Physiology

SECTION II PHYSIOLOGY Physiology is the study of life. Its ultimate object is to explain the nature of life, but at present it has to rest satisfied with describing the phenomena of life, and studying the influence of various factors upon them. Physiology like chemistry and physics is concerned with incpiiries into the causes of what takes place. It is thus distinguished from oecology (p. 6), a study which also deals with the phenomena of life ; this seeks for the purpose, or, better expressed, the uses to the organism, of adaptations or of processes, and is thus a teleological rather than a causal study. Since, however, every aspect or dis- cipline of the science cannot be separately dealt with in a text-book, this section will treat of oecological as well as of physiological prob- lems. This is further justified by the fact that physiology and oecology may and often have had a useful mutual influence. Since animals and plants are only sharply separable in their more advanced forms, animal physiology and plant physiology must have much in common. This fact emerges the more clearly when, on the one hand, the animal physiologist concerns himself with the lower animals, and, on the other hand, the vegetable physiologist studies the processes of irritability. In some respects the behaviour of the living plant does not difter from that of non-living bodies. In spite of the large amount of Avater which it contains, the plant is as a rule solid, and has the physical properties of such a body. Weight, rigidity, elasticity, conductivity for light, heat, and electricity are properties of the organism as they are of lifeless bodies. However important these properties may be to the existence and the life of the plant, they do not constitute life itself. The phenomena of life are essentially connected with THE living protoplasm. No Other substance exhibits even similar remarkable and varied properties, which we can compare to life. It is especially characteristic of the organism that the reaction which follows an external influence is a very complicated one. The connection between the causal influence and the effect induced by it 175 176 BOTANY part i is not so apparent as it is in chemical or physical processes. This depends on the part always taken by the protoplasm, so that the reaction observed is not the direct effect of an external cause, but a very indirect result. If the free end of a flexible rod is placed horizontall}^ and exposed to the influence of gravity, it will bend doAvnwards to a definite point A as the result of its weight. part of a plant will behave similarly, and if dead, as, for instance, a withered stem, Avill remain in the position it thus assumes. If, however, a living growing stem has been used in the experiment it will exhibit an eftect of gravity Avhich is very surprising in comparison with the purely physical effect. The growing portion of the stem curves, and by its own activity becomes erect again ; it thus moves against the force of gravity. If the ex- periment is made with a tap-root, this will curve vertically downwards A much further than its own weight would cause it to do. rhizome, on the other hand, will place its growing tip horizontally when it has sunk by its own weight out of the hoi'izontal plane. In these three experiments the physical conditions are the same. The weight of the earth acts on a horizontally placed portion of a plant. The results in the three cases are as difterent as possible. The explanation of this remarkable behaviour of the plant is to be sought in the fact that while, to begin with, gravity influences it as -- it would influence an inorganic structure giving weight to the mass --this primary physical change then acts as what is called a stimulus. This liberates inner activities of the plant Avhich neither quantitatively nor qualitatively have a recognisable connection with the force of gravity. Such relations become clearer if the organism is compared with a mechanism. The connection between the light pressure of the finger on the trigger of a gun and the flight of the bullet is not a simple one. The pressure first liberates a trigger ; the energy thus obtained drives the hammer on to the percussion cap ; this explodes and causes the powder to explode ; the gases liberated by the explosion force the projectile from the barrel. It is clear that the force of the hammer bears no relation to that of the pressure of the finger of the marksman, and there is just as little connection between the amount of force generated by the expansion of the powder and that exerted by the hammer of the gun. There are energies present, those of the trigger and powder, which are set free. Such liberations of energy, especially when they follow in order and constitute a chain of processes, are of very frequent occurrence in the organism. They are knov/n as phenomena of irritability, and the factor which starts them is termed the stimulus. They are found always when the specific piienomena of life are concerned. Just as the action of a machine is only comprehensible when its construction is known, a knowledge of the external form and internal structure of the plant is a necessary })reliminary to its physiological 4 SECT. II PHYSIOLOGY 177 study. It has been seen, however, that it is not possible to understand the function from the structure to the same degree in the case of the plant as it is in that of a machine. This holds still more strongly for the more specific vital phenomena. While it is true that the phenomena of life can as yet not be thoroughly explained, this does not negative the conviction that they only differ from the processes in inorganic bodies by their much greater complexity ; in principle a physico-chemical explanation of vital phenomena can be attained. -- The most important phenomena of life are exhibited