ON SEEDLINGS AND THEIR CARE
The word "evolution" chances to have nine letters. Suppose that these letters were penciled on nine blocks of wood that are otherwise identical, and these little blocks were put in a bag and mixed together. Suppose then that you were asked to put your hand in the bag and bring forth one block after another, placing them in sequence as you brought them from the bag. What probability is there, do you think, that your blindfold selection of the blocks would result in bringing them out in such sequence as to spell the word "evolution"? A mathematician could doubtless figure out the exact probabilities, but you need not be a mathematician to realize that the chances are almost infinitely against you. Now I think I am right in saying that the plant developer who expects to find a considerable number, let us say nine, of particular qualities of any given flower or fruit or vegetable combined in just the desired proportion in any single seedling selected at random, stands about the same chance of having his expectations gratified that you have of spelling out the word "evolution" correctly with blocks drawn at random. But it is obvious that your chance of successful drawing of the blocks would increase in proportion as the number of attempts you are permitted to make increases. So would the plant experimenter's chance of finding several desired qualities of his fruit or flower combined in just the right proportion increase somewhat in proportion to the number of seedlings among which he can select. Yet I suppose the mathematician would assure us that the number of attempts you must make with the blocks before you could hope, according to the theory of chances, to bring out all the letters in just the right sequence would be so large as to tax your patience beyond endurance and I can testify that the same thing holds true with regard to the experiment of the plant developer. Though he had thousands of seedlings among which to choose he is not likely to find any one in a given fraternity that fully meets his ideal. But if in making your experiment of choosing the lettered blocks you were permitted to retain the blocks bearing the letter "E" when you chanced to draw it first; and if then you were permitted to retain the letter "V" when that was first drawn from the remaining group of eight blocks; and so in sequence with "O" and "L" and the rest, it is obvious that each new test would find you with a smaller number of letters from which to select, and hence with an increasing probability of successful selection. When, finally, there remained only two letters in the bag, your chance of securing the right one in the first draw would obviously be an even one. And when only the final "N" remains, you could make no mistake your selection of the right letter then becomes a certainty. Now I make this illustration because I think it has peculiar application to the case of the plant developer. His method is not unlike the method of selection just suggested. As the result of his first hybridizations, he does not dare to hope that he will secure the exact combination of qualities he would like to see aggregated in his ideal fruit or flower. But by having a large number of seedlings from which to select he may reasonably hope to secure one that will present some one at least of the desired qualities in superlative degree. This selected seedling he may nurture and use as part of his equipment for further experiment just as you retained the letter "E" as marking the beginning of your success in spelling the word "evolution." And as the plant developer continues his experiment with successive hybridizings and successive selections, he will be able in later generations to find individual seedlings that combine successively more and more of the qualities he is seeking. When, finally, he reaches the stage where the parent forms have between them all the desired qualities in superlative degree, he is somewhat in the position that you were in when only two of your lettered blocks remained in the bag. There is at least an even chance that he will find among his seedlings of the next generation one that will approximate his ideal, even though the number from which he selects is far smaller than the earlier groups. Thus by advancing step by step and using the ground gained as a new starting point the experimenter attains his end with comparative celerity, even though there would have been scarcely more chance of attaining that end with a single experiment than you would have had of spelling out the word "evolution" at a single series. But it must be fairly remembered that the probability of success is enhanced if at any of the earlier stages of the work you have opportunity to select the best plant among a large group instead of being restricted in choice to a few individuals; just as the chance of securing the block you seek in each successive drawing increases with the number of tests you are permitted. And in point of fact, this, or something like this, is the actual method