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Of course you have heard of the feat of growing a mango tree, as performed by the Hindu jugglers. The trick consists, according to those who claim to have witnessed it, in causing a mango tree to grow to fair proportions before the eyes of the audience from a pot which at first contained no visible plant. The plant appears first as a small sprout and then grows to tree-like proportions under the manipulation of the conjurer before the very eyes of the astonished witnesses. I believe modern skepticism, aided by the camera, has demonstrated that what the juggler really does is to throw a hypnotic spell over his audience and to cause them to confuse the magic picture of his word-conjuring with actual vision. But even if we were to take the feat of mango growing at its face value, it would still be no more miraculous, properly interpreted, than things we may observe everywhere about us-say in any vegetable garden-or that you may yourself perform at any time in your own room. Suppose, for example, that you were to take a tiny seed no larger than a grain of sand, and place it in a bowl on the window-sill. You may leave it there indefinitely and it will give no sign that it differs in any wise from the grain of sand. Yet if you wish to perform a miracle along the lines of that alleged to be performed by the grower of the mango tree, you have only to pour a tumblerful of water over the seed. Then in due course a transformation will be effected. The little seed will germinate and put forth a sprout and a system of rootlets and lift its head into the air and presently develop a bud that will swell and open into a beautiful flower. This, surely, is a feat of conjuring that more than duplicates the alleged miracle of the Hindu fakir even though we were to take that performance at its face value. To be sure, we have required more time for our miracle than he required for his; but what, after all are a few days more or less in the performance of such a feat? And, indeed, are we not entitled to a little latitude of time considering that our miracle, which includes the creation of a beautiful flower, is so much more wonderful than his? Perhaps you are inclined to demur, and to say that your miracle of flower-growing is no miracle at all because you had nothing to do with the matter. The growing of the plant, with its ultimate production of the flower, you will perhaps allege, was altogether the work of nature; a work in which you had no share. Not so; for had not you supplied the cupful of water, nature would have been as powerless to transform the seed into a flower as you would be to transform the water into a flower without the aid of Nature. Your feat of jugglery, like that of any other conjurer, required appropriate paraphernalia and the aid of an accomplice. You chose as paraphernalia a tiny seed and a cup of water; and for accomplice you chose Nature herself. You invoked the aid of natural laws, just as every other conjurer must do; and the results you finally achieved were surely more wonderful, more mysterious, more inexplicable than the results of any other kind of trick that human ingenuity could devise. In effect, you held a cup of water before your audience, waved your hand over it with magic incantations, and transformed the water into an exquisitely petalled and perfumed blossom. Who could ask to witness a more marvelous feat of jugglery than that? Yet such miracles as this are matters of everyday observation with the gardener. Is it strange that he finds peculiar fascination in his work and sees in his plants something more than the mere combinations of root and stem and tuber and seedpod that they present to the casual observer? Rather to the gardener who goes about his task with the right spirit must every plant appear as the most wonderful of laboratories in which miracles of transformation, outmatching the utmost feats of the most skillful conjurer, are being performed every hour.


