Most of the plants referred to belong to the order Scrophulariaceæ, and among them we may mention the Eyebright (Euphrasia), the Yellow Rattle (Rhinanthus), the Cow-wheat (Melampyrum), and the Lousewort (Pedicularis). They generally appear in large numbers close together, often in such abundance as to determine the general colour of the ground on which they grow, and yet they do not apparently cause much damage to the grass and other plants which they rob.
These green parasites are described in various chapters, according to their habitats and their flowering seasons; so we shall do no more here than to briefly refer to their parasitic habits.
The Eyebright (p. [274]) grows on heaths and downs, where it derives organic food from the roots of the neighbouring grasses. The Lousewort, too (p. [118]), which grows in marshes and moist meadows, is parasitic principally on the roots of grasses, apparently without affecting the latter. The last-named species is a perennial, the roots of which have to find hosts that are capable of supporting it year by year. If the host of the present year should happen to die in the autumn, the suckers that were attached to its roots soon die, and the parasite has to seek a new source of supply. This it does by extending its roots until it reaches a new host, and then producing new suckers. Thus we are able to understand the origin of the long roots so often seen on the Lousewort, and also the reason why these roots never grow downwards into the soil, but always horizontally, just beneath the surface. Further, since the roots extend themselves in search of food at times when the supply is temporarily diminished or stopped, it is clear that some reserve is necessary for the elongation referred to. Such a reserve exists in the older, thick portion of the perennial root, near the base of the stem.
In the case of the Cow-wheat (p. [146]) no suckers are produced until the lateral branches of the root of the seedling reach a moderate length; but in order to increase the chances of finding a suitable host these branches are developed in large numbers, and extend themselves in all directions. The suckers produced on them cling very firmly to the root-fibres of the host, which they almost completely embrace.
The suckers of the Yellow Rattle (p. [118]) are globular, often nearly one-eighth of an inch in diameter, and partly surround the root-fibres of the plants to which they are attached.
[XXIV]
CARNIVOROUS PLANTS
Quite a number of plants, belonging to different orders, are provided with the means of capturing small animals, and of digesting their prey and absorbing the nutrient matter thus obtained into their own systems. In this way they are enabled to obtain nitrogenous material which, in the ordinary way, is absorbed in the form of mineral solutions, from the soil, by the agency of the roots. The greater number of these carnivorous plants are to be found in tropical lands; but a few are British, and are of such an interesting nature that we propose to devote a short chapter to a description of their peculiar structure and habits.
The plants to which we refer are often spoken of as insectivorous species; but although in nearly all cases the animal food consists almost entirely of insects, it is not entirely derived from this one group of animal life, and therefore the term carnivorous is rather more appropriate.