[CHAPTER III.]
History of the Doctrine of the Movements of Plants (Phytodynamics).
It will scarcely be doubted at the present day, that the mechanical laws of growth, of geotropic and heliotropic curvatures, of the various kinds of periodic movements, of the twining of tendrils and climbing plants, and of movements dependent on irritation, may be referred to a common principle, and that in all these movements besides the elasticity of the cell-walls the still unknown qualities of the protoplasm play the most important part, and that consequently the ‘streamings’ of the protoplasm, the movements of swarmspores and similar occurrences must be ranked with these phytodynamical phenomena. From this point of view phytodynamics would appear to be one of the most important foundations of vegetable physiology. The recognition of this fact is however of very recent date, and to imagine that such a conception of the movements of plants was present to the minds of the early physiologists would be to attribute to the past ideas to which it was entirely a stranger. These movements were scarcely noticed even as curiosities in former ages, and it was not till the end of the 17th century that some attention began to be paid to them; and very slow progress was made at a later time in disentangling the relations which come under consideration and which are some of them very complicated, in determining the dependence of the phenomena on external influences, and explaining to some extent their mechanical conditions.
Single movements of parts of plants are noticed in a cursory manner by some early writers. Varro was the first who mentioned the heliotropic movements of the stalks of many flowers, which he says were for that reason called heliotropic flowers; in the following century Pliny says that the leaves of clover close when bad weather is approaching; Albertus Magnus in the 13th century, Valerius Cordus and Garcias del Huerto in the 16th, thought the daily periodical movements of the pinnate leaves of some Leguminosae worth recording; Cesalpino noticed the movements of tendrils and climbing plants, and was surprised to see that the latter to some extent seek for their supports. These were every-day phenomena, but the striking sensitiveness of the leaves of Mimosa pudica introduced from America could not fail to attract attention, and so we find an essay on the causes of it in Robert Hooke’s ‘Micrographia’ of 1667. The irritability of the stamens of Centaurea had been already mentioned by Borelli in 1653.
1. We meet with the first speculations on the subject at the end of the 17th century. Ray in his ‘Historia Plantarum’ (1693) commences his general considerations on the nature of the plant with a succinct account of phytodynamical phenomena, and introduces the whole by a sentence of Jung: ‘Planta est corpus vivens non sentiens,’ etc. Though Ray, like Cesalpino, seems to believe in the Aristotelian soul of plants, yet he does on the whole endeavour to explain the movements which he describes by physical and mechanical laws; he thinks that the irritability of Mimosa in particular is not due to sensation, but to known physical causes; the movement of the leaf when it is touched is caused by a contraction, which again is due to a withering or relaxation of its parts. He endeavours to apply the knowledge of his time to the explanation of the mechanical process: leaves, he says, remain tense only because the loss by evaporation is kept constantly supplied by the water that flows to them from the stem; if then in consequence of a touch the sap-passages of the leaves are pressed together, the supply of water is not sufficient to prevent their becoming relaxed. Ray mixes up together the movements from irritability and the daily periodical movements, as was done till recent times; the latter, he says, occur not only in the leaves of Leguminosae, but in almost all similar pinnate leaves, and with these periodical movements of leaves he places also the periodical opening and closing of the flowers of Calendula, Cichorium, Convolvulus, and others. That these last movements are due to changes of temperature appeared to him to be proved by an experiment of Jacob Cornutus on flowers of Anemone, which, when cut off and placed in a well-closed box in a warm place, opened at an unusual time if the flower stalk only was dipped in warm water. This fact, afterwards forgotten and discovered again a few years ago, of the dependence of the movements of flowers on changes of temperature, was applied by Ray to explain the periodical movements of leaves, which, to use his own expression, fold themselves together as the cold of night draws on, and open again with the day, and as he thought that these movements are of the same kind as the movements of irritability in Mimoseae, he tries to explain how cooling has the same effect as a touch. It was natural in the existing state of science to assume that changes of temperature were the first causes of various movements, for a thrust was at that time almost the only recognised cause of motion. Hence Ray explained the movements of growing stems which are now called heliotropic by a difference of temperature on the opposite sides. A certain Dr. Sharroc had observed the stem of a plant on which he was experimenting grow towards that part of a window, where the air found free entrance through an opening; from this circumstance, and from the rapid elongation of the stems of plants growing under cover, which he ascribed to the higher temperature, Ray concluded that cold air hinders the growth of the side of a stem on which it falls, and that this side must become concave. Thus Ray used the etiolation of plants grown under cover to explain their heliotropic curvatures, as De Candolle did one hundred and forty years later, only with this difference, that he described the rapidity with which forced plants shoot up to the higher temperature, De Candolle to want of light. On the other hand Ray knew perfectly well that the green colour of leaves is not produced by the access of air but by the light, for, as he says, plants become green under glass, and not under an opaque cover; and if they become less green under glass than in the open air, this is because the glass absorbs certain rays of light and reflects others. Ray however, like almost all later observers till quite recent times, did not keep the elongation and bleaching of etiolated plants sufficiently distinct; his account of this phenomenon is spoilt by the presence of much that is obscure.
