236 BOTANICAL GAZETTE [APRIL 
After the elongated palisade cells had broken through the epi- 
dermis, the pressure against their side walls being thus somewhat 
relieved, the sac-like cells bulged out laterally and became club- 
shaped at the tip. The central cells stood up perpendicularly to the 
surface of the leaf, but the cells around the edge of the intumescences 
curled over toward the leaf surface, thus causing the outside whitish 
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Fic. 4.—Cross-section through a normal potato leaf. Fic. 5.—Cross-section 
through a young intumescence, showing hypertrophy of palisade layer and formation 
of cross-walls. Frc. 6.—Later stage; the swollen cells have broken through the 
epidermis. Fic. 7.—Later stage; some of the cells were’ cut through in sectioning. 
ring of the intumescence, mentioned before. The hypertrophied 
cells were not confined to the palisade layer, but often included cells 
in the layer of spongy parenchyma below. In the largest intumes- 
cences the swollen cells extended back to one or two rows from the 
under epidermis. As the intumescences grew old their walls became 
cutinized. 
