Part 7 (1/2)
It is customary to cla.s.sify the causes of disease in plants into two princ.i.p.al groups--(1) those due to the action of the non-living environment--soil, atmosphere, physical conditions such as temperature, light, etc.; and (2) those brought about by the activities of living organisms--plants and animals of various species. Before pa.s.sing to further subdivisions under these two heads, however, it is necessary to observe that no disease can be efficiently caused by an organism alone, since its powers for injury as a parasite, or otherwise, are affected by its non-living environment as well as by the host-plant. For instance, the spores of a parasitic fungus which would infect and rapidly destroy a potato plant in moist warm weather may be showered on to such a plant with impunity if the air remains dry and cool--or on to a cabbage under any circ.u.mstances as far as we know.
Again, probably no one factor of the non-living environment ever suffices to induce a disease, possibly because no such thing as only one change at a time ever occurs. For instance, it is difficult to say, when a soil becomes sodden with water, whether the excess of water and dissolved matters, the want of air displaced by the water, the lowering of the temperature, or the acc.u.mulation of foul products, etc., is the princ.i.p.al factor in causing the damage which results, and we have to determine by the balance of experimental evidence which is the dominant factor in all such cases.
The study of aetiology of disease is in fact only a particular case of that of aetiology in general. Plants at high alt.i.tudes in the Alps acquire very different characteristics from the same species in the plains. Is this due to the low temperature, the rarer atmosphere, the more intense illumination, the changes in moisture, etc., etc.? The question is more difficult than it appears at first sight, and we must remember that, complex as are the factors working on the host, they are equally complex in their actions on a parasite attacking the host, whence the resulting disease becomes indeed a tangled problem of natural selection.
Finally it remains to say a few words about a numerous cla.s.s of cases where no external cause of disease can be discovered. It was formerly the custom to group such cases of ”Internal Causes” by themselves, but apart from the fact that many of these mysterious diseases have subsequently been shown to be due to the action of external agencies, the whole question of internal causes resolves itself into one of relations between the plant and its surroundings, and it becomes evident that no inherited or internal disease can be regarded as explained until we know the external causes which have so modified the structure and working of the living cells as to make them abnormal in their reactions to other parts of the plant. ”Internal causes” of disease, therefore, is a phrase expressing our ignorance, but somewhat more emphatically than usual. If this is clearly understood there seems no reason against its employment for the time being in the artificial scheme of cla.s.sification we require. With regard to external causes due to the non-living environment, excess or deficiency of materials in the soil, water, or atmosphere plays an important part, and--since we may neglect purely aquatic plants--it is customary to speak of diseases due to unsuitable soils or to injurious atmospheric influences. For instance, any deficiency in the supplies of the necessary mineral salts (compounds of calcium, magnesium, pota.s.sium with sulphuric, nitric and phosphoric acids, etc.) leads to pathological changes, as also does the lack of the necessary traces of iron. But it is equally true that the presence of such ingredients in excess or in combinations unsuited to the plants also leads to disaster, as also does the presence of minerals or other compounds which poison the root-hairs--_e.g._ products of decomposition, soluble salts of copper and other poisons. That these matters are bound up with the whole question of manuring and of proper soil-a.n.a.lyses will be evident.
Another essential factor is the nature and quant.i.ty of organic materials in the soil, whether leaf-mould and decomposing vegetable remains, stable manures, or other animal matters, all of which affect different species very differently, and produce very different results in different soils. It is necessary to apprehend in this connection what has been stated above: that soil is not a mere dead structureless medium, and that the root-hairs of ordinary plants cannot deal with large quant.i.ties of putrefying organic matter: that a good soil must abound in useful bacteria and fungi to render such substances available--and in very various ways--and that it must be open and aerated, of proper temperature and suitably supplied with water, and so forth, or disaster will result. Here, again, then we are brought into close contact with all that is known of fermentation, nitrification, and the various biological changes going on in soil, and the application of such knowledge to the practice of manuring and tillage in all its forms.
In view of the above remarks, the danger of ”over-feeding,” in this sense, has a real meaning for horticulturists, though it must not be forgotten that no substance is really a food until it is a.s.similable into the protoplasm: manures, etc., are food-materials, not food. The futility of mere chemical a.n.a.lyses to prove what a plant requires is now well known, and it is only on the basis of long and carefully conducted experiments that we can ever discover what a particular plant in a particular soil, situation, and climate requires for healthy development. Again, the quant.i.ty of water in soil may be too great or too small for given species, and this either on the average for the year, or during critical periods only; and it is obviously important whether the excess or deficiency is due to improper supplies of water, the depth or shallowness of the soil, its retentive powers, or the nature of the sub-soil and so on, again bringing the whole matter into connection with our understanding of the physical const.i.tution and structure of soils, and the nature of soil-drainage.
For instance, a common way of killing ferns is to keep the roots and soil wet and the air and fronds dry, whereas the natural habitats provide for wet and shaded fronds and well-drained soil.
It may be noted here that in most cases where gardeners speak of plants being killed under the ”drip” of trees--_e.g._ Beech, the injury is due, not to the effects of water but to the shade: the loss of light is so great that the shaded plants die of inanition because their leaves are not able to provide sufficient carbohydrates.
