Malformations And Injuries
( Originally Published 1915 )
Malformations arising as sports or by malnutrition. Healing of wounds. Injuries due to lightning, frost, etc. Harmful effect of smoke and fog.
Abnormal development of various parts of plants whether of vegetative or of reproductive organs may be regarded as Malformations. Many of these arise as sports or mutations, and may be transmitted to the progeny of the plant. In the case of the vegetative organs such occurrences as pitcher-shaped leaves have been noted in many instances. Leaves of this type are of no special advantage to the they may indeed be a disadvantage, causing water plant, collect on the leaf surface and thus increasing the liability to fungal attacks. Among ferns an excessive development of the margin of the frond leads to the development of so-called crested varieties, which being pleasing to some tastes, have been perpetuated by artificial selection. We have no knowledge of the causes of such excessive development of the leaves, nor are we certain of the origin of similar unnatural developments in stems, such as appear in the case of " fasciation " (from the Latin fascis, a bundle). This term is applied to the abnormal development of the stem into a broad flattened structure, often caused by the fusion of several stems, or by the union of lateral shoots with the main stem. Superabundance of food supply may in some cases be responsible for this abnormal growth. Sometimes these forms occur spontaneously without apparent cause, and have been cultivated by horticulturists, as in certain species of Echinocactus and in the flowering shoots of the Coxcomb (Celosia cristata).
Almost all cultivated plants are known to produce occasionally variegated leaves, and in some cases leaves which are entirely devoid of green colour. In some instances these have been traced to lack of nutrition. We know of course that if seedlings are raised in the dark or potato, tubers are allowed to sprout in the absence of light,. the leaves will be of a sickly yellow nature, as light is essential for the production of the green colouring matter characteristic of foliage. Similarly, absence of iron salts in the soil will prevent the formation of chlorophyll. It has also been noticed that young shoots arising below a graft in the case of hollies are often white in colour. This must be regarded as due to an interference in the down-ward conduction of material caused by the artificial union of the tissues at the graft. Some cases are also known of wild plants with variegated foliage, which under cultivation in richer soil have developed normal green leaves. It has also been possible by growing plants in the green-house at higher temperatures to change the variegated into green leaves, and to prevent the formation of further variegated leaves in the case of some plants.
Some botanists have considered that a special substance or " virus" is developed in certain parts of a leaf which has prevented the formation of the green colouring matter ; but as we do not know anything of the nature of such a substance it is perhaps simpler to consider that the chlorophyll granules in certain portions of the leaf have remained in a more youthful condition, in which they produce the pale yellow colour which always precedes the green colouring matter. Apparently this stoppage in the development occurs more commonly in plants under cultivation than in their natural condition. Variegated leaves are obviously less efficient as nutritive organs, and variegated plants are therefore often less resistant than normal forms. Their foliage is more easily affected by heat or frost and the leaves are less long lived.
There are some cases in which variegation is considered to be a disease produced by bacterial action. The so-called "mosaic disease " of Tobacco plants and of Tomatoes is supposed to be due to this cause, and certain experiments indicate that it is highly infectious.
Most gardeners will have come across examples of abnormal formations in flowers. Often the sepals may become leafy as in Jack-in-the-Green Primroses, sometimes the petals have a leaf-like appearance as in the Green Rose.
To the Rower-lover these transformations are more curious than beautiful, but to the botanist they are of considerable interest as cases of reversion. For we must assume that the various floral organs are all modifications of leaves which have become adapted to the special function of reproduction, and in such foliaceous developments we may see a retrogression to a more primitive type of leaf. More rarely, but still occasionally, we may find stamens or pistil becoming transformed into vegetative leaves ; the latter is often the case in the flowers of the Double Cherry. In both cases sterility of the flower is caused. In other sports the outermost leaves of the flower, the calyx, may become coloured and delicate in texture like the corolla. This is the condition in the hose-in-hose variety of the Polyanthus.
More frequently it is the inner leaves of the flower, the stamens, that become petaloid. The " doubling " of flowers is sometimes found to take place in wild plants when transplanted into garden soil. In all probability the tendency to doubling is independent of the cultivation of the plant, but the rich nutrition which the plant receives accentuates the effect. Certainly double varieties to be kept in perfect condition require an abundant supply of food, and are liable to degenerate when grown in poor soil.
