/ PLANT GROWTH by L. EDWIN YOCUM Professor of Botany, The George Washington University Illustrated THE JAQUES CATTELL PRESS LANCASTER, PENNSYLVANIA 1945 Y VI Copyright, 1945, by THE JAQUES CATTELL PRESS PRINTED IN U.S.A. THE SCIENCE PRESS PRINTING COMPANY LANCASTER, PENNSYLVANIA PREFACE This book has been written in an attempt to bring together the knowledge necessary to answer (as far as possi- ble) the many technical questions which the plant lover may ask about growing plants. It is an attempt to make clear the "how and why" of plant growth. The principles of the laws of nature as applied to plants growing in the soil are stressed. Many of the newer theories used in plant culture are described; others, not so well established, are suggested as possible future developments. The illustrative material has been selected, when possible, because it is found around most homes, and can be examined by the reader. The privilege of using numerous illustrations, as acknowl- edged in each case, has been highly valued. The careful work of Mrs. Marian Manning in making the original photo- graphs is deeply appreciated. The author is happy to thank Dr. R. F. Griggs and Dr. Mary Reid for frequent encouragement and suggestions dur- ing the preparation of the manuscript. For the suggestions and the improvement of the chapters on plant breeding he expresses his heartfelt appreciation to Dr. and Mrs. Jack R. Harlan. The author is deeply indebted to his many teachers, students, and associates who have helped him to get the necessary knowledge to attempt a work of this kind. The critical reading of the manuscript and the many helpful suggestions of my wife, Mildred Yocum, are most deeply appreciated. L. Edwin Yocum CONTENTS Chapter One INTRODUCTION The inspiration to write about growing plants (1). Science used to explain growth (2). Some of the facts to be known about plants (3). Guides in the form of references (4). The glossary (5). Chapter Two SEEDS AND SEEDLINGS Seeds are resting plants (6). Seed structure (7), Origin of seeds from flowers (8). Kinds of seeds (10). How the seed develops into the seedling (11). Seedlings pushing out of the soil (14). Parts of the seedHng (14), Chapter Three GERMINATION OF SEEDS Conditions necessary for germination (15). Planting seeds to meet these conditions (16). The physiology within the seed (17). Influ- ence of light on seedlings (18). Planting seeds (19). Seed testing (20). Chapter Four CELL STRUCTURE AND PROTOPLASM The parts of the cell (21). Nature and function of the cell wall (24). Protoplasm (24). The nucleus (25). Chromosomes and genes (25). Cell division (28). Chapter Five ROOTS Tap and fibrous root systems (30). Root structure (30). Food stor- age in roots (37). Root hairs (38). Mycorrhiza (39). Conditions favoring root growth (39). Chapter Six ABSORPTION OF WATER AND MINERAL SALTS Diffusion (41). Composition and concentration of the cell sap (42). The soil solution (42). Movement of water and salts through the cell walls and protoplasm (43). Turgor pressure (44). Absorption of salts by plant energy (45). 58563 Chapter Seven THE SOIL Definition (46), Mineral soil (46). Soil water (48). Movement of water in the soil (49). Water-supplying power of the soil (50). Soil air and its use (50). Value of organic matter (51). Living organisms of the soil (51). Chapter Eight STEMS AND BUDS Kinds of stems (53). Stem structure (54). Stem growth in length and in thickness (55). Structure of buds (56). Kinds of buds (56). Bud and branch growth (57). Chapter Nine PRUNING AND TRAINING PLANTS Reasons for pruning (59). When to prune (62). Amount to prune (62). Pruning and the stimulation of bud growth (64). Pruning and vigor (64). Tree surgery (64). Chapter Ten PROPAGATION Sexual by seeds (66). Asexual by vegetative methods (67). Cuttings {67). Grafting (68). Budding (70). Layering (71). Chapter Eleven LEAF STRUCTURE External structures (72). The epidermis as a protective covering (72). Stomata (74). Size and distribution (74). Diffusion through stomata (75). Mesophyll (76). Chloroplasts (76). Air spaces (76). Some modified leaves {77). Chapter Twelve FOOD MAKING Photosynthesis described (79), The amount of food made (80). Con- ditions that favor photosynthesis (81). The balance of CO2 and O2 (83). Lengthof day (83). Chapter Thirteen TRANSPIRATION Description (87). Conditions affecting transpiration rate (88). Sto- matal regulation (89). Water requirement of plants (89), Transpi- ration and soil moisture (90). Watering plants (91). Chapter Fourteen BALANCE OF ROOT AND SHOOT Interdependence of root and shoot for food and water (92). Carbon and nitrogen ratio (93). Pruning and the nitrogen balance (94). Transplanting (95). Chapter Fifteen INSECTS AND DISEASES The problem (97). The epidermis as a protection against insects and diseases (98). How diseases spread (98). Spraying to kill insects according to the way they injure the plant ( 101 ). Contact and stomach poisons (101). Chapter Sixteen WEEDS Competition of weeds and desired plants ( 104). Annuals, biennials and perennials (104). Weed seed germination (106). Weeds propagated by stem or root cuttings (107). Weeds in the lawn (108). KilHng weeds with chemicals (111). Chapter Seventeen FLOWERS Described (112). Parts of the flower (115). The stamen (116). The pistil (116). The clover type (117). The sunflower type (118). The grass type (118). The pollen (118). Pollination (119). FertiHza- tion(119). Xenia(120). Metaxenia (120). Chapter Eighteen HYBRIDIZING PLANTS Illustrations (122). The work of Mendel (123). His seven experi- ments (125). Mendel's second law (129). Reduction division (131), Work of Burbank (137). Methods of hybridizing for the gardener (138). Hybrids (139). Chapter Nineteen HEREDITY AND VARIATION IN PLANTS Progress of early peoples (141). Heredity and variation (142). How employed by plant breeders (143). Mutations (143). Selection and pure lines (145). Methods of increasing the number of mutations (146). Colchicine (147). Examples of plant improvement (148). [ISIlIBRARYI ai f^Aikt: Chapter Twenty PLANT HORMONES Hormones as growth regulators (151). Vitamins (152). Plants as a source of hormones ( 154). Work with the oat coleoptile ( 154). Root- ing cuttings with chemical stimulants (156). Possible advantages of root stimulation (156). Chapter Twenty-One SOIL IMPROVEMENT Importance of good texture (158). Water-holding power of the soil (160). Value of humus (161). Compost (161). Living organisms (162). Soil erosion (162). Chapter Twenty-Two FERTILIZERS Essential elements (164). Why plants need fertilizers (165). The function of each element (169). The application of fertilizers (170). Lime (171). The newer methods of growing plants in water solutions of fertilizers (172). Chapter Twenty-Three NITROGEN The carbon/nitrogen ratio of Kraus and Kraybill (175). Nitrogen and growth (176). Means of supplying nitrogen (176). The nitrogen cycle (178). Chapter Twenty-Four SOME SPECIAL CONSIDERATIONS OF PLANT GROWTH Light and Growth (181). Description of growth (182). Temperature and growth (183). Water and growth (184). Growth cycles (186). Regulation of growth (186). Chapter Twenty-Five REST PERIOD OF PLANTS Seeds, bulbs, grass and trees in the dormant condition (188). Physiol- ogy of dormancy (189). Breaking the rest period (190). Using the rest period (191). Glossary ( 193 ) ILLUSTRATIONS Figures Page 1. Section of fruit (grain ) of corn 7 2. Seed of bean 8 3. Iris. Fruit and seed 9 4. Seedling of hollyhock and oats 12 5. Seedling of bean and pea 13 6. Resting cell and cell division 22 7. Linkage map for the fruit fly 27 8. Lima bean root 32 9. Cross section of a root 34 10. Longitudinal section of barley root 35 11. Young root with root hairs ., 36 12. Structure of a stem 54 13. The seasonal history of an elm twig 61 14. Methods of grafting 70 15. A section of a leaf 73 16. Stomatal penetration of fungus hyphae 99 17. Cross section of an infected radish leaf 100 18. A biennial plant, carrot 105 19. Vegetative propagation of weeds by stems 106 20. Zinnia flower 113 21. Petunia flower 114 22. Rose flower 115 23. Domesticated varieties derived from wild cabbage 144 24. Auxin and growth of Avena coleoptiles 153 25. The nitrogen cycle 178 ILLUSTRATIONS I. Tomato Seedlings 22 II. Fruiting Tomato Branch 23 III. Starved Cabbage Plants 54 IV. Whip Graft 55 V. Length of Day Effect 86 VI. Grass Improved with Fertilizer 87 VII. Mendel's Garden 118 VIII. Methods of Applying Colchicine 119 IX. Colchicine Treatment of Plants 118 X. Rooting Holly Cuttings 119 XL Buckwheat with Salt Deficiencies 150 XII. Cowpeas with Salt Deficiencies 151 XIII. Grass Treated with Fertilizers 150 XIV. Hydroponics 151 XV. Influence of Light on Growing Seedlings 182 XVI. Influence of Moisture on Growth 183 Chapter One INTRODUCTION Recreation must be a pleasure in order to give the most valuable relaxation from the daily business routine. The growing plant provides interest and pleasure for those who grow plants as a hobby or in their "Victory" gardens, be- cause both their minds and their bodies are stimulated. This kind of activity is only the summer recreation of some but the year-round interest of many who during the winter read of new plants and new cultural methods and plan ways of improving on the result of their past season's effort. The seed catalogues offer the beginner general guidance in plant culture but as his knowledge grows he is soon beyond the catalogue stage. The many questions of friends concerning their growing plants stimulated the author to select what seems to be the fundamental knowledge necessary for the solving of their problems. These people are already growing plants success- fully, but they are interested in the requirements of plants and the best cultural methods for meeting them. Our scien- tific age has been an important stimulus to study the basic reasons for the many plant responses. The following is an introductory survey of the point of view and the material covered in these various fields. To deal with the laws of nature as they govern the growth of plants requires a wide range of knowledge because growth involves the response of many activities of all parts of the plant to soil, water, temperature and air, as well as to the plant's enemies and other factors of its surroundings. This response of plants to their environment has been developed to the extent that it is known as the science of "Ecology." Z PLANT GROWTH The results of discovery are cumulative and no attempt will be made to trace the development of our knowledge of growing plants. The principles involving the chemistry and physics of growth will be explained in terms which will not assume a knowledge of these sciences on the part of the reader. In most cases only the