The
frequent, sudden and complete disappearance of one or both of the
rudimentary leaflets is a rather singular fact; but it is a much more
surprising one that the leaves which are first developed on seedling plants
are not provided with them. Thus, on one seedling the seventh leaf above
the cotyledons was the first which bore any lateral leaflets, and then only
a single one. On another seedling, the eleventh leaf first bore a leaflet;
of the nine succeeding leaves five bore a single lateral leaflet, and four
bore none at all; at last a leaf, the twenty-first above the cotyledons,
was provided with two rudimentary lateral leaflets. From a widespread
analogy in the animal kingdom, it might have been expected that these
rudimentary leaflets would have been better developed and more regularly
present on very young than on older plants. But bearing in mind, firstly,
that long-lost characters sometimes reappear late in life, and secondly,
that the species of Desmodium are generally trifoliate, but that some are
unifoliate, the suspicion arises that D. gyrans is descended from a
unifoliate species, and that this was descended from a trifoliate one; for
in this case both the absence of the little lateral leaflets on very young
seedlings, and their sub-
[page 364]
sequent appearance, may be attributed to reversion to more or less distant
progenitors.
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