158 OUTLINES OF PLANT LIFE. 



capsules of mosses illustrate this inequality. It may be pres- 

 ent, however, without manifesting itself in the external form. 

 If the rapidly growing flower-stalk of the dandelion or the leaf- 

 stalk of rhubarb be carefully split lengthwise the parts will 

 curve or even curl outward. Separating the pith and the 

 cortex of a young elder shoot from the wood and carefully 

 measuring them shows that the pith elongates and the cortex 

 actually shortens. The experiment, therefore, shows that the 

 pith really grew more rapidly than wood, but were com- 

 pressed in the uncut stem, while the cortex was slightly 

 stretched. The strains thus set up are spoken of as longitudi- 

 nal tensions. Similar tensions due to unequal transverse 

 growth may be shown to exist. If a thin transverse slice 

 from the fleshy leaf-stalk of the rhubarb be divided into 

 equal parts by a longitudinal cut it will be found in a few 

 moments that the halves can no longer be made to touch 

 throughout the line of the cut, because it has become convex. 

 Both sorts of tensions will be exaggerated if the parts be 

 placed for a few moments in water. 



EXERCISE XXXIX. 



To show the existence of longitudinal tensions of tissues due to unequal 

 growth or turgor. 



A. Cut a young internodeof elder 10 cm. long, making ends as square 

 as possible. Measure accurately. Remove wood all around and meas- 

 ure pith. Place pith in an atmosphere saturated with moisture and re- 

 measure after I hour. Compare measurements. (If elder is not at hand 

 use young shoots of grape, wild or cultivated. ) 



B. Split a scape of dandelion lengthwise with a sharp knife into four 

 strips. Note immediate effect upon their form. Lay the strips in water 

 for a few minutes. Observe form. Transfer them to 5 per cent salt so- 

 lution. What effect? What causes these changes of curvature ? (The 

 young stems (hypocotyls) of castor bean may be substituted for dandelion 

 scapes, but are not so responsive.) 



To show the existence of transverse tensions of tissues due to unequal 

 growth. 



A. From a piece of willow or poplar stem separate a ring of bark i cm. 



