Disease in Plants

A point to be emphasised to the utmost where agricultural students are concerned is that the essential process of feeding is the same in a green plant, a fungus, and an animal; the greatest confusion still exists with regard to this matter, owing to misconceptions as to the real meaning of the functions of the chlorophyll-corpuscles when supplied with carbon-dioxide and water and the energy of the sun's rays. The plant does not feed on carbon-dioxide, any more than it feeds on oxygen—it feeds on the organic material after it has been constructed, and the chlorophyll-function is merely one mode of obtaining supplies of such organic substance.

Notes to Chapter II

In addition to the references in the last chapter, the student should consult Sachs' Lectures, XVII.-XIX., and Pfeffer's Physiology, pp. 287-329, for the further development of this subject. An excellent résumé, with new facts and points of view, will be found in Dr. Horace Brown's "Address to the Chemical Section," British Association Reports, Dover, 1899; and "Chemistry and Physiology of Foliage Leaves" in Trans. Chem. Soc., 1893, p. 604. See also Blackman, "Experimental Researches on Vegetable Assimilation and Respiration," Phil. Trans., 1895; and Parkin, "Formation, etc., of Carbohydrates in Monocotyledons," Phil. Trans., 1899.

CHAPTER III.

THE PLANT A LIVING MACHINE

The plant a machine into which energy and material are taken—Carbon assimilation—Feeding—Accumulation and transformations in the plant. The action of light—The chlorophyll-function.

The relations of the plant to the environment can only be understood by taking into account the results of modern physiological discoveries. These teach us that the living plant is a highly complex machine, the details of its organisation and structure being much more numerous and much more closely correlated at numerous points, than the parts of any other machine known to us.

They also teach us that it is supplied with energy from without, as any other machine; and that when so supplied, and properly working, the living structure or machinery does work, also as other machines. But modern physiology goes further, in that it renders some account of the ways by which the external energy is taken into the plant, and there applied to do work, or stored up for a time in order that it may be used to do work at some future time.

The accumulation of energy thus ensured is associated with corresponding changes of material substance, and the principal means for bringing this about is recognised in the assimilation of carbon-dioxide—photo-synthesis.

In this process energy enters the chlorophyll-corpuscle in the form of the radiant energy of the sun, it is there directed in the mechanism of the protoplasm, so as to do work on the molecules of water and carbon-dioxide which have also been brought into the machinery; this it does, breaking asunder their stable structure into unstable bodies, which then re-combine in different ways to form a carbohydrate, such as starch, and this starch is temporarily stored as grains, while oxygen escapes.

Each starch-grain, therefore, is to be regarded as a packet of matter and of potential energy, as it were, capable of yielding up the latter at any future time, when put under such circumstances that it must do so. Such stores of energy-yielding substance, if I may use the much-abused phrase, form the principal food of the plant—or of an animal, if it steps in and takes them—and we now see that the process of carbon-dioxide assimilation, as it has perhaps unfortunately been called, is not the same thing as the process of feeding, for the feeding—i.e. the nutrition proper—of the plant does not begin until the food has been thus obtained.

We now see what the real position of the plant is, to its environment, whether the latter be living or dead. From our point of view, the plant serves as a centre for bringing together the substances obtainable from the soil, and those derived from the atmosphere, and so focussing and directing the radiant energy of the sun upon these substances, that they are broken up, and some of their constituents synthesised, with absorption of energy, into a body, such as starch, containing more energy than did the original substances taken together or separate. It matters little whether the actual carbohydrate thus synthesised is starch, or sugar or inulin: the point is that energy has been gained from outside and bound up with the acquired material for further use. But modern physiology has carried matters much further than this, and especially in the three following directions.

In the first place, it has shown that much of the energy thus stored from without in the plant is again liberated in the process of oxygen respiration, and expended partly as appreciable heat and partly as driving force for stimulating the machinery of the living plant to further activities.

In the second place, part of it is rearranged with the rearrangement of the molecules with which the energy is bound up, as it were, so that work of various kinds is done in the machinery of the plant: I refer to various metabolic and surface-actions resulting from the peculiar mode of presentment of the resulting substances, for instance the production of osmotic pressures in the cell.

And, thirdly, part of the synthesised substance is worked up into higher bodies, by processes which obviously entail the further doing of work on the constituents.

The further pursuit of this theme would evidently carry us beyond the more immediate subject of this book; but I want to make clear that recent researches render it more and more certain that the living plant is a complex piece of co-ordinated machinery which brings together matter and energy from the external universe, and then gets work out of these.