I have chosen the imagined incident of the flower seed grown in the bowl on your windowsill because I wished to emphasize the important principle that the one essential element without which no plant can maintain life or take on growth is water. The plant grower has always given much heed to soil. He talks of sandy loams and clayey earth, and of humus and fertilizers. And all these, as we shall have occasion to see presently, have vast importance. Yet in the last analysis the constitution of the soil-the very existence of the soil itself-is of incidental or subsidiary significance only in the plant economy. The richest soil that was ever prepared would not grow a single blade of grass or the tiniest weed if that soil were absolutely dry. Nor could the hardiest weed maintain existence for a single day if transplanted into a soil, be it never so rich, that is absolutely devoid of moisture. There must be water in the soil, to dissolve out and transfer its elements, in order that the rootlets of the plant shall be able to make the slightest use of these elements. Every essential constituent of plant food may be present in just the right proportions in soil that is packed about the roots of the plant with just the right degree of firmness, and yet the plant would perish as inevitably as if it were uprooted and suspended in the air, if there were not water present to bring the food materials into a state of solution. But on the other hand, as we have seen, a plant may grow and thrive for a time quite without the presence of soil of any kind or quality if its roots are placed in water. If we look a little farther into the intimate structure of the plant, utilizing the knowledge gained with the aid of the microscope and the studies of the chemist, we shall quickly come to understand why it is that water plays this all-important part in the functions of plant life. For it appears that the essential basis of life itself, namely, protoplasm, is a substance composed largely of water and having the physical constitution of a viscid liquid. We find, moreover, that no particle of solid matter can, under normal conditions, penetrate the walls of the cells that make up the minute compartments in which the individual masses or protoplasm lie. Ramifying everywhere among these are spaces and tubules that convey water and air. And portions of this water and air are absorbed by the bits of protoplasm through their cell walls. With the water they gain the mineral constituents that are essential for their nourishment. But these include no minerals that are insoluble. It is true that the plant rootlets may on occasion secrete certain fluids that aid the water in bringing into solution some intractable chemicals. But these secretions themselves are watery fluids and they would be ineffective if there were not water present to complete the work that they begin. In a word, then, the all-essential element for which provision must be made by the gardener or other plant developer is water. Where water is present, anywhere in the world, we find plant life luxuriating. Where it is absent, we find the deserts. There is no acre of soil anywhere that might not produce its crop of vegetation if properly watered. And, on the other hand, some of the richest soils in the world are those that are absolutely barren and fully merit the designation of desert lands because water is lacking. Of course the gardener in many regions is supplied with water in adequate quantity for his plants by the natural rainfall and may disregard the question of artificial irrigation. But even in regions where the rainfall is usually adequate, there are almost certain to come periods of drought and the wise gardener who wishes to make sure of his crop will make provision for the meeting of this emergency. Even where the soil is fairly moist, it is often possible to force the growth of a plant by additional watering. You may readily test this for yourself by the free watering of alternate plants in a row in a time when the rainfall is only moderate. You may thus produce giants and dwarfs, say in a row of tomatoes, from the same lot of seed, under conditions which are absolutely identical except as to the matter of water supply. Of course it is possible to overdo the matter, super-saturating the soil and so shutting off air from the plant roots. But that aspect of the subject will claim our attention in another connection.


If we would have a clear comprehension of the function of water in a plant, we must go a little more fully into the physiology of plant growth, following the water, with its salts in solution, from the rootlet by which it is absorbed up through the stem of the plant to the leaf. In an earlier chapter something has been said as to the forces that operate to make the water rise in seeming defiance of gravitation from the root to the leaf system of a plant of whatever size. The rise of the watery juices in a garden plant does not seem, perhaps, quite as mysterious as the rise of the sap in a tall tree. But there is no difference in principle. The laws that govern the movement of the sap are quite the same in each case. We saw that there is reason to suppose that the principle of osmosis, acting between the cells, has an important share in transferring water from one cell to another, and ultimately, step by step, from the root to the topmost leaf. It should be added, however, that the entire subject of the rise of sap in the tree has been matter for debate, and that there is not entire unanimity among plant physiologists as to the forces that are involved. That osmosis has a share, no one doubts. But it is alleged that the principle of capillarity through which liquids are drawn into minute tubes also has a share in elevating the water in the