_Sagina apetala_ and _patula_, [53]
_Spergula media_ and _salina_ and many other pairs of allied species
have differentiating characters of the same value as those of the
elementary species of _Draba verna_. _Filago_, _Plantago_, _Carex_,
_Ficaria_ and a long series of other genera afford proofs of the same
close relation between smaller and larger groups of species. The
European frost-weeds or _Helianthemum_ include a group of species which
are so closely allied, that ordinary botanical descriptions are not
adequate to give any idea of their differentiating features. It is
almost impossible to determine them by means of the common analytical
keys. They have to be gathered from their various native localities and
cultivated side by side in the garden to bring out their differences.
Among the species of France, according to Jordan, _Helianthemum
polifolium_, _H. apenninum_, _H. pilosum_ and _H. pulverulentum_ are of
this character.
A species of cinquefoil, _Potentilla Tormentilla_, which is
distinguished by its quaternate flowers, occurs in Holland in two
distinct types, which have proved constant in my cultural experiments.
One of them has, broad petals, meeting together at the edges, and
constituting rounded saucer without breaks. The other has narrow petals,
which are strikingly separated from one another and show the sepals
between them.
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