This proposition is the more important because the whole question of the enrichment of our planet with new food, new building materials, and new fuel, to compensate the daily losses, depends on it, and is of course to be referred fundamentally to the acquirement of new supplies of energy from the sun. Enormous activity has been displayed by physiologists, since 1860, in attempting to solve the question, which of the many different rays known to proceed from the sun are absorbed by the chlorophyll-corpuscle, and directed to the performance of the work above referred to.

The names of Draper, Sachs and Pfeffer stand forth prominently as pioneers in this; while those of Lommel, Engelmann, Timiriazeff and Langley have been among the most active in making important contributions to the subject, and in attempting to answer the further questions connected with the mode in which the chlorophyll is concerned in utilising the energy of the solar radiations. The point is one of supreme importance, because it goes on all fours with modern questions as to the rays of light absorbed or dispersed in our atmosphere at different seasons of the year, or in special climatic conditions, to say nothing of its other scientific aspects. Unfortunately, however, we have no satisfactory explanation of the actual rôle played by the chlorophyll substance itself, in spite of much industrious work which has been done in the subject in this country and elsewhere. As regards the rays employed, it was first proved that the most effective belong to the red end of the visible spectrum, and that the effect as measured by the amounts of oxygen given off, and of starch formed in given periods of time, is more or less proportionable to the intensity of the solar light. Then it was established that no monochromatic light is so powerful as the white light from which it was obtained, though the relative numbers expressing the activity in the red and yellow regions may stand to those in the blue as something like 12:1. The latest results place the maximum assimilation in the red-orange, and this coincides with the maximum absorption in the chlorophyll. If we may accept the current views as to the distribution of energy in the spectrum of solar light, which depends on the complete absorption of all the rays by a black body, where they are estimated as heat, we have the interesting result that the agricultural or forest plant is adapted to catch and retain, broadly speaking, just those particular rays which possess most energy.

The probability is increasing that the protoplasmic machinery is the really effective mechanism in the process, and we may figure this machinery as so holding or presenting the molecules of carbon-dioxide and water to the impact of the light-vibrations, that the latter are enabled to undo the molecular structure; the atomic combinations thereby liberated may then be supposed to form a body like formic-aldehyde, which by polymerisation becomes a carbohydrate of the nature of a sugar such as glucose, which the protoplasm then builds up into its substance and subsequently deposits as starch, and stores temporarily in the form of grains or as amorphous material.

This is partly hypothetical, and is largely due to the careful deductions of the chemists, but there are very many facts now to hand which bear out its probability, especially the recent advances in our knowledge of the sugars, and the experimental feeding of leaves and plants deprived of starch with such substances as dextrose, levulose, maltose, and other sugars, as well as glycerine and other bodies which should be convertible into, or yield them, if the theory is true. In this last connection, the careful and extensive experiments of Acton, A. Meyer, Boehm, and Laurent should be mentioned. It would be interesting to enlarge upon Engelmann's beautiful physiological experiments in connection with this subject of absorption of solar energy, where the maximum accumulation of oxygen-loving bacteria at those parts of a green alga which lie in the red-orange of the spectrum, are used as indicators of the maximum oxygen evolution (and therefore of the maximum carbon-dioxide assimilation), but space will not admit of this. For a similar reason I must also pass over the same observer's experiments with plants which assimilate in protoplasm behind a red instead of a green substance, and which absorb chiefly other rays between the yellow and blue, with the remark that they also seem to imply that it is the protoplasmic machinery which turns the energy on to the carbon-dioxide molecule, the coloured screen being secondary in the matter. Recent experiments which show that green plants will not assimilate carbon-dioxide in a light which has passed through a solution of chlorophyll—and therefore left its red rays behind; nor behind a screen of iodine dissolved in carbon-dioxide—which lets no visible rays between the red and blue pass—should be noticed, as showing the importance of the chlorophyll and the special rays referred to, however; and I ought at least to mention Timiriazeff's beautiful proof, published in 1890, that if, on the leaf of a plant left in the dark long enough to render it free of starch, a bright solar spectrum is steadily projected for 3-6 hours, the chlorophyll then removed by alcohol and the decolorised leaf placed in iodine, the image of the spectrum is reproduced by the different intensities of the starch bands, blue with iodine, in the different parts. Here, again, the maximum coloration coincides with the maximum absorption in and near the red.

Microscopic observations and photo-chemical experiments alike convince us that the chlorophyll-corpuscle is itself a complex piece of protoplasmic machinery, working for and with the rest of the plant, and there can be little question as to the greater accuracy of our reasoning on the whole question I am discussing, since Meyer, Schimper, Pringsheim, and others have established the importance of its structural peculiarities.

I must now pass on to consider another aspect of the question of carbon-assimilation.

Notes to Chapter III

In addition to the references in the last chapter, the reader may be referred to Sachs' Lectures, XXV., and Pfeffer's Physiology, pp. 329-356, where the voluminous literature is given.

