ndred species, found chiefly
in north temperate regions, but also, like other north temperate genera,
on the mountains in the tropics. The plants are small annual or
perennial herbs with trifoliate (rarely 5- or 7-foliate) leaves, with
stipules adnate to the leaf-stalk, and heads or dense spikes of small
red, purple, white, or rarely yellow flowers; the small, few-seeded pods
are enclosed in the calyx. Eighteen species are native in Britain, and
several are extensively cultivated as fodder-plants. _T. pratense_, red
or purple clover, is the most widely cultivated.

This plant, either sown alone or in mixture with rye-grass, has for a
long time formed the staple crop for soiling; and so long as it grew
freely, its power of shooting up again after repeated mowings, the bulk
of crop thus obtained, its palatableness to stock and feeding qualities,
the great range of soils and climate in which it grows, and its fitness
either for pasturage or soiling, well entitled it to this preference.
Except on certain rich calcareous clay soils, it has now, however,
become an exceedingly precarious crop. The seed, when genuine, which
unfortunately is very often not the case, germinates as freely as ever,
and no greater difficulty than heretofore is experienced in having a
full plant during autumn and the greater part of winter; but over most
part of the country, the farmer, after having his hopes raised by seeing
a thick cover of vigorous-looking clover plants over his field, finds to
his dismay, by March or April, that they have either entirely
disappeared, or are found only in capricious patches here and there over
the field. No satisfactory explanation of this "clover-sickness" has yet
been given, nor any certain remedy, of a kind to be applied to the soil,
discovered. One important fact is, however, now well established, viz.
that when the cropping of the land is so managed that clover does not
recur at shorter intervals than eight years, it grows with much of its
pristine vigour. The knowledge of this fact now determines many farmers
in varying their rotation so as to secure this important end. At one
time there was a somewhat prevalent belief that the introduction of
beans into the rotation had a specific influence of a beneficial kind on
the clover when it came next to be sown; but the true explanation seems
to be that the beans operate favourably only by the incidental
circumstance of almost necessarily lengthening the interval be