244 
IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
cells resulting from mitoses which establish walls parallel to the 
wounded edge of the leaf. All cell-layers of the blade share in 
this work, including the epidermis in case its cells are large. 
There are occasional divisions of epidermal cells in Pittosporum 
(fig. 6) and Lonicera (fig. 2) and numerous divisions in Bryo- 
phyllum (fig. 3) where several cells may be derived from either 
an upper or lower epidermal cell. 
The palisade cells divide first transversely into shorter units, 
and these may later divide at right angles by periclinal walls 
(fig. 2) . Meanwhile divisions have closed the spaces in the spongy 
mesophyll thus completing the zone of healing tissue. 
Quite striking is the appearance of a section through a porous 
leaf of the Horse-radish type (fig. 5), where the solid cicatrice 
offers strong contrast to the loose mesophyll of this leaf. Equally 
remarkable is the regular series of new cells bordering the 
wounded edge of Bryophyllum leaf (fig. 3) and extending from 
surface to surface across all of its tissues. 
The completed cicatrice reveals the cambium-like behavior of 
those cells which by mitosis established the healing layers. Such 
activity seems strangely foreign to a leaf long since passed to its 
functional maturity. 
Department of Botany, 
University of Iowa. 
