546 Blackman and Matthaei . — Traumatic Stimulation . 
existent cells directly into cork-cells by a renewal of meri- 
stematic activity, and wound-callus, on the contrary, as arising 
by an apical growth in the direction of the wound, taking 
place in the underlying cells and leading to the formation of 
unseptate tubes or rows of cells, which form a scar at the 
wounded surface. 
Massart 1 considers this distinction not to be strictly ap- 
plicable to leaves, as growth — a callus character — is nearly 
always combined with the regular wound-cork formation. 
That there should be a third alternative in the forma- 
tion of an absciss-layer, determined merely by difference in 
external conditions, suggests that all these processes are con- 
ditional variations of the fundamental reaction to a traumatic 
stimulus which is directed to breaking the continuity of organ- 
ization between the dead and the living cells. 
1 Massart, loc. cit., p. 53. 
EXPLANATION OF PHOTOGRAPHS IN 
PLATE XXIX. 
Illustrating Dr. Blackman’s and Miss Matthaei’s paper on the Reaction 
of Leaves to Wounds. 
The plate gives reproductions of six natural-sized photographs taken from the 
living leaves. 
Figs. 1, 2 are from one leaf showing the collective treatment of dead areas that 
are close together. Fig. 1 is an early state, but in Fig. 2 the process is nearly 
complete and the piece is hanging. 
Fig. 3 is a late stage in the abscission of circular dead patches. The inner hole 
in each spot has been punched out and the outer concentric ring of separation is 
the work of the absciss-layer. The rings thus cut out are mostly still attached at 
one point. 
Figs. 4, 5 are from the same leaf; showing firstly the stage at which the 
occlusion-line is visible running round each brown area of dead cells, and secondly 
the final state in which all the pieces have been exfoliated and the new edges have 
finished growing. 
Fig. 6 shows a leaf with two killed areas in the shape of longitudinal 
strips. This is in an intermediate state between those of the first and second 
figures and the pieces to be exfoliated are not quite free yet. 