plant. And it is further suggested that the constant transpiration of water from the leaves of the plants acts as a sort of suction force drawing the water upward. It should be understood, however, that this alleged suction power, when analyzed, is nothing more than a drying out of the cells of the leaf which makes them more absorbent and thus brings into play the principles of osmosis and capillarity through which they take up a new supply of water from neighboring cells. Thus, properly understood, the effect of transfusion of water from the leaves is to be interpreted in terms of osmosis, and capillarity. So also must be interpreted the so-called root pressure through which water is forced upward into the stem of the plant at a time when the plant has no leaves-as in case of a tree in the early spring time. Such root pressure undoubtedly exists, but this also is explicable as due to the absorption of salts in solution by the rootlets from the water in the soil about them, leading to osmotic action between these superficial cells and the adjoining cells, which in turn pass the water, with its modicum of nutrient salts, to yet deeper layers of cells, and ultimately up along the stem of the plant or tree-constituting the familiar phenomenon of the "rise of sap." Regardless of the precise explanation, however, the fact is obvious and long familiar that water bearing a certain quantity of minerals in dilute solution is absorbed by the roots of the plant and is carried up in due course to the ultimate buds and growing tips and leaves. It has been known for a good while also that the leaves of the plant have on their under surface vast numbers of little mouths or stomata, through which a certain amount of the water that has come to them from the roots is transpired or exhaled, and through which also air is inhaled. But it has only somewhat recently been learned that the air which thus enters the structure of the leaves is transmitted everywhere throughout the tissues of the plant, through little crevices or canals that may be likened to the bronchial tubes of an animal or of man, except that they are infinitesimal in size. Through these channels, air is brought in contact with all the cells of the plant, and, during periods of growth, there is a constant, even though slow, interchange between the air in the intercellular spaces and the structure of the protoplasm within the cells. This interchange includes the absorption of oxygen and the giving out of carbonic acid on the part of the plant cell, which is precisely the same thing that occurs in the functioning of the cells in the tissues of an animal. In point of fact the essential properties of protoplasm are the same, whether that protoplasm is found in the tissues of a plant or in the tissues of a man. Plants, like animals, in breathing take in oxygen and exhale carbonic acid gas.


This fact, as was said, has not been clearly understood until somewhat recently. The phenomenon of the absorption of oxygen and the exhalation of carbonic acid has been obscured in the case of the plant by the further fact that the plant leaf absorbs constantly from the air during the daytime, under the influence of light, a relatively large quantity of carbonic acid gas from the minute quantity in the air, so that the net result is that it takes up from the air more carbonic acid than it exhales. It was only by studying the plant in the dark, when the elaborate processes through which it utilizes the excess of carbonic acid are in abeyance, that the fact of the close analogy between vegetable protoplasm and animal protoplasm as to the ingestion of oxygen and the giving out of carbonic acid as a waste product was demonstrated. Now it is known, however, that the protoplasm of a plant cell, as it exists in the root and trunk of a tree, for example, and indeed in any part of a plant where there is no green matter, not only functionates in the same way as the protoplasm of animal cells, in regard to absorbing oxygen and giving out carbonic acid, but that the two have precisely the same food habits in general. The average plant cell, as it exists in the root or stem of the plant, is in precisely the same position as the cells of an animal, in that it can secure nourishment only from food that has been prepared in a particular way. It can no more take a crude solution of mineral salts and extract nourishment from them than can the animal cell. All the necessary constituents that go to make up the best food may be present, but neither the plant protoplasm nor animal protoplasm can make use of these constituents unless they have been compounded in a unique and extraordinary way. But when we consider the matter one stage farther we come upon this vital difference: the plant, unlike the animal, has provided a special mechanism-a unique laboratory-through which it is able to manufacture from the crude salts in watery solution, with the aid of another element taken from the air, a new compound which will serve the protoplasmic cell with food. That is to say, the plant organism as a whole, comprises a laboratory for compounding the crude elements, which by themselves cannot be used as nourishment, into a substance that can be used as nourishment. Stated in slightly different terms, every well-organized plant has a food factory as part of its regular equipment. Here indeed is a difference and a very vital one between the plant and the animal. For no animal is equipped with such a food factory as this. And when we add that the food factory of the plant is the only place in the world where food stuffs are manufactured, and that no animal of any kind could live an hour without nourishment that was originally manufactured by some plant, the vital importance of the matter will be manifest.


