168 BOTANY 



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 represents the cell sap, as 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 pres- 

 sure of the cell sap. The tension thus arising from the mutual resist- 

 ance of different tissue systems acts upon the various plant organs 

 like the turgidity of the single cells, and keeps them firm and rigid. 



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 both united 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 lengthen to 55-60 cm. 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 

 assist, as in the leaves and stems, in maintaining the rigidity of the plant body. 

 Longitudinal and transverse tensions occur, particularly when, through secondary 

 growth, newly formed growing tissues have to overcome the resistance of other 

 tissues. In this way the primary and then the secondary cortex of trees become 

 greatly stretched by the new cambial growth, so much so, that if a ring of bark 

 be removed from a stem and then placed round it, its edges cannot be brought 

 together again, even by the expenditure of considerable force, on account of the 

 tangential contraction which has taken place. 



In the meristematic tissues of growing points there is scarcely any 

 perceptible tension, while, on the other hand, in regions which are in 

 a state of elongation the tension of the tissues attains its highest limit. 

 After an organ has completed its growth the elasticity of the cell walls 

 and the turgescence of the cells decrease ; and the tension of the 

 tissues is therefore also diminished. The requisite rigidity is, however, 



