It occurs also in pinnate
leaves, and complete sets of all the intermediate links may often be
found on the false or bastard-acacia (_Robinia Pseud_Acacia_).
Opposed to this increase of the number of leaflets, and still more rare
and more curious is the occurrence of "single-leaved" varieties among
trees and herbs with pinnate or ternate leaves. Only very few instances
have been described, and are cultivated in gardens. The ashes and the
bastard-acacia may be quoted among trees, and the "one-leaved"
strawberry among herbs. Here it seems that several leaflets have been
combined into one, since this one is, as a rule, much larger than the
terminal leaflet of an ordinary leaf of the same species. These
monophyllous varieties are interesting also on account of their
continuous but often incomplete reversion to the normal type.
[344] Pinnate and palmate leaves are no doubt derivative types. They
must have originated from the ordinary simple leaf. The monophylly may
therefore be considered as a reversion to a more primitive state and the
monophyllous varieties may be called atavistic.
On the other hand we have seen that these atavistic varieties may revert
to their nearest progenitors, and this leads to the curious conception
of positive and negative atavism. For if the change of compound leaves
into single ones is a retrograde or negative step, the conversion of
single or ternate leaves into pinnate and palmate ones must evidently be
considered in this case as positive atavism.
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