The singular fact of
the cotyledons of this plant not sleeping at first is therefore due to the
pulvinus not being developed at an early age.]
We learn from these two cases of Lotus and Oxalis, that the development of
a pulvinus follows from the growth of the cells over a small defined space
of the petiole being almost arrested at an early age. With Lotus Jacobaeus
the cells at first increase a little in length; in Oxalis corniculata they
decrease a little, owing to self-division. A mass of such small cells
forming a pulvinus, might therefore be either acquired or lost without any
special difficulty, by different species in the same natural genus: and we
know that
[page 123]
with seedlings of Trifolium, Lotus, and Oxalis some of the species have a
well-developed pulvinus, and others have none, or one in a rudimentary
condition. As the movements caused by the alternate turgescence of the
cells in the two halves of a pulvinus, must be largely determined by the
extensibility and subsequent contraction of their walls, we can perhaps
understand why a large number of small cells will be more efficient than a
small number of large cells occupying the same space.
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