WOUND RESPONSES OF LEAVES 
239 
and this condition inhibits the development of cortication that 
would markedly cut down the interior illumination. Neither cork 
nor bark may be regularly employed, so foliar organs are denied 
the major defenses of stems. Examination of leaves always sug- 
ests that the protective covering is reduced to the minimum so as to 
favor the utmost light relation. It may be that the marked effi- 
ciency of the deciduous trees is due to their large, thinly covered 
leaves which while most efficient through the summer become 
a menace in autumn and are frankly abandoned at the end of the 
growing season. Such tropophytes possessing a rhythmic foliage 
in harmony with the seasonal changes have leaves that are most 
susceptible to injury. 
To determine the nature of the healing tissues employed, 
wounded leaves were collected in early September from a number 
of plants, the injured areas were cut out together with a margin 
of normal blade, and the pieces prepared for sectioning. This 
material was killed in 1% chrom-acetic acid, imbedded in paraffin, 
and sections prepared 12 micra in thickness. Serial sections were 
used as large numbers of cuts had to be studied to find one show- 
ing what might be an average condition for the plant in question. 
Not only are the sections modified through any obliquity to the 
surface or margin but the structure of the blade and the nature 
of the cicatrice vary with relations to veins and islet-borders. 
