414 MANUAL OF BOTANY 



that it escapes in drops or in a slow stream. This may often be 

 seen on the edges or apices of blades of grasses in the early 

 morning. It is of great use also in forcing water into the axial 

 woody cylinder of roots, as will appear later. Occasionally the 

 turgescence becomes so great as to lead to rupture of the cell- 

 walls, as is the case sometimes in fleshy fruits. 



That the condition of turgidity in cells is attended by a 

 stretching of the cell- walls can be seen by taking a piece of a 

 plant which is turgid, such as the stalk of a rhiibarb leaf, and, after 

 carefully measuring its dimensions, steeping it for some time in 

 a 10 per cent, solution of salt. On removing it, it will have 

 become flaccid, and a remeasurement will show that both its 

 length and thickness have diminished. Turgescence is not, 

 however, solely due to physical causes ; the protoplasm which 

 lines the cells has a regulating influence over the passage of the 

 water in and out of the cell. If a turgid pulviniis of such a 

 plant as Robinia, or Mimosa, be stimulated by rough handling 

 of the leaf, the latter falls from its expanded position towards 

 the stem, and the fall is found to be due to the escape of water 

 from the cells of the lower side of the pulvinus. The original 

 state of equilibrium has been disturbed by the shock to the 

 protoplasm administered by the stimulation, and the latter allows 

 water to pass outwards. 



The active influence of the protoplasm is seen also in 

 another class of phenomena. Certain structures known as 

 nectaries have been described as occurring in many flowers. 

 These are aggregations of cells of a particular kind which exude 

 a sugary fluid upon their surfaces. If the petals of certain 

 flowers bearing these nectaries be out off and their cut ends 

 immersed in water, the glands continue for some time to exude 

 the nectar. There can be no question here of any filtration of 

 water through the tissue, as there is no pressure acting on 

 the base of the petal. The protoplasm causes the stream to 

 continue to flow by producing osmotic substances, in this case 

 chiefly sugar. It differs still more in its behaviour in that it 

 pours out not only the water thus absorbed, but a certain 

 amount of the sugar in addition. If the gland be killed by 

 alcohol, the sugar already there is retained in the cells, and no 

 exudation of nectar or even of water takes place. 



The vital activity of the protoplasm is thus seen to be 

 intimately connected with the presence of water in its substance. 

 The importance of the ready access of the latter is seen further 

 from other considerations. In the foregoing pages we have 



