The Encyclopedia Americana (1920)/Dodder

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DODDER, a genus of plants, Cuscuta, of the family Custcutaceæ. The characteristics of the group are filiform twining stems, parasitic on other plants, to which they attach themselves by suckers. They have lost all trace of leaves, even the cotyledons of the embryo being no longer distinguishable, while chlorophyll is almost completely absent. In one American species a slight trace of coloring matter has been noticed. The seed germinates very late in spring, and as the seedling rises from the ground a yellow or pink stem soon begins to show the sweeping movements of circumnutation of a climbing plant. If no plant known as the “host” is in the neighborhood for it to take up its quarters on, it falls to the ground, but retains its vitality for some weeks, by which time a victim may probably have germinated. As soon as it touches a living plant it twines firmly round it, and a series of small wart-like adventitious roots (haustoria) are developed, from the centre of each of which a bundle of suctorial cells force their way through the epidermis and cellular envelope into the bast and press against the woody tissue of the host. The portion of the dodder stem below this attachment now dies off and there is then no longer any connection with the ground. The growing point again circumnutates until it finds a new base of attachment upon the same or a different stem of the host, there to repeat the formation of suckers. In this way a tangled skein of threads is formed, over which, late in the season, the yellow or white flowers develop in dense clusters and the black seeds are shaken out of the capsule by the wind or gathered with the crop. This parasite ins often very injurious, fields of flax, clover and lucerne sometimes showing well-marked patches completely desolated by the pest. These have to be mowed down and burned before new seed has set; while pains must be taken to procure seed free from those of the parasite. Preventive measures are to make careful examination of the seed (see Seed Testing), rejecting any that contains dodder seed and any produced upon land known to be infested by dodder. When observed growing among a crop, frequent hoeing and burning are often satisfactory. Pasturing with sheep confined to the infested patches is also practised, the animals being kept for several weeks upon the land and given extra food if necessary. The most satisfactory treatment, however, is clean cultivation or the growing of a crop upon which the dodder cannot grow. There are about 100 species of dodder of wide geographic distribution, of which at least 25 are found in the western and southern parts of North America. The temperate species are all annual, but some of the tropical species are perennial. A common American name is tangle-weed. It is a remarkable circumstance that Cassytha, a totally unrelated Oriental genus of Lauraceæ, has not only assumed the same general mode of life, and the twining, leafless habit, but germinates and penetrates in a precisely similar way.