ON THE PARASITES OF CLOVER.
Of the truly parasitic plants affecting the clover crop, we have two
genera—namely, _Cuscuta_ or Dodder, and _Orobanche_ or Broomrape. Both
of these, some few years since, were comparatively rare as farm pests;
but as they are probably more abundant on Continental than on our home
farms, they are greatly increasing from the constant influx of foreign
seeds.
CUSCUTA—_Dodder_.
Of the genus _Cuscuta_ we have two species of agricultural
importance,—_Cuscuta epilinum_, the Flax Dodder, and _C. trifolii_, the
Clover Dodder. In both, the plant itself consists of a mass of pink and
yellowish tendrils, upon which are placed here and there compact bunches
of flowers varying alike in colour. The whole plant, in both species,
being entirely parasitic—that is, it lives wholly on the juices of its
foster-parent,—it has no leaves of its own; still, however, the Dodder
plant is in the first instance produced from seed, each flower being
succeeded by a capsule containing two small wrinkled seeds, which, not
being larger or lighter in the _C. epilinum_ than a linseed, or in the
still smaller seed of the clover, in the case of the _C. trifolii_, the
seed of flax or clover crops affected with dodder will never be entirely
free from it: as an evidence of its large increase, we remember once
seeing a crop of flax grown from Riga seed diminished about
one-twentieth by the dodder; but on the seed so produced being sown in
another field of the same farm, the crop of flax was well-nigh
destroyed.
Our friend Professor Voelcker had some seed of the flax dodder sent to
him for analysis, the reason being that, as his correspondent had
separated a great number of bushels of this weed pest from a single crop
of flax, he was desirous of ascertaining whether it possessed any
feeding properties or the reverse; and on this head it is satisfactory
to learn that it is considered useless, though innocuous.
It was part of this sample with which we experimented on the mode of
growth of dodder, which, although being the dodder of the flax or
linseed plant, yet its natural history will doubtless be that of the
clover dodder;[8] we shall, therefore, describe the progress of our
experiments, and their results.
[8] We are desirous of instituting special experiments on the growth of
clover dodder, but have failed to procure ripe seed, the reason being
that the seed does not ripen after the clover has been cut down for its
first crop.
Having prepared some finely-sifted soil in a garden saucer, we sowed a
small quantity of flax seed with which had been purposely mixed a few of
the seeds of flax dodder; this, on being placed in a hot-house, showed
the progress indicated in the diagram.
[Illustration: A. Seed-covering beneath which _radicle_ or young root is
pushing.
B. Leafless stem or tendril growing upwards, bearing seed-covering on
its apex.
C. Young thread-like plant freed from seed-covering, on the look-out for
a foster-parent.
D. Not finding a foster-parent, droops and dies.]
Our next diagram shows the progress of dodder-growth when the parasite
has germinated sufficiently near to a young flax plant to be attracted
to it. In such case, instead of dying, it seems all at once to be
animated by new vigour. The highly elastic thread, which now represents
the whole dodder plant, goes through the following stages:—
[Illustration: A. The dodder, having just clasped a flax plant, has made
two coils round the stem of the latter.
B. Meanwhile the flax in growing lifts the dodder out of the soil.
C. While the flax is getting still taller, the dodder sends out
rootlets, which pierce and fix themselves into the flax. During this the
dodder sends out buds upwards, which, elongating until new flax plants
are met with, explains not only how the dodder commences a growth quite
independent of the soil, but, by spreading, from plant to plant, thus
increases to an indefinite extent.]
In this way, then, the dodder of flax, commencing from seeds at
different points, spreads in more or less extended patches, which, if
such centre be few, will be distinct; if many, the pest may occupy the
greater part of the crop by spreading, and so becoming confluent.
Such is the method of growth of flax dodder, and we have no doubt but
that the dodder of the clover progresses in like manner; at all events,
we see the latter occupying more or less isolated patches in the
affected crop; and in this case, as in the former, the crop-plant is not
only starved, from having “its verdure sucked out,” but it is borne down
to the ground and ruined.
As regards its destruction, we should be careful to look at our crops in
their early growth, as, if the sickly-looking, wire-like tendril be
observed then, it is easily removed by hand; if, however, it has made
head, the best way would be to make a trench of a foot wide around the
plague-spots, which will prevent its spreading, as the plant must have
contiguous clovers to twist round if it is to extend; and then burn some
straw on the dodder plot, and it will be wasted to death. Probably,
however, the easiest plan is to depasture the crop,—certainly not to
seed it down—in which case it will be impossible for any dodder seeds to
ripen.
But here, as in other cases, the evil will be prevented by sowing pure
seed, whether of flax or of clover; and as the dodder is a small, brown,
roundish little seed, so different from that of either crop, there is no
difficulty in recognizing it where present.
OROBANCHE—_Broomrape_.
The Broomrape is now becoming a very pernicious clover weed, especially
in lighter soils. We have seen it on clover near Stonehenge so thick as
to have positively spoiled the crop; and we should expect from its
bitter, disagreeable flavour, that if cattle did not universally refuse
to eat it, it might prove mischievous to them.
The species which attacks clover is the _Orobanche minor_—Lesser
Broomrape,—which is at once distinguished in a clover field by its
upright brownish spike of dead, dry-looking, lipped flowers; the stem
without true leaves, but clothed with small brown leaf-like processes
(_bracts_ of the botanist), which, with the stem, are clothed with
hairs.
This plant, which is much larger and very different from the clover, is
parasitic on the principal division of the clover root; so that if the
soil be carefully removed from the broomrape, it will be found to swell
at the base, into which the clover root may be detected to be fastened,
and a very odd appearance indeed has the small-stemmed clover united to
so comparatively large a parasite.
The seeds of the broomrape are so small as scarcely to be detected in a
sample of clover seed; indeed, several may be fastened to a seed as
dust, so that whatever care may be used in the selection of seed will
hardly prevent this pest. Any great injury to the clover crop may be
speedily stopped by hand-picking the broomrape; for, although it will
sometimes branch up again, it will be much lessened, and the few
secondary shoots will usually be very weak.
Clovers are attacked by _Epiphytes_—that is, minute fungoid plants
growing upon the leaves; but the natural history of these is too obscure
for a general treatise, nor are they of sufficient interest to the
practical farmer.[9]
[9] To such as may be interested in the study of the “rusts” of Clover,
and some other plants, we would earnestly recommend a perusal of some
most interesting papers on the subject, by M. C. Cooke, Esq.,
beautifully illustrated by Messrs. West & Sowerby, which will be found
in the _Popular Science Review_—a serial which should have a place in
the house of every country gentleman.
[Illustration: Corn.]
HOW TO GROW GOOD CORN.