This tribe of plants is almost exclusively North American, but
the hardy MARSH SKULLCAP or HOODED WILLOW-HERB (S. galericulata),
at least, roams over Europe, and Asia also, with the help of
runners, as well as seeds that, sinking into the soft earth of
swamps and the borders of brooks, find growth easy. The blue
flowers which grow singly in the axils of the upper leaves are
quite as long as those of the larger and the showy skullcaps; the
oblong, lance-shaped leaves, which are mostly seated on the
branching stem, opposite each other, have low teeth. Why do
leaves vary as they do, especially in closely allied species?
"The causes which have led to the different forms of leaves have
been, so far as I know," says Sir John Lubbock, "explained in
very few cases: those of the shapes and structure of seeds are
tolerably obvious in some species, but in the majority they are
still entirely unexplained; and, even as regards the blossoms
themselves, in spite of the numerous and conscientious labors of
so many eminent naturalists, there is as yet no single species
thoroughly known to us.
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