54 



Elementary Plant Physiology. 



additional volume of water taken into the cavity, and the walls 

 are held rigid and firm by the pressure. Living cells are gen- 

 erally in a similar state of turgidity, and the firmness of soft- 

 bodied organs is due almost entirely to the turgidity of the 

 cells. Large plants, such as trees, secure firmness of the trunks 

 and branches by great masses of dead tissues mechanically 

 joined and arranged to secure stiffness (Fig. 31). 



The walls of some cells are more elastic and stretch farther 

 under the pressure of turgidity, and the osmotic value of the 



cell sac varies 

 greatly in differ- 

 ent instances. 

 Inequalities in 

 turgidity of tis- 

 sues fastened to- 

 gether, and joined 

 to dead cells in 

 which turgidity 

 does not exist, 

 gives rise to 



Stresses and ten- 

 Fig. 32. Behavior of tissues due to tensions, a, positions sions between the 

 assumed by the halvesof a slice through the center of a stem; different parts of 

 , diagram of a stem and relative length of pith and wood _. , 



when separated ; m,m,m,m, parenchyma. After Hansen. stems an U Other 



structures. 



49. Tissue tensions. If the parts of a tent or some 

 similar structure are cut apart, they quickly assume forms and 

 positions quite different from those in which they were bound 

 together under tension. The same may be said of the tissues 

 in a stem. 



Cut a slice a centimeter in thickness, and 10 cm. in length, 

 from the center of a petiole of rhubarb (Rheum), petiole of 

 Calla, or young stem of elder {Sambucus}. Lay flat in a dish. 

 Now divide longitudinally in the middle by a single downward 



m 



