62 VEGETABLE PHYSIOLOGY 



growing cells, and in most others in which chemical 

 changes are proceeding. Hence such cells are continually 

 absorbing water, and are consequently so full that a consider- 

 able stretching force is exerted on the cell-wall which bounds 

 them. Cells in such a condition are called turgid, and 

 the condition itself is known as turgor or turgescence. The 

 equilibrium which is attained by such a cell is reached 

 when the distension caused by the entering osmotic stream 

 is balanced by the elastic recoil of the extensible cellulose 

 wall. In some cases the tension set up in a tissue by the 

 turgescence of the cells is sufficient to force the water, by a 

 process of nitration, through the walls of the outermost 

 ones, so that it escapes in drops or in a slow stream. This 

 may often be seen on the edges or apices of blades of grass 

 in the early morning. It is of great use also in forcing 

 water into the axial woody cylinder of roots, as will appeal- 

 later. Occasionally the turgescence becomes so great as to 

 lead to rupture of the cell-walls, as is the case in some 

 pollen grains, and sometimes in certain fleshy fruits. 



That the condition of turgescence 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 rhubarb 

 leaf, and, after carefully measuring its dimensions, steeping 

 it for some time in a ten per cent, solution of common salt. 

 On removing it, it will be found to have become flaccid, 

 and a remeasurement will show that both its length and 

 thickness have diminished. Turgescence is not, however, 

 due simply to physical causes ; the protoplasm which lines 

 the cell has a regulating influence over the passage of the 

 water into and out of them. When a turgid pulvinus of 

 such a plant as Bobinia or Mimosa is stimulated by rough 

 handling of the leaf, the latter falls backward from its ex- 

 panded position, 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 or compels the water to pass outwards. 