It has been justly observed by other writers on botanical subjects that no notice is usually taken of one of the most remarkable of the phenomena of which we are here speaking, because, being a matter of every-day occurrence, it is simply accepted as something obviously in accordance with the nature of things; this is the fact, that the main stems of plants grow vertically upwards and their main roots downwards. To the French academician Dodart, whom we have already encountered in the history of the theory of nutrition, is due the great merit of being the first to find this apparently simple phenomenon really very remarkable; he convinced himself by experiments on germinating plants, that these vertical positions are caused by curvatures, and endeavoured to discover the physical reason why the main roots if placed in an abnormal position escape from it by curving in the downward direction, and the main stems in the upward direction, till they both reach the vertical line. It was a matter of minor importance that his mechanical explanation, which supposed that the fibres of the roots contract on the moister side and those of the stem on the same side lengthen, was quite unsatisfactory; it was much more important that these remarkable phenomena were made the subject of scientific enquiry, and we find that various observers soon after directed their attention to them, and exercised their acuteness in attempts at explaining them; to these attempts we shall return in a future page.
A still more universal phenomenon than the vertical growth of stems and roots is the growth of plants generally, and it required as much or even more of the spirit of enquiry to propose the question, whether this growth can be explained by mechanical laws, and what that explanation is. Mariotte touched on this question in 1679, but only incidentally, and supposed that the stretching of the pith, which meant at that time the whole of the parenchymatous tissue, was the cause of the growth of the parts of plants. This idea might have had its origin in the Aristotelian notion that the pith is the seat of the vegetable soul, but Mariotte endeavoured to give physical reasons for it. Hales in his ‘Statical Essays’ of 1727 went much more minutely into the question of the growth of plants. Following the train of thought in his doctrine of the nutrition of plants, he introduces his observations on their growth with the remark, that plants consist of sulphur, volatile salts, earth, water, and air, the first four of which attract one another, and therefore form the solid, inert part of the substance of plants; the air behaves in a similar manner as long as it is kept by the other substances in a solid condition; but as soon as it is set at liberty it is capable of expansion. On this power of expansion in the air, by which the juices of plants are quickened and strengthened, he builds his mechanical theory of growth, according to which the plastic parts of the plant assume a state of tension, and as the air enters into combination with other substances and so becomes fixed, warmth and movement are excited, and these make the particles of sap assume by degrees a form and shape. These principles supplied his starting-point. To get a clearer idea of the way in which the growth of the parts of plants proceeds, he made equi-distant punctures in young stalks and leaves, and found that the intervals between them increased by growth more in the younger intervening parts than in the older. In the course of these observations he is particularly struck by the great longitudinal extension which accompanies growth, because, as he says, the vessels still continue hollow, as a glass tube when drawn out to its utmost extent retains its canal. He finds Borelli’s idea confirmed, that the young shoot grows by the extension in length of the moisture in the spongy pith; and he endeavours to explain the fact that the growing shoot does not extend equally in the transverse direction, and so become spherically rounded off like an apple, from the nature of the structure of the cell-tissue. That the air enclosed in the tissue and the sap with it presses into the shoot with sufficient force to produce so great an extension, he thinks is proved by his experiments, which show him the great force with which the water rises in the bleeding vine, and forces itself into swelling peas; it is known, he says, that water acts with great force when it is heated in a vessel, for water can be driven into the air by heat; the sap in plants is composed of water, air, and other active ingredients, and makes its way with great force into the tubes and cells, when it is heated by the sun.
