Il6 THE YEGETATIVE 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, (Photo by the author.) 



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 