CHAPTER IV.

METABOLISM

Quantities of starch formed, and their significance for the plant. The absorption of energy—the conversion of energy in the plant. The plant is a complex machine for concentrating and storing energy and material from without.

Sachs measured the increase in dry weight (due to the carbohydrates formed in the chlorophyll-corpuscles) per square meter of leaf-surface, exposed for a definite period, by drying rapidly at 100° C. equal areas of the leaves concerned, and comparing the weights.

Of course the results are not to be pushed too far, in view of the fact that some of the starch is continually passing away to be utilised, and of the difficulties of comparing the weather, the intensity of light, currents of air, hygroscopic conditions of atmosphere, and other variable factors which influence the matter. For instance, the stomata open and close to different extents according to the conditions of light and moisture, and this affects the whole mechanism of transpiration especially, and therefore the supplies of water and mineral salts. Nevertheless, some interesting and valuable results have been obtained in connection with this important subject.

It was found, for instance, that the foliage of a sun-flower or of a vegetable-marrow may be forming starch at a rate of considerably over a gram per hour in every square meter of leaf-surface exposed on a fine day; while in particularly clear and warm sunny weather Sachs obtained as much as 24 to 25 grams per square meter per diem.

When one reflects that 200 square meters is not an extravagant estimate for the area of leaf-surface exposed on a tree, for a period which even in our latitudes may be considerably over 100 days of, say, ten hours' light, we need no longer wonder at the rapidity with which wood is produced in the stems, and similar estimates (which I have purposely kept lower than the estimates for continental and tropical climates) may suffice to show how quickly potatoes or the ears of corn, etc., may fill up with the starch or other carbohydrates which render them valuable as crops. We want more measurements in these connections, moreover, for there are several ways in which they are of scientific value and practical importance.

It is evident from what has been said that every grain of starch formed represents so much energy, packed away for the moment in the storehouses of the plant; and we know that—quite apart, however, from intermediate transformations of the energy thus stored—this energy reappears in the kinetic state eventually, when the starch is burned off, in presence of oxygen, and transformed into carbon-dioxide and water. It matters not how quickly or how gradually this combustion occurs, or whether it is accomplished by burning in a fire, or by slow and complex stages in respiration or metabolism: the point is that the unit of weight of starch yields so many units of heat when its structure tumbles down to the original components, carbon-dioxide and water.

Clearly, if we know how many units of heat are yielded by the combustion of one gram of starch, we can obtain an estimate of the amount of energy, measured in terms of heat, which the foliage gains and stores up—an estimate which will approach the truth in proportion as our estimate of the total assimilative activity is correct.

A word of warning is necessary here, however, for those best acquainted with physiology recognise that however useful such calculations as the above may be, and undoubtedly are, to give a general idea of the fact that the energy represented is large, it would be a mistake to suppose that such estimates give even an approximate measure of the energy of potential which may be got from the carbohydrate, and still less of the amount of work that may be got from its employment, according to the way it is employed or presented in the plant. To take a single instance only. If the carbohydrate is rapidly burned off to carbon-dioxide and water, very little is got out of it in the way of work—most, if not all, of the energy set free escapes as heat: whereas if the carbohydrate is slowly and gradually oxydised, passing through various stages and giving rise to powerfully osmotic bodies in the process, or if it is built up into protoplasm, or into the structure of a cell-wall, relatively enormous quantities of work may be got out of its surface-energy, and heat may be absorbed. Whence it follows that we cannot measure the power for physiological work of a body by merely obtaining its heat of combustion, any more than we can infer its significance in metabolism from its chemical properties.

The general conclusion that the plant stores large quantities of energy may of course be arrived at by simply estimating the enormous quantities of food-material which we obtain annually from agricultural plants.

Modern physiologists have attempted to proceed further than this, however, in their essays to form an estimate of the relations between the available energy in the solar rays and that used and stored in the plant.

If we reflect on such phenomena as the cool shade of a tree, and the deep gloom of a forest, and on experiments which show that an ordinary leaf certainly lets very little of the radiant energy of the spectrum pass through it, it becomes evident that many of the rays which fall on the leaf are absorbed in some form, and it becomes very probable that much of the solar energy, other than that we term light, is retained in the leaf for other purposes than assimilation—or, at least, no other conclusion seems possible in view of all the facts. Engelmann's researches with purple bacteria are almost conclusive on this point, and we may regard it as extremely probable that the plant makes other uses of rays, perceived by us as heat-rays, as sources of energy. Researches on the influences of temperature on assimilation and other functions point to the same conclusion; and Pfeffer and Rodemann definitely state that heat is converted into work in the osmotic cells. And the study of the absorption bands in the spectrum of the living leaf becomes more intelligible in the light of these conclusions. Moreover, the fact that a plant still carries on processes of metabolism when active transpiration has lowered its temperature below that of the surrounding air—and the plant therefore receives heat from the environment—points to similar conclusions.