Closely bound up with this is the question of the gases in soils. Apart from the disastrous effects of poisons--_e.g._ coal gas escaping from pipes under pavements in towns, etc., diseased conditions often result from deficiency of oxygen at the root-hairs, due to imperfect aeration of soils, brought about by stagnant water, excess of animal matter, and so forth.
Unsuitable const.i.tution of the atmosphere is also a fruitful source of disease, though its effects are commoner in closed stoves and greenhouses than in the open. Nevertheless the continual exhalation of sulphurous fumes, chlorine, and other poisonous gases in the neighbourhood of manufacturing centres or of large smoky towns, volcanoes, etc., play their part in injuring plants; and excessive moisture in the form of mist, rain, etc., is also important. All these matters bring us at once into the region of physiology, and only an intelligent appreciation of what is known about the action of the atmosphere on the soil and the plant will save the peasantry of a country from a hopeless mysticism but little removed from that of the Middle Ages, when blights and other evils were vaguely referred to the river-mists, thunder clouds, and easterly winds.
If we summarise the above as the material factors of the environment, we may cla.s.sify another set of external non-living causes of disease as the non-material factors. Such are princ.i.p.ally the following:
The s.p.a.ce at the disposal of plants greatly affects their welfare. The crowding of roots in the soil and of foliage in the air, resulting in the loss of light to the leaves, involves deficiency of all the materials referred to above--minerals, organic materials, gases, and water--and no better ill.u.s.tration of the intense struggle for existence among these apparently pa.s.sive and motionless beings, plants, can be given than an over-crowded seedbed or plantation. If left to themselves such over-stocked areas exhibit to the keen eye of the trained observer all the phases of starvation, weakness, wounding, rot, and, so to speak, brutal dominance of the stronger over the weaker which it is the object of cultivation to prevent. Here, then, we are brought face to face with the true significance of thinning and weeding out, pruning, and similar processes.
Unsuitable temperature is one of the commonest of all sources of disease, for every plant is adapted to certain ranges of temperature, and best adapted to a given optimum somewhere between the maximum and minimum temperature for each function. Consequently any serious departure from the mean may bring about physiological disturbances of the nature of disease, and this in very various ways, as exemplified by the results of frost, sun-scorching, drought, hail-storms, forest fires, and so forth.
As a predisposing factor to disease abnormal temperature effects play a great part. Many wound-fungi gain their entrance through frost-cracks, bruises due to hailstones, or into tissues chilled below the normal.
No less remarkable are the diseases primarily due to insufficient or improper exposure to light, which affects the chlorophyll-apparatus and the process of carbon-a.s.similation and through these the whole well-being of the plant. Every plant is adapted to certain ranges of light intensity, and most cultivators know how impossible it is to grow shade plants in fully exposed situations, and how easily plants which live in open sunny situations are ”drawn” and killed by shade. It is equally important to have the right kind of light, as disastrous experiences with greenhouses glazed with gla.s.s which cut off certain rays of light have taught. Here, again, it is important to notice that the optimum intensity or quality of light may differ for different functions and organs of the plant, as is shown by many adaptations on the part of species growing in natural situations--_e.g._ bud protection, orientation of leaves, etc.--and it may be taken as a rule that etiolated plants are peculiarly susceptible to other diseases.
As regards other factors of the inorganic environment, disasters which come within the scope of our subject may be brought about by many agencies, the mechanical effects of snow and hail, wind, avalanches, etc., the effects of lightning, and so forth, being a few of them.
NOTES TO CHAPTER XI.
For other detailed cla.s.sifications of the causes of disease the reader is referred to the works of Sorauer and of Frank referred to in the last chapter. Also Kirchner, _Pflanzen Krankheiten_, Stuttgart, 1890.
Of more historical importance are the older cla.s.sifications of Berkeley, _Gardeners' Chronicle_, 1854, and Re, _Gardeners'
Chronicle_, 1849-50. These latter are interesting as showing the very different views held by the earlier workers, and comparison of these with the modern views helps to mark the progress of physiology during the half century which has intervened.
CHAPTER XII.
CAUSES OF DISEASE. THE LIVING ENVIRONMENT.
_Causes due to animals--Vertebrata--Wounds, etc.--Invertebrata --Insects, etc.--Plants as causes of disease--Phanerogams, weeds, etc.--Cryptogams, fungi--Epidemics, etc._
Pa.s.sing now to those causes of disease which are connected with the living environment, we may obviously divide them into two groups of agents, animals and plants.
Among animals, the various vertebrata, including man, are especially responsible for the larger kinds of wounds and wholesale destructive processes due to breakage, stripping of leaves and bark, cutting and biting, and so forth. Cattle, rabbits, rats and mice, squirrels and birds of various kinds stand out prominently as enemies to trees and other plants, to which they do immense injury in various ways by their horns, teeth, claws, and beaks; and the damage which an ignorant gardener or forester can do with his ill-guided footsteps, axe, spade, and knife can only be appreciated by one who knows the habits of plants.