in which the experiments of the plant developer are carried out, whenever he is attempting to construct a new fruit or flower or vegetable having a number of specified or clearly imagined qualities. In such a case, the wise experimenter does not hope to secure ideal results with a single hybridization; he seeks to group desired qualities of his flower or fruit together through successive crossings and selections. Keeping one supreme quality in mind and perhaps two or three others in the immediate background, he makes sure of first one and then another of these qualities, adding to them by successive crossings and selections and thus although advancing, as it were by indirection, and at first seeming to advance but slowly he may ultimately work with increasing certainty and approach his goal somewhat rapidly. For example, our first cross, say in the case of a prune, may be made between two varieties that both show a fair quality of fruit. Careful attention to the result will guide us in the matter of the next experimental crossing. We soon discover which qualities are prepotent, and which tend to remain latent, and by selecting only individuals that show a tendency to vary in the desired direction, we introduce an element of direction into the experiment. I am accustomed to speak of this as "momentum of variation." We do not always know why a certain plant tends to vary in a given direction, but we may observe the fact, and the wise experimenter is always on the lookout for this tendency, and ready to avail himself of the advantages it offers. Technical workers sometimes give the name "orthogenesis" to this tendency to vary in a certain direction, which I speak of as the plant's "momentum." Whatever aid we may gain in this way, however, the manner of our advance is often devious. In fact, it is very likely to be somewhat comparable to the progress of a sailing ship which tacks this way and that, and which at times may seem to be progressing in the wrong direction, yet which in the end forges ahead. Take by way of illustration the case of our stoneless plum. We discover soon that the stone seed is prepotent or dominant, and stonelessness latent or recessive. So we must be prepared to see the progeny of our first generation of hybrids all produce stony fruit. But a knowledge of the tendency of latent or recessive characters to reappear in successive generations comes to our aid, and we go on with the experiment with full confidence, even though for the moment we seem to be going backward rather than forward. In due course the second generation of plums appears with a number of stoneless specimens, the latent character having come to the surface. But these lack many of the good qualities that our perfected fruit must have, and in order to breed these qualities into the stock we must make a new cross; and this will involve the breeding in again of the tendency to bear stone fruit. So in three generations we shall find ourselves, as regards the essential quality of the stony seed, somewhat further back than we were in the beginning. But, on the other hand, our third generation fruit, even though it has a stony seed, has qualities of flesh that its stoneless ancestor altogether lacked; and in the fourth generation we shall be prepared to find individual seedlings that bear stoneless fruit of greatly improved quality. In each successive generation, then, we are dealing with better material-getting the chances grouped, if you will.
WINNING AGAINST ODDS
But, in a sense, we are running counter to the trend of heredity, because vastly the great proportion of the ancestors of our plum were bearers of stoned fruits. And so we must continue re-shuffling and dealing over, as it were, and watching results. We may lose in one generation what we gained in the generation before as regards the matter of stonelessness; even while on the whole advancing toward the production of a fruit of desired quality. But just in proportion as our ideal calls for the combination of numerous good qualities, does the attainment of that ideal become difficult. Even when, at let us say the fifth or sixth generation, we interbreed individuals that have the desired quality of stonelessness, we shall not at once secure what is desired; because our seedlings combine so many ancestral traits that they will not breed true. Even though they are all stoneless, there will be a great variation as to other qualities, and it is only by dealing with large numbers of seedlings that we can hope to find one or two that will show the desired combination of traits in high degree. Perhaps the comparison may be thought somewhat whimsical; but I am led to make it because I thought it might serve to suggest the complexities and difficulties that attend a plant-breeding experiment that involves the blending of numerous desired characters. And the lesson that I wish pre-eminently to inculcate is this: You must make many experiments at plant-breeding before you can hope to secure the combination-the sequence of qualities-that you desire.