The importance of the conclusion is immense, for even if the plant had no other sources of energy than the darker heat rays of the solar spectrum, it is clear that it ought to be able to do work.

The above may suffice for the general establishment of the conclusion that the plant absorbs more radiant energy than it employs solely for assimilation, and emphasises our deduction that it is a machine for storing energy.

The question now arises, how is this relatively enormous gain in energy employed by the plant? Our answer to the question is not complete, but modern discoveries in various directions have supplied clues here and there which enable us to sketch in some degree the kinds of changes that must go on.

Not the least startling result is that, important as carbon-assimilation is as the chief mode of supplying energy, it is not the only means that the plant has of obtaining such from the environment, and it is even possible—not to say probable—that energy from the external universe may be conveyed into the body of the plant in forms quite different from those perceptible to our eyes as light.

In the most recent survey of this domain, it is pointed out that we may distinguish between radiant energy, as not necessarily or obviously connected with ponderable matter, and mechanical energy, which is always connected in some way with material substance. All mechanical performances in the plants depend on transformation of some form of these, evident either as actual energy doing mechanical work, or as energy of potential ready to do work.

In so far as molecular movements are concerned, we have the special form of chemical energy. The evolution of heat, light and electricity by plants are instances of radiant energy, and so on.

Many transformations of energy in the plants are due to non-vital processes—e.g. transpiration, warping actions, etc., but we cannot always draw sharp lines between the various cases. Nor can we directly measure the work done in the living machinery; but from the effects of pressures and strains, the lifting of heavy weights, driving of root-tips into soil, osmotic phenomena, etc., it is certain that the values may be very high.

The following classes of processes in living protoplasm and cells may be taken as indicators. First we have transformation of chemical energy, without which continued life is impossible: in many cases—e.g. the processes connected with oxygen respiration—these result in the development of heat. Secondly, we have those remarkable manifestations of energy known as osmotic processes, which depend on surface actions, and with which may be associated other surface effects, such as imbibition, secretion, etc., and in connection with which heat may be evolved or absorbed. It is true the substances which exhibit the properties here referred to may be produced, or placed in position, by chemical energy, or they may be absorbed by roots, etc.; but the proximate energy exhibited by them is not derived from chemical energy, and may be out of all proportion to the chemical energy of the substance or substances concerned. Moreover it is significant to note that a highly oxydised body may develop much osmotic energy, as well as a highly combustible one.

It is of the greatest importance to realise the truth that much work can be, and is done in the living plant, by conversions of energy of potential independent of and out of proportion to the chemical energy available by decomposing the substances concerned; even the heat of respiration may be superfluous here, for the plant may absorb heat from without, and convert it into work.

Tensions often arise in the plant, and do work expressed as movements—e.g. the springing of elastic Balsam fruits, stamens of Parietaria, etc.

Osmotic energy not only results in enormous pressures and tensions, but causes movements by diffusion and diosmosis, and any given osmotic substance which carries this energy with it is not necessarily formed always in the same way in the cell—e.g. glucose may arise from starch, or from carbon-dioxide, or from oil.

Surface-energy is also expressed in the powerful attractions for water exhibited in imbibition, swelling, capillarity, absorption, surface tensions, etc.

Transpiration induces relatively enormous disturbances of equilibrium, and does work in moving water quite independent of chemical energy.

Again, what may be termed excretion-energy, as expressed in the separation of a solid body—e.g. a crystal—from a solution, may be for our purposes regarded separately. Any change in the condition of aggregation of a substance in the plant may result in movements and the overcoming of resistances.

It will be evident from this short digression—and this is the point I wish to emphasise—that in the interval between the securing of a grain of starch, representing so much energy won from the external universe, and the reconversion of this grain into its equivalent carbon-dioxide and water, by respiration, resulting in the loss of the above energy as heat, the starch referred to may have undergone numerous transformations in the living machinery of the plant, and have played at various times a rôle in connection with the most various evolutions of energy.

If we try to picture a possible case, we may take the following. A given starch-granule, after being built up in the chlorophyll-corpuscle, is decomposed, and yields part of itself as glucose, which passes down into other parts of the plant in solution. Part of it is merely re-converted into starch, and temporarily stored: another part passes into the arena of oxydation-processes, the sum of which constitute respiration, and may serve for a time in the molecules of an organic acid: yet another part may be converted into a constituent of the cellulose cell-walls; while part may be brought into play in the reconstruction of protoplasm.