in the following ways : 1. An organism, which appears to us as an individual, does not consist of the same unchanged material, even when no further growth in size is taking place. ^Yhile its external form remains constant, progi'essive changes go on internally. New substances are taken up from without, are transformed within the plant, and are again given off from it. The organism has a metabolism. Inorganic nature offers us no process analogous to this. 2. As a rule, however, metabolism does not proceed so that the absorption and giving-off of material are equal, but more is absorl�ed than is given off. The mass of the organism is increased, it grows. Growth is also known in the cases of chemical precipitates or deposits, and of crystals. In these cases it tends to proceed in such a way that no essential change of shape takes place (crystals), or that the changes in shape are accidental and irregular (precipitates). The organism, on the other hand, assumes by changes of its form quite definite shapes, which follow in regular order. It passes through a DEVELOPMENT which leads sooner or later to the production of new organisms or daughter individuals ; reproduction takes place. Growth, development, and reproduction are processes highly charac- teristic of living beings. 3. Lastly, organisms exhibit powers of movement ; they either change their positions bodily, or they bring larger or smaller parts of their bodies into other positions. Since inorganic bodies and dead organisms may exhibit movements, it is only the kind of movement and the means by which it is brought about that are characteristic of living beings. In nature the three processes mentioned above, metabolism, development, and movement, usually go on simultaneously. Meta- bolism without movement of the substances concerned is impossible ; development is liound up with metabolic changes and with movements ; and, lastly, movements cannot occur without metabolism. Neverthe- less, we may for descriptive purposes consider the three processes separately, and thus divide Physiology into the following sections : 1. The study of metabolism or chemical physiology, which may also be termed the physiology of nutrition. N 178 BOTANY paut i 2. The study of development or the physiology of form, changes of shape, and the mechanism of development. 3. The study of movement. The full vital activity of the plant is only attained when a number of conditions, which may be divided into internal and external, are fulfilled. (^) The internal causes of life are connected with the protoplasm. Its structure and organisation not only determine that the changes which take place in the organism have a Adtal character, but that the organism shows specific differences depending on the descent of its protoplasm. These internal factors are only effective, however, by continual interaction with the external world. The environment not only provides the materials from which the body of the plant is built up, but supplies energy in the form of the vibrations of heat and light ; this energy is again used in numerous processes in the life of the organism. The infiuences of the external world also act, as has been already pointed out, as stimuli to which the protoplasm of the plant continually reacts in the manifestations of its life. These external influences can only be of use Avithin definite and strictly limited ranges of intensity. The capacity of life of vegetable protoplasm is only maintained within a relatively narrow range of temperature ; full vital activity only occurs within still narrower An limits. excess of light is injurious to life, as is too little warmth, and it only requires a minute quantity of any poisonous substance to destroy beyond recovery the specific structure of the protoplasm. The lower limit for the operation of an external factor is known as the MINIMUM, the upper limit as the maximum, and the particular grade of intensity of the factor at which any particular vital phenomenon attains its highest expression is known as the optimum. Minimum, optimum, and maximum, but especially the optimum, are by no means fixed points for the organism. They are changeable Avithin certain limits, on the one hand, Avith the duration of action of the factor ; on the other, in relation to other conditions of life. These so-called " cardinal points " are usually different for the various vital processes of the same plant. i The dependence of the vital plienomena on external factors can be graphically represented and made clearer by means of curves. If the intensity of the factor (degrees of temjjerature, intensity of light) is marked on the abscissa, and the ;l AE intensity of the reaction of the i)lant on the ordinate, curves of the form (Fig. i 175), which are known as optimum curves, are usually obtained. The summit of this curve corresponds to the optimum. Curves, in which the dependence of chemical reactions, on for instance tempera- ture, are represented, usually exhibit another shape (AB) : they are for the most part logarithmic curves with no indication of an optimum. This does not express any fundamental dilference between the reactions within the organisin and inorganic reactions. Probably optimum curves are always the resultants of two kinds of elfect of the same factor, one accelerating, the other destructive. Thus, for instance, SRCT. II PHYSIOLOGY 179 a cliemical reaction in which a particular substance S is formed may be accelerated AB by temperature as is represented in the curve ; if, however, the substance S is at the same time decomposed by the rising temperature, its actual amount may be represented by the curve CD ; the effect of the rise of temperature on the chemical reaction is expressed by the curve AE, wliich is the resultant of CD and AB.