Degenerate is perhaps not a good term to use, as the plant is in a more perfect condition from the reproductive point of view when the stamens and pistil are normally developed and fertile than when they are transformed into showy petals. In that case the flower is generally completely sterile, though it may sometimes retain a few serviceable stamens or a receptive seed vessel.
From the doubling of flowers such as the Rose, we must distinguish the doubling of the so-called " Flowers " of Daisies, Chrysanthemums, and other members of the Composites. In this Family the apparent flower is really a head of small flowerlets or florets closely crowded together. These are often of two kinds, showy ray florets and small central disk-florets usually of a different colour.
When these "flowers" are said to double, it is by transformation of the tubular flowers ,of the centre into conspicuous strap-shaped flowers, like those of the margin of the inflorescence. In that case they need not necessarily become sterile, for in, many Composites like the Dandelion, all the florets are normally of the ray-floret type. Another curious freak met with in some of the Compositae when they receive abundant nutrition is the development of some of the ray-florets into small heads of flowers on stalks of their own, so that the large central head is surrounded by a number of lesser ones. This is the condition in the Hen-and-Chicken Daisies, and similar modifications are found in other plants.
A not uncommon abnormality in gardens is the development of a terminal flower of large dimensions and regular shaped at the top of the flowering spike of the Foxglove; in the Snap Dragon, too, flowers showing radial symmetry instead of the usual two-lipped condition may be formed. A plant may indeed bear nothing but "peloric" flowers as these are called.
Monstrosities may also occur in the development of fruits. Double-fruited oranges with two whorls of carpels (pegs) one inside the other are occasionally found, while in one variety known as Buddha's fingers, the various carpels are only united below and taper off above into finely divided pod-like segments.
While most of these malformations are due to little understood internal causes, many cases of excessive development of plant structures are produced as the response to irritation. Thus, numerous kinds of galls and tumours may be developed by animal and vegetable parasites insects and fungi, which injure the tissues and cause them to swell or grow more vigorously. These will be dealt with in later chapters.
Plants like animals have the power of protecting themselves against external injury and of heeding any wounds that are caused thereby. The most usual method is by the formation of layers of cork, which will protect any exposed part from the atmosphere, thereby preventing the excessive loss of moisture through the wounded surface and also reducing the chances of invasion of the tissues by disease or decay producing organisms. This development of a resistant layer of cork is due to the active growth of the living cells of the plant in the exposed region. This active growth requiring energy, which plants gain by their respiratory process, we find that the healing of wounds is marked in plants as in animals by a local rise in temperature. "The wound tissue which is developed at first is soft and termed callus; it is that excrescence of cells which is produced as we have seen around the base of a shoot in making cuttings. Within this callus layer impervious cork is then formed and this is usually sufficient for the healing of herbaceous plants or soft tissues. In trees and shrubs, however, if the stein or branches are deeply cut, the wound will subsequently be covered up by woody layers as well. The formation of these liquified tissues commences at the margin of the wound and they gradually cover over the entire wounded area. It is thus that branches broken at their base become covered up, in nature, and are found as knots buried in the wood. In cultivation, of course, much larger branches are often amputated than usually break off in nature, and though their stumps will ultimately be covered up, it takes a considerable time in the case of a thick branch, and before the wound is closed up there is plenty of time for the wood exposed by the cut to commence to decay. To prevent this from taking place, it is advisable to coat the surface of the stump immediately after the removal of the branch, with tar or some antiseptic substance, which will prevent the entrance of bacteria or, of other harmful fungi.
Injury by lightning, if It is deep and considerable, may be beyond the power of the plant to repair; but where only the outer layers have been damaged, in which case a single longitudinal fissure is generally found running vertically down the side of the stem, callus and cork usually heal up the gap completely.