THE LOGIC OF QUANTITY PRODUCTION
Now note the application: Each individual seedling of a hybrid strain represents a unique combination of ancestral traits, and constitutes in itself a new and unique experiment-equivalent to an independent deal of the cards. So the probability of securing what we seek will be somewhat proportionate to the number of seedlings. This is particularly true in the case of such variable plants as the fruit trees of our orchards. The case is far simpler when we are dealing with plants that vary little in their qualities, or where we are breeding with only a single pair or two pairs of unit qualities in mind-say "hardness" of kernel and immunity to rust, as in Professor Biffin's experiments with wheat; or good flavor and whiteness as in my white blackberry. But where the varied traits sought to be combined in a Shasta daisy are in question; or the many qualities of a commercial cherry or prune, the case assumes new complexities. Hence it is that my records tell of tests applied to about half a million seedlings of the daisy; seven and one-half million seedlings of various plums, and the like. Hence also the constant necessity of what my neighbors speak of as ten-thousand-dollar bonfires in my orchard, when we burn seedlings that have been inspected and found wanting. To burn 65,000 hybrid blackberries in one pile, as I once did after saving perhaps half a dozen individual vines, seems like willful extravagance to the casual observer, but it is an unavoidable incident in the search for perfect fruits. Such prodigal use of material implies a large measure of experience in the handling of seeds and the growing of seedlings. In point of fact, it might be said that this is the most important part of a plant-breeder's task, so far as the practicalities of experiment are concerned. It is part and parcel of his daily routine. It is highly desirable, then, that the would-be experimenter should gain a clear understanding of the essentials of method of caring for seeds and cultivating seedlings. So it is my purpose in the succeeding pages of this chapter to give a few practical hints as to various aspects of the subject. Thus summarized, the lessons I have learned in the hard school of experience may enable the reader to avoid some pitfalls and to make certain experimental shortcuts.
KEEPING SEEDS OVER WINTER
To begin at the beginning, let us note that the preservation of seeds over winter calls for careful attention. All fruit seeds except those of apricots and almonds, when removed from the fruit, are at once placed in slightly moist, coarse sand or fine gravel or in sterilized sawdust. In warm climates the boxes containing the seeds are then buried on the shady side of a building or tree where they will become neither too dry nor too wet. The object is to keep the kernels as nearly as possible in their original condition. If tree seeds, especially those of the cherry, the pear, and the plum once became thoroughly dry, it is difficult, and in some cases impossible, to induce them to germinate. An important function of the pulp of these fruits, in the original wild state, was, presumably, to keep the seeds moist until the season for germ motion. I have elsewhere called attention to the exceptional difficulty of keeping stoneless plums and prune seeds in condition for growing. Not having the natural protection of the shell, they tend to germinate too early, and of course they are peculiarly subject to the attacks of insects and of fungous diseases. Such seeds may best be placed in cold storage as soon as collected and cleaned, and kept at freezing temperature. Seeds thus cared for will sometimes germinate almost as quickly and readily as beans or corn. They must not be planted too early in the spring, lest their too prompt germination subject them to injury from late frost. Incidentally, I may note that grafts sent to me from a cold climate have often been observed to start with greater promptness, and grow better than those from our own immediate vicinity where the winters are mild. Cold seems to rest the tissues and prepare them for rapid growth, just as treatment with narcotic drugs has been observed to do in certain interesting experiments that will elsewhere be referred to more at length.
OUT OF DOOR PLANTING
In California, plum seeds are usually planted in January or February, in a little furrow about an inch deep. A furrow may be made accurately and expeditiously with the aid of a triangular bit of board an inch or so wide nailed across another longer piece, so that when drawn along a garden line it makes a narrow furrow of exact width and uniform depth throughout. Plant the seeds about one-half inch to an inch apart, and cover with a thin layer of soil; then fill the furrow with sawdust. This is an important matter with cherry and plum seeds, especially with the stoneless ones which must be given every inducement to push through the soil. A heavy, compact soil placed over cherry and plum pits prevents a large number from pushing up to the light. For this reason a sawdust covering is preferred, and it also regulates the moisture with exactness, allows for sufficient aeration, and equalizes the temperature. Moreover, the sawdust is distasteful to slugs, thrips, cut-worms, and other insect pests. Peach, nectarine, and apricot seeds are planted farther apart and a little deeper; quince, pear and apple seeds are planted about the same as plum seeds, both as to distance and depth, or in large lots may be rather thickly sown in drills or furrows six or eight inches wide and eighteen to thirty inches apart. For growing seedlings of conifers-pines and their allies-cold frames or shallow boxes are used filled with mellow sandy loam; or the seed may be sown broadcast or in rows in cold frames without boxes. The object of the cold frames is to shelter from hot sun and drying winds and in cold climates to prevent freezing. If the season is short or if warm weather comes on suddenly, it is sometimes desirable to soak seeds in water before planting. After being in the water several hours they should be drained and set in a warm place where germination can start quickly. In this way growth may sometimes be advanced by a week or more. But such forced germination is not usually necessary or desirable. If carried too far before planting, it endangers the growth. On the other hand, the very early plants often escape cut-worms and other insects by attaining a fair growth before these pests put in an appearance.
