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.