The fact that nyctitropic movements
occur in species distributed in many families throughout the whole vascular
series, is intelligible, if they result from the modification of the
universally present movement of circumnutation; otherwise the fact is
inexplicable.
In the seventh chapter we have given the case of a Porlieria, the leaflets
of which remained closed all day, as if asleep, when the plant was kept
dry, apparently for the sake of checking evaporation. Something of the same
kind occurs with certain Gramineae. At the close of this same chapter, a
few observations were appended on what may be called the embryology of
leaves. The leaves produced by young shoots on cut-down plants of Melilotus
Taurica slept like those of a Trifolium, whilst the leaves on the older
branches on the same plants slept in a very different manner, proper to the
genus; and from the reasons assigned we are tempted to look at this case as
one of reversion to a former nyctitropic habit. So again with Desmodium
gyrans, the absence of small lateral leaflets on very young plants, makes
us suspect that the immediate progenitor of this species did not possess
lateral leaflets, and that their appearance in an almost rudimentary
condition at a somewhat more advanced age is the result of reversion to a
trifoliate predecessor.
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