Holden . — Some Wound Reactions in Filicinean Petioles . 789 
resulting in a callus-like incurving at the margins somewhat 
similar in appearance to the early stages of repair seen in trees 
after a branch has been severed (PL LXXIV, Fig. 20). In the re- 
mainder the epidermal cells nearest the wound showed at the most 
an increase in general bulk rather than elongation in any particular 
direction, and were usually filled with gum (PI. LXXIV, Fig. 16). 
The walls did not exhibit any additional growth in thickness 
in any case. 
(b) The vascular bundle and bundle- sheath. Whenever a bundle was cut 
into to any great extent, degeneration of the tissues and a general 
deposit of coarse, brown, granular matter seemed invariably to 
result. Where the bundle-sheath was not actually cut away the 
cells on the side adjacent to the wound showed a growth in 
length and also division, this often leading, where the sheath did 
not show radial thickenings, to loss of identity as a definite layer. 
Where radial thickenings were present the daughter cells resulting 
from the division also showed these (PI. LXXIV, Pig. 20). 
The endodermal cells of some forms (e. g. Davallia ) are characterized by 
heavy brown thickenings, but these do not seem to be in any way affected 
by wounds, the bundle-sheath of parts remote from the wound showing 
an amount of thickening equal to those nearer to it (PI. LXXIV, Fig. 15). 
With regard to the phloem and xylem, where the differentiation of these 
elements is already complete no traumatic effects are perceptible, but 
where, as in the apical portions, the lignification of the later-formed xylem 
elements has not begun, both this and the phloem show distinct elongation 
towards the affected part. The cells of the phloem and of the bundle 
parenchyma often show both elongation and transverse division ; in the case 
of the large xylem elements elongation alone occurs. That this elongation 
is of the nature of traumatic response, and is not due to the disturbance of 
the bundle symmetry by the departure of a leaf-trace, is strongly supported 
by the fact that specimens wounded on the abaxial side of the petiole show 
this feature very markedly (PI. LXXIV, Fig. 17). 
From the foregoing it will be perceived that the more specialized 
tissues, such as the epidermis, the endodermis, and the vascular elements, 
are all responsive to a greater or less extent to traumatic stimuli, but that, 
as might be expected, the modification is of a much less pronounced character 
than that obtaining in the comparatively simple cortical parenchyma. 
A first series of experiments such as these are almost of necessity 
incomplete, since the aim has been to obtain a general idea of the mode of 
response to wounding in the Filicineae as a group, rather than a detailed 
knowledge of the behaviour of one or more species. As a result the effect 
of varying physiological conditions has not been studied, except incidentally. 
With the exception of a few of the hardiest forms, such as Pteris 
