PLANT PHYSIOLOGY AS RELATED TO PRUNING 13 



in plants. The movements are due to the tendency of 

 liquids of difi'erent densities to equalize, the weaker mov- 

 ing toward and diluting the stronger, and ^•ice versa. Root 

 hairs contain cell sap, which, besides protoplasm, con- 

 tains also usually a stronger solution of salts than does 

 the soil water. Hence the flow of water is from the soil 

 through the root-hair wall and from the root hairs to 

 other cells more and more remote from the water-absorb- 

 ing surfaces — the interior cells of the roots, the stems, 

 the leaves, etc. 



The effect of this flow or hydrostatic pressure, called 

 turgor, is the principal cause of normal stiffness of succu- 

 lent \-egetative parts of plants. Its opposite effect (wilt- 

 ing or plasniol_ysis) may be due to lack of sufficient water 

 to maintain turgor or to a sfjil water solutinn stronger 

 than that in the plant tissues, the result in this latter 

 case being the passage of water from the plant into the 

 soil water. So far as jiruning is concerned, the strength 

 of the solution is of minor importance, l3ut unless there 

 is an inflow of water from the soil to the roots sufficient 

 to maintain transpiration, wilting ^vill be a more or less 

 serious phenomenon, in some cases e\'en resulting in 

 death. 



14. Root or sap pressure is the term given to the os- 

 motic power of the roots. It may be obser\cd when 

 transpiration is not too rapid to offset its effects. The 

 most common phenomenon which pro\'es this sap pres- 

 sure is the so-called "bleeding" of grapevines, maples, 

 box elders, birches and other plants cut in spring about 

 the time that gro^^•th normally starts. If the cutting is 

 done earlier the adjacent cells become dr}' and shriveled 

 so the sap cannot exude. In other cases the wounded 

 surfaces may develop corky layers, may become clogged 

 with glutinous or solid material, or special growths 

 (tyloses) which come from adjacent cells and clog those 

 cells or tubes from which the major part of the bleeding 