2. The course of the 18th century gradually increased the number of the phytodynamical phenomena, to which physiologists paid more or less attention, and repeated attempts were made to explain them on mechanical principles. These attempts were for the most part unsatisfactory, because movements distinct in kind from one another were mixed up together, their dependence on external influences was not distinctly perceived, and the knowledge of the anatomical structure of the parts which exhibited the movements was, owing to the decline of phytotomy, extremely imperfect. Moisture and warmth played the chief part in these explanations, but their mode of operation was expressed in the most general terms; the mechanical processes in plants were described much in the way in which a person with very indefinite ideas as to the nature of steam and the construction of the inside of a steam-engine might speak of its movements. The majority of writers, in accordance with the tendencies of the age, professed their desire to refer the phenomena of life in plants not to an unknown principle called the soul, but to mechanical and physical causes; but they did not apply their minds to the examination of these phenomena with that strenuous effort, which in this subject especially could alone lead to a complete and satisfactory explanation of them.
Linnaeus studied the periodical movements of flowers in 1751 and those of leaves in 1755, but a mechanical explanation of them was not to be expected from him; he contented himself with pointing out the external conditions of these phenomena in many species, with classifying them, and giving the periodical movements a new name by calling the positions assumed by night the sleep of plants; nor did he use the word at all in a metaphorical sense, for he saw in this sleep of plants a phenomenon entirely analogous to sleep in animals. That the sleep-movements were not capricious but due to external influences was with him a necessary consequence from the nature and idea of the plant, which was that of a living and growing being, only without sensation. But it should be mentioned that he stated correctly that the movements connected with the sleep of plants are not caused by changes of temperature, or not by these only, but by change of light, since they take place in the uniform temperature of a conservatory.
Linnaeus’ account of these kinds of movement was only formal, it is true, but still it was well-arranged and clear; the treatment of the same and other movements by his contemporary Bonnet was quite the reverse. It is scarcely possible to imagine anything more shapeless, such an utter confusion of things entirely different from one another, as is to be found in Bonnet’s experiments and reflections on the various movements of leaves and stems in his work on the function of leaves, published in 1754; geotropic and heliotropic curvatures, nutations and periodic movements, are all run one into another; a person who understands something of the subject may find here and there single things in his experiments that may be turned to account, but he was himself unable to make any use of them. He set out with a preconception which prevented him from the first from understanding what his experiments showed him; it was his object to prove from a multitude of instances, that stalks and leaves so curve, twist and turn in all cases, that the under sides of the leaves are directed towards the ground, in order that they may be able to suck up the dew, which according to his theory is the chief nutriment of plants and rises from the ground. It is no great merit in him, that amid all this confusion a correct observation here and there forced itself upon him, as for instance that organs, chiefly such as are young and ductile, if they are put out of their natural position, endeavour to recover it by bending and twisting. On the other hand his conclusions with regard to the mechanical causes of these movements are utterly inane; the least skill in judging of the results of his experiments must have led him to very different ideas; warmth and moisture, he says, appear to be the natural causes of movement, but warmth is more effective than moisture, and the warmth of the sun more effective than that of the air. This explanation is unsuitable to just those cases which he chiefly studied, the geotropic and heliotropic curvatures. In one point only he arrived ultimately at a right judgment, namely that the great lengthening of the stem, the small size attained by the leaves and the want of colour in plants grown under cover, are caused by partial or entire absence of light; Ray however had shown this before as regards the colour.