facts pertaining to a particu- lar phenomenon are summarized, but a few brief descriptions are given of some of the more recent discoveries of these facts. An attempt has been made to present scientific knowl- edge in a direct and terse manner. If the reader will follow the text illustrations, or better, where possible, study his plants in the light of the text descriptions, he should find this method sufficiently detailed. Considerable thought has been given to presenting clear mental pictures of the structure and the internal work of the plant. In order to study or converse about the growing plant it is necessary to know the various parts of the seedling described in the next chapter as well as the whole plant in later chapters. Even that inner structure hidden from view because of the size of cells, as described in Chapter 4, may be studied by the aid of the microscope in order to under- stand its growth and specialization. In fact, it is the influ- ence of the environment of these cell structures which deter- mines their growth and in turn the growth of the whole plant. The many different kinds of cells found in plants are shown in the illustrations of plant structure. These cells are all inter-connected and carry on their particular functions through the protoplasm, which is a highly complex chemical containing a large quantity of water. This protoplasmic material is living and can grow or increase in quantity by its synthesis from water and the so-called food-making materials absorbed by the plant from the soil and the air. These detailed structures of the parts of typical plants INTRODUCTION 3 are stressed In various chapters to give the reader a clear understanding of the plant, necessary as a basis for the obser- vation and study of the growing plant. These parts should be compared and contrasted in different plants and in many cases described in brief notes for future comparisons. All experiments should be studied carefully and compared with normal or untreated plants. Often important differences may seem doubtful differences until such comparisons can be made. Such careful observation and checking are the final constructive steps necessary for the most successful improvement in methods or in plants. As the fundamentals of plant life are learned it becomes easier to understand fertilizer requirements, as described in Chapter 22, and to diagnose the plant's lack of certain food- making materials and of water for its absorbing roots. Water and fertilizers applied by gardeners using that knowl- edge have paid pleasing dividends in many "Victory" gardens. The soil, as outlined in Chapters 7 and 21, is such a com- mon material that it is usually thought of as only a source of water and of fertilizers for the plant. If that were the case, the use of water solutions or water gardening would be simplified. We speak of a good or of a poor soil, because of the plant's response not only to the water and the minerals, but to the many other components of a good soil, such as air, soil structure, and the life of the soil. Plants are often compared to factories in their activities. They take their raw materials from the earth and the at- mosphere and by using the sun's energy for power, convert them into many useful products. Sugars and starches are the first products, but these can be reconverted into any number of substances found in the plant, most of which are useful to man. The cell walls become wood, the proteins are useful for food or to make the parts of the seed, the coloring matters make the leaves green and give color to the flowers 4 PLANT GROWTH and fruit. We cannot enumerate the many uses we find for these manufactured products of the plant. We must, how- ever, bear in mind that as in any factory a balance is main- tained between the supply of raw material and the finished products. Hybridizing of plants is becoming so important and so popular that two extensive chapters are included to give a scientific background and a working knowledge of the com- mon methods. Of course the factual information covered in many other parts of the book is fundamental to the study of hybridizing. For example, the sections on chromosomes, flower structure, and seed development should be carefully studied before attempting any such plant improvement. Only a small portion of all the important and necessary knowledge for the reader who wants to know all about plant growth can be included in a single volume. It is hoped that with the illustrations and text diagrams and with suggestions about plants familiar to many people this may become useful as a work book, with help from the addition of selected refer- ences at the end of most of the chapters. The author has attempted to list representative titles, but realizes that many other equally important ones might have been added. Refer- ences of a general nature, such as the various textbooks of botany, review articles in special fields and research articles in small parts of special fields are included from which the reader will be able to select the ones best suited to his needs. Many of the illustrations were selected to show more detail than described in this book, in order that they might serve as an aid in terminology in more intensive reading. The illustrations should be found valuable aids in helping the reader to build a clearer mental picture of plant