Part 7 (2/2)
It is among the invertebrata, however, especially insects and worms, that the most striking agents of disease in plants are to be found, for, with the exception of certain rodents--and we may logically include also human invasions--vertebrate animals do not often appear in such numbers as to bring about the epidemics and scourges only too commonly caused by insect pests.
Insects injure plants in very various ways. Some, such as locusts, simply devour all before them; others, _e.g._ caterpillars, destroy the leaves and bring about all the phenomena of defoliation. Others, again, eat the buds--_e.g._ _Grapholitha_; or the roots--_e.g._ wire-worms, and so maim the plant that its foliage and a.s.similation suffer, or its roots become too scanty to supply the transpiration current. Many aphides, etc., puncture the leaves, suck out the sap, and produce deformations and arrest of leaf-surface, as well as actual loss of substance, and when numerous such insects induce all the evils of defoliation. Others, such as the leaf-miners, tunnel into the leaves, with similar results on a smaller scale.
It must be remembered that a single complete defoliation of a herbaceous annual, or even of a tuberous plant like the potato, so incapacitates the a.s.similatory machinery of the plant, that no stores can be put aside for the seeds, tubers, etc., of another year, or at most so little that only feeble plants come up.
In the case of a tree the case is different, and since most large trees in full foliage have far more a.s.similatory surface than is actually necessary for immediate needs, a considerable tax can be paid to parasites or predatory insects before the stores suffer perceptibly.
Still, it should be recognised that the injury tells in time, especially in seed years.
Many larvae of beetles, moths, etc., bore into the bark and as far as the cambium or even into the wood or pith of trees, the local damage inducing general injuries in proportion to the number of insects at work: moreover, the wounds afford points of entrance for fungi and other pests.
Galls and similar excrescences result from the hypertrophy of young living tissues pierced by the ovipositors of various insects, and irritated by the injected fluid and the presence of the eggs and larvae left behind. They may occur on the buds, leaves, stems, or roots, as shown by various species of _Cynips_ on oak, _Phylloxera_ on vines, etc., in all cases the local damage being relatively small, but the general injury to a.s.similatory, absorptive, and other functions is great in proportion to the number of points attacked.
Many grubs--larvae of flies, beetles, etc.--bore into the sheaths or internodes of gra.s.ses, or the pith of twigs, or into buds, fruits, and other organs of plants, and do harm corresponding to the kind and amount of tissues injured.
Various species of so-called eelworms--Nematodes--also cause gall-like swellings on young roots, or they invade the grains of cereals.
Finally, various slugs and snails cause much injury by devouring young leaves and buds and diminis.h.i.+ng the a.s.similatory area.
Plants as agents of disease or injury fall naturally into the two main categories of flowering plants (Phanerogams) and Cryptogams, among which the fungi are the especially important pests.
Beginning with weeds, we find a large cla.s.s of injurious agents. Weeds damage the plants we value by crowding them out in the struggle for existence, as already stated, and when the weed-action is simply due to superfluous plants of the same species, we speak of overcrowding. But it must not be overlooked that the compet.i.tion between crowded plants of the same species--where every individual is acting as a weed to the others--may be more dangerous than between plants and weeds belonging to other species and genera, because in the former case they are struggling for the same minerals and other necessary food-materials: a matter of importance in connection with the rotation of crops.
The question of allowing gra.s.s to grow at the foot of fruit trees, as in orchards, is a good case in point. Such gra.s.s may increase the damp and shade, thus favouring fungi at one season, and dry up the moisture of the soil to the injury of the fine superficial roots at another, as well as exhaust the soil, owing to the compet.i.tion of the roots for salts and other materials. On the other hand, the checking of surface roots by compet.i.tion with the gra.s.s has been claimed as advantageous. In this connection probably the whole question of the composition of the turf arises, as well as that of possible cropping for hay, and manuring.
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 gra.s.ses.
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 gra.s.ses 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 princ.i.p.al 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 compet.i.tion 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 advent.i.tious 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.
Examples of local action are furnished by _Schinzia_, which forms gall-like swellings on the roots of rushes; _Gymnosporangium_, which induces excrescences on the stems of junipers, and numerous leaf-fungi (_Puccinia_, _aecidium_, _Septoria_, etc.), which cause yellow, brown, or black spots on leaves, as well as by _Ustilago_, which attacks the anthers or the ovary of various plants, and so forth. In such cases the injury done by a few centres of infection is very slight, but prolonged action may bring into play secondary effects such as the gradual destruction of the cambium round a branch, when, of course, the effect of ringing results; or if the fungus becomes epidemic and myriads of leaf-spots are formed, the destruction of foliar tissue, gradual taxing of the a.s.similatory cells, etc., may end in rapid defoliation, and renewed attacks soon exhaust the plants and lead to sterility and death, as often occurs with Uredineae--_e.g._ the coffee leaf-disease.
It is highly probable that such fungi are particularly exacting owing to their exhausting demands for compounds of pota.s.sium, phosphoric acid, and other bodies.
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