f>2 VEGETABLE PHYSIOLOGY 



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 cells has a regulating influence over the passage of the 

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

 such a plant as Robinia or Mimosa is stimulated by rough 

 handling of the leaf, the latter falls backwards 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. 

 The active influence of the protoplasm is seen also in 

 another class of phenomena. Certain structures known as 

 nectaries occur conspicuously in many flowers. They are 

 aggregations of cells of a particular kind which exude a 

 sugary fluid upon their surface. The liquid in the cells 

 contains a little sugar, and this weak solution is capable 

 of passing through the protoplasm, not by osmosis, but by 