Now of course the plant in operating its wonderful food factory is functioning to supply its own needs. It must supply nourishment to the multitudinous cells that make up its root and stem and branches, which, as we have seen, are quite incapable of extracting nourishment from the crude salts in solution that they are constantly transporting. But incidentally, in manufacturing food for its own cells, the plant is producing a supply of food that will be available for the sustenance of animal cells also. Thus the entire animal world may be said to be a vast parasitic colony as absolutely dependent upon the vegetable colony for its essential food supplies as any other parasite is dependent upon its host. When we consider the matter in this light, it is pretty obvious that about the most interesting thing in the world, from the standpoint of animal economy-which of course includes human economy-is the wonderful laboratory or factory of the plant where alone is effected the transformation of the crude inorganic elements into such combinations as are available for the sustenance of life. When we reflect that the plant laboratories in which this wonderful and vitally essential transformation is effected are chiefly located in the leaf of the plant, it appears that the thoughtful person must regard this structure-the most ordinary green leaf of tree or shrub or vine or the tiniest blade of grass-as in some respects the most wonderful thing in the world. When the wise plant developer goes into his garden or orchard, therefore, his eyes turn always first and foremost to the leaves of the plants with which he works. The reader will perhaps recollect that over and over I have called attention to the predictions that may be made as to the future fruiting powers of a given plant-apple seedling or pear seedling or grape seedling or what not-from observation of the leaves. The reason for this will now perhaps be more apparent. It will be still more clearly evident if we inquire a little more in detail as to the exact processes that take place within the structure of the leaf-laboratory in which is brought about the all-essential manufacture of food on which the future growth of the plant itself and its fruiting possibilities must absolutely depend. No one needs to be told that all normal leaves are green in color. But perhaps it may not have occurred to you what a really remarkable fact this is. The trunks and branches and roots of plants may vary widely in color; and flowers and fruits may show all diversities of the rainbow. But from one Arctic circle to the other and around the circumference of the globe, plants of every tribe (with the rare exception of parasites which take food predigested by the green plants), from the minutest creeper to the most gigantic sequoia or palm or eucalyptus, have leaves of the same primary color. And the reason for this is that the leaf derives its color from the massed effect of little structures called chlorophyll granules that nestle in its individual cells, constituting the really essential part of its food-forming laboratory. These have adopted a green uniform as the insignia of their office, and they hold as rigidly to this color as if their very lives depended upon it. And for aught we know to the contrary, their lives may depend on it; for no one has yet been wise enough to say just what relation the color bears to the wizard-like powers of the so-called chlorophyll granules that wear it, and that, seemingly with its aid, effect the marvelous transformation of inorganic elements into food-stuffs of which they alone of all created things are capable. As I say, no one knows just what relation the green color of the chlorophyll granules bears to their work because no one knows just how their work is performed. That is to say no one at all understands why it is possible for the plant cell that bears within its substance one of these green chlorophyll bodies to combine certain inorganic elements into nutritious foods, a feat that no human chemist can perform. But on the other hand, we do know, thanks to the analysis of the chemist-who can sometimes tear things to pieces and find out what they are made of even when he cannot put them together again-what the chlorophyll granule accomplishes, even though we cannot understand just how or why it is able to perform its work.


What takes place within the structure of the leaf, then, with the aid of the wonderful green workmen, is this: A certain number of molecules of water, brought to the leaf from root and stem, are taken in hand and compounded with a certain number of molecules of carbon extracted from the air that has been brought into the leaf laboratory through its mouths or stomata from the outside atmosphere. When the compound has been effected, we still have the atoms of hydrogen and oxygen that composed the water molecules and the atoms of carbon, but they are so marvelously put together that they no longer constitute the liquid water or the gas in which the carbon was imported. They now constitute an altogether new substance which is termed sugar. Thus only three elements are dealt with and these very familiar ones. It would seem as if almost any chemist should be able to manage a simple combination like that. But in point of fact no human chemist knows how to manage it. There are forces to be invoked in effecting that combination of which no chemist has any knowledge. Only the chlorophyll grains in the plant leaf have learned the secret, and up to the present they have kept their secret well. There are other feats of atom-juggling performed with the new compound that are wonderful enough. For example, the sugary compound is ordinarily transformed, in