CHAPTER XXIII.
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.
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.
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].
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:—
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.
Corn.