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Rh impulses in animals. Again, the degree of differentiation is very slight anatomically, but delicate protoplasmic threads have been shown to extend through all cell-walls, connecting together all the protoplasts of a plant. These may well serve as conductors of nervous impulses. The nervous mechanism thus formed is very rudimentary, but in an organism the conditions of whose life render locomotion impossible great elaboration would seem superfluous. There is, however, very great delicacy of perception or appreciation on the part of the sense organ, stimuli being responded to which are quite incapable of impressing themselves upon the most highly differentiated animal.

The power of response is seen most easily in the case of young growing organs, and the parts which show the motor mechanism are mainly the young growing cells. We do not find their behaviour like that of the motor mechanism of an animal. The active contraction of muscular tissue has no counterpart in the plant. The peculiarity of the protoplasm in almost every cell is that it is especially active in the regulation of its permeability by water. Under different conditions it can retain it more strongly or allow it to escape more freely. This regulation of turgor is as characteristic of vegetable protoplasm as contraction is of muscle. The response to the stimulus takes the form of increasing the permeability of particular cells of the growing structures, and so modifying the degree of the turgidity that is the precursor of growth in them. The extent of the area affected and of the variation in the turgor depends upon many circumstances, but we have no doubt that in the process of modifying its own permeability by some molecular change we have the counterpart of muscular contractibility.

The response made by the adult parts of plants, to which reference has been made, is brought about by a mechanism similar in nature though rather differently applied. If the leaf of Mimosa or Desmodium be examined, it will be seen that at the base of each leaflet and each leaf, just at the junction with the respective axes, is a swelling known as a pulvinus. This has a relatively large development of succulent parenchyma on its upper and lower sides. In the erect position of the leaf the lower side has its cells extremely turgid, and the pulvinus thus forms a cushion, holding up the petiole. On stimulation these cells part with their water, the lower side of the organ becomes flaccid and the weight of the leaf causes it to fall. The small pulvini of the leaflets, by similar changes of the distribution of turgidity, take up their respective positions after receiving the stimulus. In some cases the two sides of the pulvini vary their turgidity in turns, in others only the lower side becomes modified.

Similar turgescence changes, taking place with similar rapidity in the midrib of the leaf of Dionaea, explain the closing of the lobes upon their hinge. More slowly, but yet in the same way, we may note the change in turgidity of certain cells of the Drosera tentacles, as they close over the imprisoned insect.

Organic Rhythm.—It is a remarkable fact that during the process of growth we meet with rhythmic variation of such turgidity. The existence of rhythm of this kind has been observed and studied with some completeness. It is the immediate cause of the phenomena of circumnutation, each cell of the circumnutating organ showing a rhythmic enlargement and decrease of its dimensions, due to the admission of more and less water into its interior. The restraint of the protoplasm changes gradually and rhythmically. The sequence of the phases of the rhythm of the various cells are co-ordinated to produce the movement. Nor is it only in growing organs that the rhythm can be observed, for many plants exhibit it during a much longer period than that of growth. It is easy to realize how such a rhythm can be modified by the reception of stimuli, and can consequently serve as the basis for the movement of the stimulated organ. This rhythmic affection of vegetable protoplasm can be observed in very many of its functions. What have been described as “periodicities,” such as the daily variations of root-pressure, afford familiar instances of it. It reminds us of a similar property of animal protoplasm which finds its expression in the rhythmic beat of the heart and other phenomena.

.—Sachs, Lectures on the Physiology of Plants, translated by Marshall Ward; Vines, Lectures on the Physiology of Plants; Pfeffer, The Physiology of Plants, trans. by Ewart; Reynolds Green, Introduction to Vegetable Physiology; The Soluble Ferments and Fermentation; Detmer, Practical Plant Physiology, trans. by Moor; Darwin and Acton, Practical Physiology of Plants; Davenport, C.B., Experimental Morphology, vols. i. and ii.; Verworn, General Physiology, trans. by Lee; Butschli, Investigation on Microscopic Forms and on Protoplasm, trans. by Minchin. (Author:Joseph Reynolds Green)

“Phytopathology” or plant pathology (Gr., plant), comprises our knowledge of the symptoms, course, causes and remedies of the maladies which threaten the life of plants, or which result in abnormalities of structure that are regarded, whether directly injurious or not to life, as unsightly or undesirable. In its systematized form, as a branch of botanical study, it is of recent date, and, as now understood, the subject first received special attention about 1850, when the nature of parasitism began to be intelligible; but many disjointed references to diseased conditions of plants had appeared long before this. The existence of blights and mildews of cereals had been observed and recorded in very ancient times, as witness the Bible, where half a dozen references to such scourges occur in the Old Testament alone. The epidemic nature of wheat-rust was known to Aristotle about 350, and the Greeks and Romans knew these epidemics well, their philosophers having shrewd speculations as to causes, while the people held characteristic superstitions regarding them, which found vent in the dedication of special festivals and deities to the pests. Pliny knew that flies emerge from galls. The few records during the middle ages are borne out by what is known of famines and pestilence. Shakespeare's reference in King Lear (Act III., sc. iv.) may be quoted as evincing acquaintance with mildew in the 17th century, as also the interesting Rouen law of Loverdo (1660). Malpighi in 1679 gave excellent figures and accounts of leaf-rolling and gall insects, and Grew in 1682 equally good descriptions of a leaf-mining caterpillar. During the 18th century more academic treatment of the subject began to replace the scattered notes. Hales (1727-1733) discussed the rotting of wounds, cankers, &c., but much had to be done with the microscope before any real progress was possible, and it is easily intelligible that until the theory of nutrition of the higher plants had been founded by the work of Ingenhouss, Priestley and De Saussure, the way was not even prepared for accurate knowledge of cryptogamic parasites and the diseases they induce. It was not till De Bary (1866) made known the true nature of parasitic Fungi, based on his researches between 1853-1863, that the vast domain of epidemic diseases of plants was opened up to fruitful investigation, and such modern treatises as those of Frank (1880 and 1895), Sorauer (1886), Kirchner (1890), were gradually made possible.

Plant pathology embraces several branches of study, and may be conveniently divided as follows:—

1. The observation and accurate description of symptoms (Diagnosis).

2. The study of causes or agencies inducing disease (Aetiology).

3. The practise of preventive and remedial measures (Therapeutics).

In plants, however, the symptoms of disease are apt to exhibit themselves in a very general manner. Our perceptions differentiate but imperfectly symptoms which are due to very different causes and reactions, probably because the organization of the plant is so much less highly specialized than that of higher animals. The yellowing and subsequent casting of leaves, for instance, is a very general symptom of disease in plants, and may be induced by drought, extremes of temperature, insufficient or excessive illumination, excess of water at the roots, the action of parasitic Fungi, insects, worms, &c., or of poisonous gases, and so forth; and extreme caution is necessary in dealing with amateur descriptions of such symptoms, especially when the untrained eye has taken no cognisance of, or has only vaguely observed, the numerous collateral circumstances of the case.

The causes of disease may be provisionally classified somewhat as follows, but it may be remarked at the outset that no one of