Il6 THE VEGETATIVE FUNCTIONS OF PLANTS 



in others the crisping may denote a diseased condition 

 of the leaf. 



119. Tissue -Tension. If a thin surface strip of tissue 

 is cut away for a short distance from a stalk of celery, 



FIG. 74. Longitudinal tissue-tension in leaf-stalk of rhubarb. In C 

 the strip of outer tissues, entirely removed from the main piece, is seen to 

 have shortened, showing that, before being removed, it was in a state of 

 longitudinal tissue-tension, owing to the fact that, in growth, the inner 

 tissues elongated more rapidly than the outer, thus stretching the latter 

 lengthwise. 



FIG. 75. Portion of dandelion scape, showing "curls" resulting from 

 longitudinal tissue-tension. 



or the thick petiole of a burdock or other leaf, the strip 

 at once curves outward, on account of its longitudinal 