BOXES FOR SEEDLINGS
Valuable plants to be grown in large quantities from rare seeds, may best be started in small boxes or "flats" indoors, under glass or in sheds made of laths or slats so spaced as to allow free entrance to air and sunshine. Boxes of the right design and construction are far better for this purpose than pots or earthen pans. The boxes or "flats" that I have used for twenty years are made of redwood lumber. Where this cannot be obtained, cypress is nearly as good, but soft pine is not durable and should be avoided. Eighteen inches square, outside measure, four and one-half inches deep, inside measure, is a good size. Two opposite sides are of common board lumber three-quarters or seven-eighths of an inch thick; the other sides, are a little less than half an inch thick. The bottoms are made of redwood "shakes" which are about one-fourth of an inch thick; two or more spaces of an eighth of an inch being left for drainage. Across the bottoms are nailed three strips which add rigidity and strength as well as affording better ventilation and drainage. After all the parts are carefully fitted, the joints are sometimes dipped in linseed oil, before being strongly nailed together. This gives durability and tends to prevent the nails from rusting out. These redwood boxes may be used for many years if sterilized once a year by being placed for about three or four minutes in boiling water. A suitable soil is the first requisite in raising seedlings in boxes. The mixture which I have generally found best for use in the early winter for raising seedlings in boxes in the greenhouse, is compounded about as follows: One-half clean, rather coarse, sharp sand; with about forty percent of some good pasture or forest soil which generally contains more or less leaf mould. To this is often added ten percent finely powdered moss or peat. These mixtures, with the addition of about one or two percent of fine ground bone meal or superphosphate, make soils in which seeds of almost any kind of plants from any part of the earth will germinate. Seedlings thrive in this soil until they are ready for transplanting. If seeds of choice plants are to be grown, the soil is sterilized by a thorough scalding to destroy any fungus or insect pests. Usually we find it suits the plants better if a part of the soil last prepared is left over for use with the new mixture, like yeast for a loaf of bread, and I always prefer to have a little of the old on hand for this purpose. Common sharp sand, if the right texture can be obtained, is far better for cuttings than the soil just described. The sand found along creek or river banks is generally free from injurious insects or fungous diseases. But for rare cuttings and very choice seeds, this should be rinsed by pouring large quantities of water through it, at the same time stirring or jarring the material. In filling the boxes, coarse gravel, such as will just pass through a half-inch mesh, or a little smaller, is placed one-quarter to one-half inch deep over the bottom of the box. This ensures perfect drainage and sufficient aeration, both of which are of the utmost importance. The box is then filled, to within about an inch of the top, with the sand or special soil. Make the filling a little shallower for fall planting, when we expect much cool, damp weather, and slow growth, to prevent drowning or "damping off" of the seedlings during the winter; a little deeper for spring planting, to prevent too sudden drying out, and otherwise to regulate the amount of moisture. This may seem like a matter of small consequence, but such details often determine success or failure.
