The Clover Dodder, with a Separate Cluster of Flowers Representing the Natural Size.
Again, should the young plant fail to reach a favourable support, so that it is of necessity compelled to trail along the ground, the filaments which would soon produce suckers when attached to a living plant have no power to form any structures capable of extracting food material from a damp soil.
Circumstances being more favourable, however, the upper filament eventually finds a stem, and immediately begins to twine itself round it, making a few close coils in a clockwise direction. Should the support prove to be a dead stem, little wartlike swellings are produced at points where the two touch, and these serve as a means of attachment for the climbing filament, but no suckers are formed. If, however, the filament surrounds a living stem, each of the swellings gives rise to suckers that penetrate into the tissues of the latter, and withdraw the organic food necessary for the continued existence of the plant.
The Dodder now grows rapidly, giving off branches which search in all directions for additional supports, sometimes climbing from one plant to another, and producing new suckers whenever a favourable situation has been reached. The plant has now all it requires both in the way of mechanical support and nourishment, and its lower part, thus rendered useless, soon withers, breaking all connexion with the soil on which the seed originally germinated. New branches continue to form, each one producing additional suckers for the extraction of food from the host or hosts, until a tangled mass of clinging stems is the result. Then the globular clusters of little flowers appear, followed by balls of small capsules which throw off their lids when ripe, allowing the seeds to be shaken out by the wind. The Dodder plant now withers, leaving, in the autumn, its dead tangles of climbing filaments still attached to the withered herbs on which it fed, or to the branches of the tree which served as its host.
Other parasitic plants possessing no chlorophyll, and therefore incapable of building up organic compounds for themselves, derive their food from the roots of trees and shrubs.
Among these is the Toothwort (Lathræa), which is carnivorous as well as parasitic, and is described in our chapter ([XXIV]) dealing with carnivorous plants, so that we need only refer here to its habit as a parasite.
The seed of this plant germinates on the damp ground to which it falls in early summer. The young root penetrates into the soil, deriving its nourishment entirely from the food reserve that was stored up in the seed, and soon sends out lateral branches in search of the roots of a suitable host. If it fails to attain this end by the time that the reserve is exhausted, it dies; but if it succeeds in reaching the root of an Elm, Hazel, Hornbeam, Ash, Poplar, or other tree, it fastens itself to it, and develops suckers which penetrate into the substance of the root to extract its sap. The parasite now grows very rapidly, producing its underground stems, with their fleshy, overlapping scales, as described on p. [352].
The Broomrapes of the same order (Orobanchaceæ) are very similar in their parasitic habits to the Toothwort, and, like the latter, they possess no chlorophyll. The seeds germinate on the damp soil, producing a long, narrow embryo that grows downward into the ground until it reaches the root of some herb or shrub. It then gives off suckers which penetrate into the root, and, with the aid of the organic food thus obtained, forms a tuberous swelling on its surface. Flowering stems are afterwards produced, and these, rising above the soil, bear terminal spikes of lipped flowers, followed by capsules containing many seeds.
The Great Broomrape.