose displacement 

 they make for. In the case of freely motile pulvini, for example, we find the motor 

 apparatus consisting of thick masses of cortical parenchyma, around a cord of passive 

 tissue consisting of modified wood and pith, or of the former alone. The precise dis- 

 position of the pads of cortical parenchyma in relation . to the passive tissue on which 

 they act varies in different cases; but we can always recognise them as consisting of 

 two opposed sets, differing from one another in structural and functional properties 

 and making for displacement of the passive tissue in opposite directions. In certain 

 cases the relation of the opposing tissues, however, more closely resembles that which 

 we find present in the case of the guard-cells and ordinary epidermal cells. The 

 movements in the opposite halves of the laminae of the pinnae of many nyctitropic 

 leaves at a certain stage of their evolution are effected under the influence of structural 



