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Other examples frequently occur both in individual leaves and along the
course of axes. Examples of such occurrence are contained in several of the
tables ; for example, in cases 91 and 92 of Table I, we have examples of
discontinuous order in the action of the pinnules of individual pinnæ, in cases
6 and 8 of Table VII we have action in secondarily affected leaves commenc-
ing in the pinnæ before any action had manifested itself in the primary pulvinus,
in cases 5 and 6 we have discontinuous action along the axis as the result
of the application of heat, and in Table XI we have numerous examples of a
similar phenomenon as the result of section of the extremities of axes. Especi-
ally in these latter cases we see that in certain cases we have to deal, not
merely with discontinuity in order of occurrence of action, but with absolute
discontinuity of action due to certain intermediate parts failing to act at all
when others both distal and proximal to them have acted in greater or less
degree. A closely related phenomenon frequently presents itself to observation
under certain circumstances in cases of heating axes or section of the extrem-
ities of axes where spreading action extends along the axes for a considerable
distance but is confined to the primary pulvini alone. In such cases the
limitation of action to the primary pulvini is readily explicable as the result of
the relatively excessive strain there already alluded to, and the same holds
good in regard to the numerous instances in which section of distal pinnules
is followed by action confined to the injured pinna and the primary pulvinus.
The cases of pure discontinuity in actior in the pinnules of a pinna or the leaves
on an axis are more difficult to explain, but, in the light of the facts which
we have just been considering, it appears certainly more justifiable to ascribe
the phenomena to differences of structural power, or in amount of strain in the
different parts, than to totally inexplicable and undemonstrated differences in
conductivity, irritability, or contractility in the protoplasm.
In dealing with the subject of discontinuity in action there is one thing
more which calls for special note, and that is the frequency with which the
secondary pulvini fail to act even where complete action has taken place
throughout the whole of the rest of a leaf. This is specially the case in cases
of action following section of the pinnules, for in such cases, under the con-
ditions in which the experiments dealt with here were conducted, action in the
secondary pulvini occurs so infrequently as to be almost exceptional. In
cases of action due to the application of heat to pinnules, action in the
secondary pulvini is generally well pronounced, but even here exceptions
frequently present themselves. This is not the only difference in the phenomena
attending heating as compared with section of the pinnules. A reference
to the tables will show that premature action in the primary pulvinus is
relatively very rare in experiments on heating, and that there is a distinct
tendency to wider propagation of action in them than in cases of section. The
phenomena of recovery, too, are different according to the exciting cause. In
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