CHAPTER XIX. CLOVER DODDER. Cuscuta Trifolii, Bab. CLOVER DODDER is such a familiar, and, as some observers say, increasing pest in our fields, that any detailed description of its superficial appearance and habits is unnecessary. Clover dodder is probably perfectly familiar to every observant person who has walked through clover fields. All the dodders, and there are some forty or fifty species-belong to one genus of parasitic plants termed Cuscuta, a name said to be derived from Chassuth, the Arabic name for dodder plants. The Kadytas of Theophrastus and the Cassytas of Pliny are believed to be dodder. These names, as well as the Arabic name, signify to hold fast, to stitch, and to oppress. The popular name, dodder, is an English form of the Dutch and German names Dodern, Touteren, and Todern. Dodd signifies a bunch, and dot, a tangled thread. Trifolii indicates that the plant now under description invades clovers. The dodders are commonly termed scald-weeds, hell-weeds, or strangleweeds, and in some districts devil's-guts; the popular names indicate the strong hatred rustics bear towards these weeds. Cuscutas are closely allied to the Convolvuli of our gardens, and some botanists place them in the same natural order with the convolvulus; others relegate them to a natural order by themselves, named Cuscuteœ. Part of a plant of Cuscuta Trifolii, Bab., growing parasitically on clover, is shown at Fig. 47, twice the size of nature. Most of the characters of the parasite that are visible to the unaided eye are given in this sketch. Dodders are plants with yellowish or reddish leafless threadlike stems, the leaves being represented by a few small transparent scales. The small, usually pinkish, bell-shaped, sometimes sweet-scented flowers, as in clover dodder, are collected in little closely-packed heads or FIG. 47.-CLOVER DODDER. Cuscuta Trifolii, Bab., growing on Clover. clusters, as shown at A, B, Fig. 47. Each floral perianth is usually divided into four or five segments. The flowers are commonly succeeded (but not in the case of clover dodder in this country) by four small seeds. The threadlike stems are furnished with numerous very small suckers, as at C, D, with which the parasite attaches itself to its host. Dodders grow in all hot and temperate regions, and they fix upon a great variety of plants in addition to field clovers and lucerne. The best known of these host plants are flax, thyme, broom, heath, furze, cabbages, nettles, hops, cranberry, rock-rose, centaury, scabious grass, bracken, yellow-rattle, eyebright, bastard toad-flax, yellow bedstraw, camomile, sow thistles, tomatoes, and even the vine. It is extremely common to find seeds of dodder amongst impure clover seeds imported from the Continent. In some instances it is easy to sift dodder away from the larger-seeded varieties of clover, and we know that most seed-merchants are very particular in this respect. In other instances the dodder and clover seeds approach each other so nearly in size that sifting one from the other is impossible, and the No. 17 sifter becomes quite useless. B X.5 Seeds of Red Clover, Yellow Trefoil, Dutch Clover, and Clover Dodder. Enlarged 5 diameters. For the purpose of comparison, two seeds of perennial red clover are illustrated at Fig. 48, A; at B the seeds of yellow trefoil; at C of white Dutch clover; and at D the seeds of clover dodder, all to the same scale, viz. five times the size of nature. All clovers in cultivation vary in size between the limits shown by A and C in Fig. 48. The examples for measurement were kindly forwarded by Messrs. Sharpe and Co. of Sleaford, Messrs. Sutton and Sons of Reading, and Messrs. Edward Webb and Sons of Wordsley, Stourbridge. The dodder seeds were sifted out of impure foreign importations. In a pound of average clover there are 250,000 seeds. A dodder seed is brown, dull, and minutely granular outside when seen with the aid of a strong lens, whereas clover seed, although often brown, is smooth and shining, with a minute scar or protuberance at one point of the circumference at E, Fig. 48. At this point the radicle, the first or elementary root of the plant, emerges at the time of germination. This scar is almost invisible in dodder seeds. When seen in section a clover seed materially differs from a dodder seed. The interior of the clover seed shows the presence of cotyledons or seedleaves, as at Fig. 49, A, enlarged ten diameters; and the first rudimentary rootlet or radicle at B. The illustration at C shows a clover seed after it has been planted for three days or a week in moist sand. The testa, or outer integument of the seed, has burst, and the first Seeds of Perennial Red Clover and Clover Dodder, seen in section, and germinating. Enlarged 10 diameters. rootlet of the infant clover plant at D is descending to the sand. Clover-dodder seed is engraved to the same scale at E and F; at E the seed is shown in section; there are no seed leaves, but the young plant within consists of a simple thread, spirally coiled round a little central mass of fleshy albumen. At the period of germination the thread emerges with a dilated end as at G, and the granular coat of the seed frequently breaks up as shown. If a germinating seed in this condition is transferred to a slip of glass and held before a strong light, the spiral embryo will be seen through the cracked testa as here illustrated. Sometimes the cotyledons or seed leaves are represented by one or two very minute scales in germinating dodder. Clover and dodder seeds, farther advanced in growth, are shown at Fig. 50, twice the size of nature. The clover seeds on the left are sending their first roots deep into the earth, whereas thedodder seeds on the right are sending their threads into the air in search of a host on which to live parasitically. The ground line is indicated at A. Clover and dodder seeds generally germinate after the lapse of a week or less; but certain seeds of clover dodder from the X.2. FIG. 50. Clover and Dodder seeds germinating. Twice the size of nature. sample we experimented with did not germinate till two months had passed. The variation in the time of rest in and on the ground before germination is an obvious advantage to the dodder, for, if the first dodder seedlings find no hosts ready for them, other seedling dodders, during a period of two months, still have a good chance. Clover often remains without producing other than the two first seed leaves for one or two weeks, and during that period the young dodders often attach themselves to the clover seedlings, as illustrated at Fig. 51, enlarged ten diameters. |