part at least, into granules of starch to be stored away for safe keeping. And this transformation implies a bit of juggling that is by no means easy. But after all it is only the changing of one organic compound into another, and the human chemist can do some extraordinary feats in that line. The really wonderful work done in the leaf laboratory is the original transformation of inorganic materials into an organic compound. Of course there are other important stages of the work through which final assimilation is accomplished. To make starch or sugar into protoplasm it is necessary to bring another element into the combination. This element is nitrogen. There must also be incorporated small quantities of a number of minerals; notably compounds of phosphorus and potash and lime, but including six or eight others that must be present in infinitesimal amounts. And the building of these substances into combination with the sugar in such a way as to produce the substance called protoplasm, the basis of all life, constitutes the culminating stage of the miracle. But the way in which this is effected is even less clearly understood. We do know, however, that all these substances are brought to the plant in watery solution. Nitrogen constitutes about four-fifths of the atmosphere, as everyone knows, and hence it seems rather strange that the plant does not draw what nitrogen it needs from this source, in particular since it gets its carbon from the air. But in point of fact the plant, no less than the animal, might starve to death from lack of nitrogen even while its tissues are everywhere bathed in nitrogen gas. To make the nitrogen available for the purpose of nutrition it must be made into soluble compounds called nitrates, and must be supplied in dilute watery solution. Such nitrates, therefore, are among the most important of the soluble compounds that must be contained in the medium surrounding the roots of the plant. Sucked up by the rootlets in dilute solution, along with much smaller quantities of phosphorus and potash and the other essential minerals, it is carried to the plant cells and ultimately compounded with sugars made in the leaf laboratory to make living protoplasm and thus to promote the growth and development of the plant.


This protoplasm is, of course, in the last analysis the vitally important substance. Without it there is no life. Even the chlorophyll body is itself a protoplasmic substance and establishes its workshop in a protoplasmic cell. All the life processes-growing, flowering, fruiting-are linked with the protoplasmic activities, just as are all the life processes of animals of every kind. But from the standpoint of the gardener, which furnishes our present outlook, interest may be said to center on the production of the non-nitrogenous carbon compounds, starch and various sugars, the creation of which in the leaf of the plant we have just witnessed. For the chief products of the vegetable garden (with the notable exception of peas and beans) contain only a small proportion of the nitrogenous matter which the food specialist names protein. We depend for our nitrogenous foods largely upon the animal world. The products of the vegetable garden are stores chiefly of carbohydrates, that is to say of starches and sugars. These make up the chief bulk of such tubers and roots as potatoes and carrots and parsnips, and the main nutritious matter of the principal garden vegetables, except, as just intimated, that peas and beans have a relatively high proteid or nitrogenous content. After what has been said, it will be understood that the starch and sugar content of the potato, for example, is not developed in the tuber itself, but is manufactured in the leaf of the plant and is then carried down in the elaborated sap that runs as a sort of return current to the roots and is there deposited for the uses of the new plant next season. In the case of the carrot and parsnip, the same thing, of course, is true. Here a large root, with its deposit of starch and sugar, is designed to live through the winter and next season to supply such nourishment for the plant as will enable it to take on rapid growth and to develop a large quantity of seeds. These plants are biennials and do not fruit in their first season. It is this fact that has been taken advantage of by man in developing their roots and diverting them to his own uses.


In all this, it will appear, we have said nothing as to practical methods of gardening. But I have thought that a clear outline of the principles involved in the all-important matter of the nutrition of the plant, and in particular a full presentation of the reason why the leaf structure of the plant is of paramount importance, might serve better to prepare the would-be gardener for his task than a mere categorical citation of methods, unexplained as to their final purpose. Whoever has carefully followed the outline just given will have a clear notion of the needs of the plant and might depend, were it necessary to do so, on his own ingenuity to devise means for meeting these needs. But, as a matter of course, we shall have occasion to deal more at length with specific methods of procedure with reference to the different types of garden vegetable when we take up in successive chapters the story of my work in the development of plants of the vegetable garden. And the general methods of soil preparation, drainage, irrigation, and fertilization are elsewhere treated in detail.

-I wish to emphasize the important principle that the one essential element without which no plant can maintain life or take on growth, is water.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 7 Chapter 1