n6 
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 
