The branches of the tree do not, as a rule, produce flowers as long as they are able to climb; but as soon as they reach the summit of the tree or wall to which they cling, or reach a situation where there is a sufficient abundance of light and air, they change their character in a remarkable way. They now become bushy, cease to produce suckers, and give rise to undivided leaves that turn in all directions for light and air. At the tip of each twig is formed a cluster of yellowish-green flowers, arranged in a short raceme or in an umbel. These flowers have an inconspicuous calyx which forms a border round the middle of the ovary, and five short petals. There are also five stamens, and united styles. The fruit is a smooth, black berry, containing from two to five seeds.
[XXIII]
PARASITIC PLANTS
A number of plants extract more or less of the organic material they require from other plants, and thus save themselves the labour of building up this material themselves. These are termed parasites; but we must be careful to distinguish between them and certain other plants which, though apparently parasitic, are not really so. One plant may climb on another, perhaps even producing "rootlets" by which it clings to its living support, and yet it may not be a parasite in the proper sense of the term, for it may not absorb the slightest amount of nutritious matter except from the soil and the air. It is not at all uncommon for the Honeysuckle to twine its stems round the trunk and branches of a young tree, with the result that the tree becomes stunted, and assumes a starved appearance, especially in its lower parts; and yet the Honeysuckle is not a parasite. It has withdrawn nothing from the tree which supports it, but has coiled itself so tightly round it as to interfere with the circulation of its sap. The lower part of the tree is especially affected because the strangulating coils of the climber prevent the downward flow of the sap contained in the vessels of the bast or inner bark, and this is the sap which holds the constructive materials that have been built up in the leaves, under the influence of light.
Many of the parasitic plants are of microscopic dimensions, and others are larger species belonging to the Fungi or Mushroom group. Some, however, are flowering plants, and these only fall within the scope of our work.
We shall first deal with parasites which have no green leaves or chlorophyll, and are therefore entirely dependent on outside sources for their supply of organic material, starting with the interesting Dodders (Cuscuta), which coil themselves round herbs, shrubs, or even trees, and produce sucking organs on their stems that come in contact with their host.
These are all smooth plants, with globular clusters of yellowish-pink flowers, the calyx being of the same colour as the corolla. The former is deeply divided into four or five parts, and the corolla has four or five spreading lobes with as many scales inside its broad tube. The ovary has two distinct styles, and the fruit is a globular capsule. The following summary of distinguishing features will enable the reader to identify the British species of the genus:—
1. The Greater Dodder (Cuscuta europæa).—A plant of a greenish yellow colour, generally more or less tinged with red, with flowers in sessile, globular clusters nearly half an inch in diameter, each individual flower being about a tenth of an inch. This species is not abundant. It may be met with in hop-fields, and is also parasitic on nettles, various shrubs, and trees, including the elder and the ash.
2. The Flax Dodder (C. Epilinum).—Very much like C. europæa, but the flowers are fewer in number, larger, and more fleshy. The calyx is nearly as long as the corolla, with sharply-pointed segments; and the corolla tube is always globular. This species is not indigenous, but is sometimes met with in flax-fields.