THE SEEDLING KINDERGARTEN
All ordinary seeds are sown quite thickly in the boxes and covered lightly with the same soil, according to the size of the seed-just a dusting of soil for the finest of seeds, and an eighth to a quarter of an inch for the larger ones. In testing new varieties, ten or twenty different kinds of seeds may be planted in sections in one box, each marked with a small wooden label, tacked on the upper edge of the box with the name, or the reference-book number, of the seeds. After the seeds are planted, the surface is pressed down with a flat piece of board until it is level, smooth, and solid. Instead of watering the surface by any sprinkling process from above, the boxes are placed after the seeds are planted, into a square pan containing water sufficient in depth to rise nearly or quite even with the surface of the soil. In a few minutes the water saturates the soil and entire contents of the box, without disturbing the seed, and without packing the soil in the least. The boxes are then removed and tilted to one side so that the superfluous water can slowly drain out. A thin layer of moss sifted over and under the seeds acts as a non-conducting blanket, equalizing the temperature and retaining moisture. A layer of gravel above the moss protects the seeds or young plants from being washed about when they are watered from above, as they are usually sprinkled after a few weeks of growth. The thin covering of gravel also wards off fungous diseases which afflict tender seedlings. All this may seem like unnecessary trouble, but it is absolutely necessary if one wishes to attain the best success. No part of the program can be omitted without risk of loss or injury to the seeds. When the seedlings have two to four leaves it is best to transplant them into another box, whether they are large or small, in order to give them more room in which to develop. In each box used for raising fruit seedlings we put about sixty-four, or sometimes late in the season as many as one hundred specimens. They are allowed to grow until toward spring when the weather becomes warm, about the time of the blooming of fruit trees, when they are ready to be transplanted to the open fields. Some of the smallest plants raised in greenhouses, like calceolarias, lobelias, begonias, ferns, etc., may readily be transplanted, even when they can hardly be seen, by lifting them on the end of a moistened quill, pencil, or small knife blade, placing them on the soil which has been previously moistened as before described, then covering with a glass for a few days until the young plantlets can get established. This is the quickest and best method of transplanting some of the smallest seedlings, and though apparently tedious is often the most speedy and profitable mode.
GOING UP A GRADE
In transplanting all small seedlings, they are placed in straight rows in the boxes; usually eight rows with eight plants in a row in the eighteen-inch boxes; but, for larger individuals, six rows of six plants; or, on the other hand, ten rows of ten or even twelve rows of twelve in case of the smallest ones. After standing in the greenhouse for a week or two, the boxes of seedlings are removed, usually to a sheltered place out-of-doors, in order that they may continue growth and become hardened through exposure to sunshine and outdoor air. Later, they may be safely transplanted into other boxes, giving them more room for growth, or to the field where they may be planted in long rows about four feet apart, so that they may afterwards be cultivated by horsepower in the usual way. In general the treatment here described is employed for cactus, berries, lilies, begonias, grasses, potatoes, roses, ferns, or any of the thousands of species of domestic, foreign, arctic, or tropic seeds which are received from collectors. In transplanting, it is best to have the boxes of plants carried into the field, and with most plants it is best to saturate the soil in the boxes, letting them drain a little before attempting to transplant. Then with a trowel they may be taken up with the dirt surrounding the roots and set out. After marking the rows with a garden line, a long narrow crevice is cut by inserting a flat spade and moving the handle back and forth a few inches. The plants can be rapidly placed in the crevice thus made. One side of the soil is pressed down with the foot or with a tamper, and packed quite firmly against the roots. Then more soil is drawn in with a hoe or rake and carefully placed about each plant, after which a common garden rake is used in leveling and loosening up the soil along each side of the row, which prevents "baking" and helps to keep the temperature equable and the soil moist. The most tender plants treated in this way are saved almost without exception.
