WORK OF THE SHOOT 97 



and, being released from the rest of the stalk, have expanded. 

 Make another cut, at right angles to the first, and note the 

 result. Now place the cut end in water and note what 

 happens. The inner surfaces have become still more 

 convex. 



Our previous experiments in osmosis will help to explain 

 this. The water, through the attraction of the cell-contents, 

 has been absorbed by these cells, increasing the internal 

 pressure and stretching the elastic cell-walls. The dis- 

 tension of the cell by the internal pressure of sap in plants 

 is called turgidity. Next place the cut stalk in strong 

 salt solution and note the change in shape. The salt 

 solution, attracting water from the cells, reduces the internal 

 pressure ; the walls contract, the cells become smaller, 

 and consequently what was before the convex side becomes 

 now the concave side. Wash the cut stalk thoroughly in 

 water, and allow it to remain in water a short time. Do the 

 cells regain their turgidity and the strips resume their 

 former shape ? It is by such changes in the internal 

 pressure of the cells of plants that shoots are at one time 

 fresh and turgid, at another limp and wilted. The pith cells 

 of herbaceous stems, and of quick-growing shrubs, show 

 the same tendency to elongate, and though unable to do 

 so, owing to the resistance of the surrounding woody tissue, 

 they help considerably to maintain the rigidity of the 

 shoot. 



Root-pressure. We have already seen, in our observations 

 on Wheat seedlings (p. 33), that the roots may absorb more 

 water than a plant can utilize in a given time, and that 

 the excess is forced out of the tips of the leaves (Fig. 58). 

 Drops of water may often be seen on the leaves of certain 

 plants in the early morning, e. g. on the leaf-teeth of 

 Fuchsia and Lady's Mantle, and at the ends of the main 

 veins in the Garden Nasturtium. Sometimes the salts 

 in solution are so abundant as to leave a deposit on the leaf 



1296 r- 



