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This latter fact, as well as the
presence of the rudiments, both lead to the conclusion that M. albida is
descended from a form the leaves of which bore more than two pairs of
leaflets. The second leaf above the cotyledons resembles in all respects
the leaves on fully developed plants.
When the leaves go to sleep, each leaflet twists half round, so as to
present its edge to the zenith, and comes into close contact with its
fellow. The pinnae also approach each other closely, so that the four
terminal leaflets come together. The large basal leaflets (with the little
rudimentary ones in contact with them) move inwards and forwards, so as to
embrace the outside of the united terminal leaflets, and thus all eight
leaflets
[page 381]
(the rudimentary ones included) form together a single vertical packet. The
two pinnae at the same time that they approach each other sink downwards,
and thus instead of extending horizontally in the same line with the main
petiole, as during the day, they depend at night at about 45o, or even at a
greater angle, beneath the horizon.


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