As regards any particular weed, the cultivator should learn all he can respecting its duration, seeding capacity, method of dissemination, the depth and spread of its root-system, and any other particulars which enable him to judge when and how to attack it. It is only necessary to see the victory of such drought-resisting weeds as _Hieracium pilosella_, Plantains, _Hypochaeris_, on lawns to realise how weeds may win in the struggle for existence with the finer grasses. Many so-called weeds are, however, partially parasitic, with their roots on the roots of others--_e.g._ _Rhinanthus_, _Thesium_, etc., and much damage is done to meadow grasses and herbage by the exhaustive tax which these semi-parasites impose. This is carried still further in the case of such root-parasites as _Orobanche_, where the host-plant is burdened with the whole support of the pest, because the latter, having no chlorophyll, is entirely dependent on the former for all its food. Even ordinary climbing plants may injure others by shading them, either by scrambling over their branches--_e.g._ Bramble, or twisting their tendrils round the twigs--_e.g._ Bryony, or twining round them--_e.g._ Woodbine, _Convolvulus_, etc. The principal direct injury is in these cases owing to the loss of light suffered by the shaded foliage, but the weed-action is often increased by the competition of their roots--_e.g._ briars; and in the case of woody climbers the gradually increased pressure of the woody-coils round the thickening stems compresses the cambium and cortex of the support and induces strictures and abnormalities which may be fatal in course of time. Epiphytes, or plants which support themselves wholly on the trunks, branches, or leaves of other plants, also injure the latter more especially by shading their foliage--_e.g._ tropical Figs, Orchids, Aroids, etc.; and similar damage is done by our own Ivy, the main roots of which are in the soil, but the numerous adventitious roots of which cling to the bark. When the climber or epiphyte is also parasitic, as in the case of the Dodder, _Loranthus_, Mistletoe, etc., the direct loss of substance stolen from the host by the parasite comes in to supplement any effect of shading that the latter may bring about if it is a leafy plant. Of Cryptogams, apart from a few epiphytic ferns, and the intense weed-action of certain Equisetums, the rhizomes and roots of which are as troublesome as those of twitch and other phanerogamic weeds, it is especially the fungi which act as agents of disease, and which, as we now know, are _par excellence_ the causes of epidemics. The action of fungi may be local or general; and restricted, slow and insidious, or virulent and rapidly destructive.