The union taking place underground, while the bulk of both partners in the symbiosis rises into the air, renders the association a little difficult to see, but there is no doubt that the plants in question do afford each other assistance, forming, as it were, a kind of partnership. The most pronounced case of parasitism, that of Cuscuta, the dodder, which infests particularly clover fields, appears to differ only in degree from those mentioned, for the plant, bare of leaves as it is yet contair.s a little chlorophyll.
These enenlies are as a rule so conspicuous that we do not look on their depredations as diseases, though the gradual deterioration of hay under the exhausting effects of root-parasites like Rhinanthus, and the onslaught of Cuscuta when unduly abundant, should teach us how unimportant to the definition the question of size may be.
The dodder is a genus (Cuscuta) of leafless parasites with slender thread-like twining stems. The flowers stand singly in the leaf-axils or form few or many flowered cymose inflorescences; the flowers are sometimes crowded into small heads.
Cuscuta has a thread-like, spirally twisted embryo with no trace of cotyledons.
Cuscuta contains nearly too species in the warmer and temperate regions; two are native in Britain.
Convolvulus arvensis (bindweed) is a pest in fields and gardens on account of its wide-spreading underground stem, and many of the dodders (Cuscuta) cause damage to crops.
- Cuscuta europaea, Dodder.
- Cuscuta glomerata.
In Cuscuta (Dodder) (q.v.), however, we have an exception.
Such aberrant forms are to be regarded in the same light as Cuscuta and Orobanchaceae, for example, among Phanerogams. As these non-green plants do not cease to be classed with other Phanerogams, so must the forms in question be retained among algae.