Frost is in all probability one of the most frequent of external factors causing injury to plants. Though we cannot as a rule prevent the havoc wrought by frost, it is not without interest to note what is the effect of freezing upon plant structures. It is particularly the young growing parts of plants which are nipped by frost, while the mature leaves are often undamaged.. On the other hand, somewhat fleshy plants like the Nasturtium, do not resist frost very well. The cell sap containing a good many organic acids and often sugar in solution, does not freeze readily. The denser the sap is, the less danger there is of its being frozen. As a protection against the effect of frost, we find that with a lowering of temperature some of the water from the cell-sap passes into the spaces which exist between the cells in mature tissues. Thus the sap becomes more concentrated and less liable to injury, while the water which may become frozen in the intercellular spaces does no damage if the latter are of fair size. If they are small, then the expansion of the water in freezing may tear the tissues asunder and thus injure them. In the growing parts of the plant there are tiny intercellular spaces, so that little' water can be passed out of the cells, and consequently the sap itself may be frozen, in which case the protoplasm, the living substance of the cells, is killed and the tissues become blackened. A very sudden frost is always more harmful than a long spell of frost preceded by a gradual lowering of the atmospheric temperature, as plants are able to prepare themselves by a condensation of their cell sap. It is the same with recovery from frost. Plants in a frozen condition may recover readily if they are slowly thawed. When exposed to bright sunshine on a frosty morning they may he permanently injured as often happens with Wall flowers. It is therefore well to shade frozen plants front the direct rays of the sun, so that. they are thawed less rapidly. We must remember that in bright sunshine the pores of the leaves open and the plants are rapidly transpiring, and lf this takes place while the ground is still frozen and the roots arc unable to absorb water, the consequences may be serious.
Those of us who have labour( d in town gardens are only too familiar with disappointment due to general sickliness of some of our plants and to that harmful effect of of a smoky atmosphere. Worse in winter than in summer it is nevertheless pronounced even in the latter, and records taken show that there arc quite a number of gloomy days in July and August, and that air pollution is quite appreciable during these months. From actual measurements made during the month of July, it has been a calculated that if we draw a circle of a mile radius around the Town Hall of Manchester, 195 tons of impurities would be collected during the month from this area, and of this 100 tons would consist of soot or other insoluble matter; and Manchester is by no means the worst town in this respect. It is gratifying to know that efforts are being made in many of our industrial centres to understand, and let us hope, also to cope with this problem of air pollution, and all such movements deserve the hearty support and co-operation of gardeners and flower-lovers.
From what has been said in earlier chapters, it will be clear that bright sunshine is the most important factor in the nutrition of plants, as it is, only in the presence of light that the green chlorophyll of the leaves is able to form starch, the food so 'essential for the further growth of the plant. A murky atmosphere therefore lowers considerably this prime nutritive activity of the leaves. If in addition to that the leaf, as is always the case with town evergreens, has become coated with a more or less opaque layer of sooty matter, the normal power of leaf nutrition is still further reduced. What wonder then that our town and suburban gardens suffer many serious losses.
It is only hardy plants which can survive these adverse conditions. Conifers are usually regarded as fairly resistant plants, and so they are to drought or cold, but smoke is most harmful to them as their breathing pores or stomata are situated at the base of a depression on the surface of the leaf, and this pit becomes partially filled with soot and the pore is thus blocked. In ordinary leaves these pores are usually on the protected under surface and are therefore not endangered by the smoke, but in the case of the needle-shaped leaves of the conifers they are freely exposed to smoke and fog. As a consequence we find that conifers do not thrive in the neighbourhood of our industrial towns. A blackening of leaves is, however, not the only noticeable effect of air pollution on vegetation.
We observe, particularly on evergreens, that many of the leaves have brown spots or dead margins, and in both cases this is traceable to the acidity of the atmosphere, which kills the tissues, particularly those near the stomata or breathing pores. It is particularly in winter fogs that the air becomes very acid, owing to the sulphur contained in the coal, and our own sense organs enable us easily to detect the presence of the sulphuric acid. Even in summer the acidity of the atmosphere causes the discolouration and the early fall of the leaves of trees in our parks and town gardens.
It is obvious that the rain washes this acid into the soil, so that not only the leaves but the roots also suffer from its effect. Rain water collected in or near our towns is always slightly acid, and a great difference can be observed in the growth of seedlings, some of which are watered with ordinary rain water and others with rain water in which the acid has been neutralised. A considerable number of striking experiments on this point have been made by Mr. Ruston, of the Leeds University. The lesson they teach us is that the soil of gardens or allotments near our large towns should be well treated with lime, so as to neutralise as far as possible the acidity which is caused by the rain washing the smoky atmosphere.
A consideration of the fact that this smokiness and acidity of the air is greater in winter than summer suggests that a dressing with lime in the spring before growth recommences would be especially beneficial. Experience has also taught us that many biennials are better wintered under glass than when left in the open, even when they readily withstand the frost. A glance at the condition of the lights of a cold frame will show us what a coating of soot we have prevented from collecting on the leaves.
A really penetrating fog, however, will get into frames and greenhouses, and may often do considerable damage to the delicate blooms of Orchids and to many other flowers, besides causing in some plants the leaves to become discoloured or to fall.