PHYSIOLOGY 181 



In the tension produced by turgidity we see how purely physical 

 processes determine the rigidity of plants. These PHYSICAL processes 

 are, however, dependent upon the VITAL functions of plants, inasmuch 

 as they can only be called into action by living protoplasm in 

 conjunction with the structure and properties of the cell. Living 

 plant cells have thus power to regulate the physical effects of 

 osmotic pressure by increasing or diminishing, or even suddenly 

 overcoming their turgidity. (The osmotic pressure is increased three 

 times by the change of glucose into oxalic acid.) It will also be 

 apparent, in considering the operation of other physical forces, that 

 the primary and essential result of the vital action is to give rise to 

 the operation of physical processes, and to favour, constrain, or vary 

 them in such a way that they become of service to plant life. 



Tension of Tissues. The rigidity of parenchymatous tissue, 

 although to a large extent dependent upon the tension arising from 

 the turgidity of its individual cells, is nevertheless considerably 

 enhanced by the opposing pressure between the inner and outer 

 tissue systems, in particular, between the pith and the epidermal 

 and cortical tissues. The pith in this case plays the same part as the 

 cell sap, since it is continually striving to increase its volume ; the 

 epidermal and cortical layers, on the other hand, by the pressure of 

 the internal pith cylinders, are stretched and distended, just as are 

 the cell walls by the osmotic pressure of the cell sap. 



The tension of tissues is easily demonstrated by removing a strip 

 of the peripheral tissue from a piece of a turgescent stem (of a Sun- 

 flower, Helianthus, for example), and cutting out the pith. It will be 

 found that the outer tissue at once becomes shorter, and the pith 

 longer than when they were united together in the stem. If 

 the length of the stem experimented upon was 50 cm., the cortical 

 strip would shrink to 46 cm., and the pith suddenly lengthen 

 to 60-70 cm. 



According to J. C. MULLER a pressure of 13 atmospheres would be needed to 

 prevent the expansion of the isolated pith. 



From this experiment it will be seen that the natural length of a stem represents 

 the equilibrium maintained between the tendency of the pith to elongate and of the 

 outer tissues to contract. The cortical tissue between the epidermis and the pith 

 affords a transition between the two extremes of tension, the inner cell layers are 

 compressed like the pith, and the outer layers stretched like the epidermis. The 

 tension of tissues is also demonstrated by the fact that each strip of a fresh shoot 

 which has been split longitudinally will curve outward, so that the pith forms the 

 convex, the epidermis the concave side. 



There is often a great difference in tension even between the outer and inner 

 layers of the tissue of hollow organs, such as the stalks of a Dandelion ( Taraxacum 

 officinale), which, when split longitudinally, curl into helices of many turns, 

 especially if placed in water. A tension exists wherever resistant and unequally 

 strained tissues are in contact, and often occurs in parts of plants where it does not 