OUT IN THE OPEN
Nearly all plants should be set out in the field somewhat deeper than they grow in the boxes. When plants have long roots these should be straightened out and placed as deeply as possible in the soil to give them a good start by the time the dry summer weather commences. Otherwise the young plants could not, in some cases, extend their roots fast enough to keep up with the gradually disappearing moisture, and so might die of thirst. When seedlings are removed from the protection of the glass house to the open air, or in transplanting in the fields, it is best, if possible, to choose a time when there are no severe winds, and when the sun is not too hot and the atmosphere neither too dry nor too chilly. Generally in California tender plants best withstand moving from the greenhouse to the open air just before or during a warm rain. At such times the atmosphere is similar to that in the greenhouse. Even under the most favorable circumstances they must be shielded from winds or bright sunlight to which they are not adapted. To accustom the tender seedlings to outdoor conditions, the flats are placed in square frames about six feet wide and a foot or two high. These are covered with a portable covering made of common laths nailed on narrow strips of board, so placed that the space between the laths is about equal to the width of a single lath. When the boxes of plants are placed in these frames, it is best to have some slats underneath so they will not rest on the ground; otherwise fungous diseases are often communicated from the earth to the soil in the boxes and to the tender plants. When the slat covering is kept over the frames for five to twenty days according to the season, the little plants will have adjusted themselves to their new environment so that the slats can be removed. After a few more days of growth they will probably be strong enough to be removed to the open ground.
RUNNING THE GAUNTLET
Many tiny seeds, just as they are germinating, may be destroyed in a short time by a cold dry wind, or they may be killed even more quickly by too much moisture and too little air. Young seedlings may be killed by a common fungus which causes "damping off." This is very destructive where plants are grown too thickly in the seed boxes especially in a close atmosphere before transplanting. Sometimes a whole box containing thousands of valued seedlings will be destroyed in this way in a few hours, the trouble generally commencing in little spots or patches from which it rapidly spreads in all directions. The tiny plants may most often be saved after the fungus starts by applying a dusting of sulphur or of coarse, dry sand or gravel. Sometimes if placed in a cool, dry atmosphere so that the excess of moisture is evaporated they may be saved. The gravel, mentioned as sprinkled over the moss when the seeds are planted, is the first and best preventive of damping off. It covers the soil with a substance on which the fungus cannot readily establish itself, and thus separates the unhealthy from the healthy plants. If good care in general is supplemented by the use of this dry sand or gravel, the fungus has little chance to spread from plant to plant. Of course, one is obliged to be on the lookout for insect pests, slugs, cut-worms, crickets, aphides and thrip, which are sometimes very destructive. Slugs, cut-worms, and crickets require instant attention when they first attack the young plants. The appetites of these pests often increase to greater proportions than can be appeased by the growth of the remaining plants and they must be carefully sought in or under the boxes. Sometimes slugs may be headed off for a time by sprinkling lime, red pepper, quassia, or tobacco dust in their paths. Thrip and the aphides are best destroyed by fumigating the houses once a week or twice a month with tobacco smoke; the frequency may be regulated according to the abundance and the persistence of the enemy. All in all, it is a severe gauntlet that the little seedling is called upon to pass. Yet if the methods described in this chapter are carefully followed out, it is possible to grow successfully any seed, from whatever climate or soil or location, that has the least germ of life within it. These methods have been successfully used with the seeds I am constantly receiving from numerous collectors in Siberia, Brazil, Chili, Argentina, Patagonia, Mexico, Central America, the Philippine Islands, Alaska, British Columbia, North and South Africa, Europe, India, South Sea Islands, Australia, New Zealand, Central and Western China, Japan, and Korea. By sedulous attention to the details above outlined, the raising of seedlings becomes so certain a procedure that the loss should not exceed one plant in a hundred. And this, obviously, is a most important consideration, especially with rare foreign seeds or seeds produced by hybridizing experiments that have involved exceptional care and labor. To such priceless stock, any amount of time and labor may be given ungrudgingly. And even in planting common nursery stock one soon learns that a thorough knowledge of the requirements of the plants is essential to success, and that cheap, careless work is always the most expensive in the end. A perusal of the foregoing pages will perhaps serve, better than almost any other exposition, short of inspection of the work itself, to give the reader an inkling of the enormous amount of mere mechanical labor-in addition to ceaseless watching and patient waiting required to bring the seedling plant through the time of its tender infancy. When it is further reflected that seedlings must be handled by thousands; and that this care is, after all, only one of many essential stages of each individual series of experiments in plant development, perhaps a fairly clear notion will be gained of the laborious-even though fascinating-character of the task that confronts the person who would develop a new fruit, a flower of modified color, or a plant of altered structure.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 4