(2) The cardinal points of temperature are usually on the average much lower for the plants of cokl climates than for those of temperate or tropical regions. Thus the geographical distribution of plants is in the first instance dependent on these cardinal points. The highest cardinal points are found not in tropical plants, but in certain Bacteria which can live in fermenting substances at a temperature of 70� C. At low temperatures in the neighbourhood of the freezing-point of water many plants are killed by being frozen. This may happen to susceptible plants at a temperature above 0� C. and long before ice is formed in tlie tissues. Other plants can endure the formation of ice within these, and may be thawed from a hard, frozen condition still alive. The resistance of lower organisms to extreme cold is noteworthy. In Pictet's experi- ments Diatoms endured for a long time a temperature of - 200� C. ; various Bacteria, according to the investigations of Macfadyen and others, can support a six months' exposure to 200� and even to 250� of cold, produced by means of liquid air and liquid hydrogen (�^). The demands of plants on light also show much variety. Some require full Fig. 175.-- Explanation in Text. sunlight in order to thrive successfully, while others prefer the shade of woods or of caves or clefts in rocks (shade-plants). By increase of the intensity of light any cell can be killed ; in different cases the action of the light may be either mainly chemical or mainly thermal. Many Bacteria are killed even by bright daylight ; on this depends the important hygienic effect of light in houses and dwelling-rooms. The need of light not only changes from one species of plant to another, or from individual to individual, but the optimum effect of light may change for the same individual as it develops. Many of the cultivated plants of the tropics, e.g. Colfee and Cocoa, require shade when young, and require to be at first protected by shade-giving trees (species of Albizzia) planted for this purpose. When older they bear or even require exposure to the full tropical sun. In addition to the energy which the plant obtains from the rays of light and heat, numerous influences of substances in the plant's environment have to be considered. This is not the place to enumerate the elements necessary to plants, but the need of water, which is familiar to every one, may be mentioned. 180 BOTANY part i Sub-Section I METABOLISM (^) I. The Chemical Composition of the Plant {^) Any consideration of the metabolic changes in the ^^lant requires a knowledge of its chemical composition. This is studied by chemical -- methods. Water and Dry Substance. Some insight into the composition of the plant can l)e obtained without special means of investigation. Every one who has dried plants for a herbarium knows that the plant consists of water and dry substance. He also knows how the removal of the water influences such fundamental physical properties of the plant as its rigiditj^ and elasticity. By means of Aveighing it is easy to show how large is the proportion of water in the total weight of the plant. For this purpose it is not sufficient to expose the plant to the air, for Avhen air-dried it still retains a considerable proportion of water, which must l)e removed by drying in a desiccator or at a temperature of over 100^ C. It can thus be ascertained that the proportion of water is very considerable ; in woody parts some 50 per cent, in juicy herbs 70-80 per cent, in succulent plants and fruits 85-95 per cent, and in aquatic plants, especially Algae, 95-98 per cent, -- of the weight of the plant consists of water. Ash. While we can thus distinguish by drying between the water and the dry substance of the plant, we are able by burning to distinguish between the combustible or organic material and the incombustible substance or ash. The fact that the plant leaves an ash is evident in the burning of wood or in the smoking of a cigar ; the microscope further shows that even minute fragments of cell-wall or starch grains leave an ash on burning. Information as to the quantitative relations of the ash is afforded by analysis, which shows especially that the various organs of a plant differ in this respect ; leaves, for example, tend to contain more than stems. It has thus been found that the dry substance of the leaves of Brasdca rapa contains about 20 per cent of ash, Avhile the stems have only 10 per cent. The constituents of the ash also vary according to the nature of the soil and other external influences. On the other hand, distinct species may accumulate different quantities of mineral substances, even when exposed to the same external conditions. While the majority of the more common elements occurring in the earth are found in the ash of plants, only a few elements are present in sufficient amount to be quantitatively estimated. These SECT. II PHYSIOLOGY 181 are the non-metals CI, S, P, Si, and the metals K, Na, Ca, Mg, and Fe. Besides these tlie following may be met with in the ash of plauts : Iodine, Bromine, Fluorine, Selenium, Tellurium, Arsenic (which may be introduced into cultivated soils with superphosjihates). Antimony, Tin, Titanium, Boron, Lithium, Rubidium, Strontium, Barium, Zinc, Copper, Silver, Mercury, Lead, Aluminium, Thallium, Chromium, Manganese, Cobalt, and Nickel. -- Organic Substance. Chemical analysis is not needed to show that the plant contains carbon in a combined form. Every burning log or match shows by its charring that it contains carbon. The examination of a piece of charcoal in which the finest structure of the wood is retained, shows further how uniformly the carbon is distributed in the plant, and how largely the substance of the plant consists of this element. Accurate weighing has shown that carbon constitutes about one-half of the dry weight of the plant. The enormous masses of coal in the earth's crust are the carbonised remains of plants that lived in earlier geological periods ; lignite and peat and also coal, when pre- pared by special methods, exhibit their vegetable origin by their macroscopic and microscopic structure. On combustion of the dry plant the organic substance is changed, and passes off in the form of carbon dioxide and water, ammonia or free nitrogen. It contained the elements H, 0, N, and C chemically combined ; some of the elements mentioned as occurring in the ash -- may also occur in organic compounds. Source of the Materials. There are thus oplYJliirteen elements, found in considerable quantity in the plant. When the plant is growing their amount is continually increasing in the plant, and they must therefore be continually absorbed from without. The mode of life of a plant affords clear indications as to the source of the materials from which it is built up. Plants are known which live wholly in the soil ; others float freely in water ; others, while attached to a substratum, are eventually dependent on the air ; while, lastly, others live upon the bodies of other plants or of animals (parasites). Thus the substances which are found within the plant must have been derived from the soil, the water, the air, or from other organisms. Rarely, however, does a plant obtain all that it requires from one only of these media. The ordinary terrestrial plant sends its root down into the soil, and expands its leafy shoots in the air. Many aquatic plants have, in addition to the submerged organs, others which reach the atmosphere. Parasites also are able to absorb substances from the atmos})here. As a rule, only gases and liquids can enter the plant ; solid substances have to be brought into solution before they can pass through the firm cell-walls. When, however, cell-walls are absent, as 182 BOTANY part i in the Myxomycetes, the naked protoplasm is able to surround and thus to absorb solid particles. II. Absorption and Movement of Substances A. Absorption of Substances into the Cell('') We may best commence our consideration of the absorption of substances with the free-living cell, and investigate in what manner -- this absorbs water, solid substances, and gases. Water. All parts of a plant and all the parts of its individual cells are saturated with water. The cell-membrane has the water so freely divided between its minute particles that the water and the solid substance are not distinguishable under the highest magnification. If the water is allowed to evaporate, air-filled cavities do not appear in its place, but a contraction of the cell-wall takes place. On the other hand, the absorption of water by dry or not fully saturated cell- walls causes a swelling of the latter ; this takes place with consider; able energy, and can overcome considerable resistance. The walls of lignified cells absorb about one-third of their weight of water. The i| walls of some Algae and of the cells of some seed-coats and fruits [ consist more largely of water than of solid substance. The behaviour of the protoplasm is similar. Both wall and protoplasm are capable of swelling, and in the actively living cell are always in this condition. So long as the protoplasm is not saturated with water, it removes water from the cell-wall ; owing to this, the wall which may before have been saturated becomes poorer in water, and seeks to replace the loss by taking up water from its surroundings. In this way the loss of water by the protoplasm affects the outer world by the medium of the cell-wall, AYith the cell-sap it is different. This is a solution of various organic and inorganic substances in water. For simplicity we may assume that we are dealing with a solution of salts. If such a solution is enclosed by a cell-membrane, if we, for instance, fill a tube of cellulose with the solution and fasten the ends firmly, and then this "cell" is placed in water, the salts will diffuse into the water and water will diffuse into the cell ; this goes on until the same concentra- A tion is attained at all points both within and without the cell. partition which is equally permeable to water and salts has no effect on the movements of diffusion which take place in every free mass of fluid ; everything goes on as if the partition were not present. If this cell-membrane is rejolaced by one, which, while | being permeable to both water and salts, lets the water pass through much more rapidly than the salts, more water will at first pass into the cell than salt passes out ; thus a pressure will arise within the cell expressed, in an increase of the volume of the cell, and in SECT, ir PHYSIOLOGY 183 a stretching of the cell-Avall. Naturally this condition cannot be maintained in the end the same result must be reached as in the ; former case, viz. a uniform distribution of the water and salts through the whole space. When this is attained the increase in volume of the A cell is again lost. third case is essentially distinct : when the membrane consists of a substance which is permeable to water but impermeable to salts. If such a semi-permeable substance is employed, a condition of tension must again result, but in this case it is permanent, since diffusion of the salts outwards can never take place. In the vegetable cell itself the cell-wall is completely permeable. The layer of protoplasm applied to it, on the other hand, is more or less semi -permeable, at least so long as it is living. As a result of this there is a one-sided passage of water into the vacuole without any A corresponding passage outwards of salts. further result is the pressure of the cell-contents on the protoplasmic sac and through it on the cell -wall. The protoplasm becomes stretched under this pressure (turgescence, osmotic pres- sure) without much resistance, but the cell-wall, by virtue of its elasticity, exerts a considerable