struc- ture. The work of the plant can be understood and enjoyed only as it is considered in relation to structure. Finally, in order to give further aid in the study of the growing plant, a glossary of difficult terms has been included. INTRODUCTION 5 It is necessary to use at least a limited terminology to de- scribe the subjects properly, but it has been restricted as much as seemed possible. The glossary also gives a reference to the further description and use of each term. REFERENCES Bailey, L. H., The Standard Cyclopedia of Horticulture, in six volumes, The Macmiilan Co., 1922. , Hortus, The Macmiilan Co., 1935. Peattie, Donald Culross, Flowering Earth, G. P. Putnam's Sons, 1939. Chapter Two SEEDS AND SEEDLINGS A seed is a potential plant living in the seed coat with stored food and awaiting favorable conditions of moisture and temperature for further growth. Seeds have many re- markable characteristics which have enabled them to repro- duce their kind through many past ages after withstanding conditions unfavorable to the mature plant. These charac- teristics, in many cases, have been and are important to the survival of the species. Many, but not all, cultivated seeds retain these characteristics, and this fact must be considered in their culture. Those that have lost some of these old habits under cultivation cannot survive among the wild con- ditions if they escape from the garden and the care of man. The breeding of plants for domestic use has made many changes in the seeds as well as in other parts of the plant. Seeds, such as beans, which are used for food have been developed to increase the amount of plant food stored in them; other seeds have been improved by the development of strains that germinate quickly and in a uniform time. Most mature seeds of wild plants are in a resting con- dition for varying lengths of time during which they are unable to germinate. Seeds of some cultivated plants such as petunia and portulaca fall to the ground during the sum- mer and remain dormant until spring; others, such as lark- spur, are dormant for a short period after which they grow a good tap root system and a rosette of leaves in the late summer; but the seeds of many cultivated plants have no resting period or one of only a few days. This lack of a r€st period explains the sprouting of farm crops when unfavor- able weather occurs at harvest time. Since seeds are capable 6 SEEDS AND SEEDLINGS. of germination, it is necessary to store them under conditions unfavorable to growth activities until planting. Cool, dry storage is advisable for most seeds. The seed coat consists of several layers of tough-walled cells. In some families, notably sunflower, zinnia (Fig. 20, C and E) and grass, a single seed grows in an ovary (see Fig. 1 showing it as fruit coat) the wall of which is so closely Fruit Coat Homy, fjndosperm Starchy, Endo$pet7n Cotyledon Fig. 1. Lengthwise section of fruit (grain) of corn showing the embryo with its parts embedded in the endosperm. (After Smith et al., A Textbook of General Botany. By permission of The Macmillan Company, publishers.) attached to the seed coat that they appear to be one. The gardener calls these seeds, but since the ovary wall is in- cluded, the botanist speaks of them as fruits. Hence we plant the fruits of corn and sunflower. The relation of seeds to the ovary and the fruit is further explained in Chapter 17. The seed coat is a protection against mechanical injury, and in a small degree against moisture loss or absorption. Some seeds, notably those of the clover family, have a nar- 8 PLANT GROWTH row layer in the outer row of cells, which is impervious to water. A few other seeds have seed coats so tough that the seedling cannot break it to escape. If seeds do not swell with a few hours of soaking it is best to break the seed coat, since it so frequently is a cause of delayed germination. Some seeds of wild plants have their germination delayed several years by their naturally tough seed coats, and thereby some of them may germinate at a more fortunate time for their survival than if all germinated at the same time. The hilum is the scar on the seed coat which was caused by the attachment of the seed to the ovary, and through which the food passed to the seed. This is very obvious in bean seeds (Fig. 2). The micropyle (Fig. 2) is a small pin- MICROPYLE HILUM HYPOCOTYL RADICLE PLUMULE Fig. 2. Seed of bean. A, side view. B, as seen from inner (attached) edge. C, from the outer edge. D, embryo; seed coat and 1 cotyledon removed. point pore below the hilum through which the pollen tube entered and toward which the radicle points inside the seed coat. Every seed must grow from a fertilized ovule in a flower, as described in more detail in Chapter 17. In the common garden pea each flower has an ovary which will develop into the mature pod. Each pea or seed in the flowering stage was only a small ovule consisting of a mass of cells with a single female sex cell. A pollen grain must fall on the stigma and grow the pollen tube bearing two male sex cells, one of which must unite with the female sex cell of each ovule. This is called fertilization and results in a powerful stimulation for SEEDS AND SEEDLINGS the growth of the ovary and seed. These two sex cells will become the embryo plant of the seed. If an egg cell remains unfertilized, the ovule will remain about as large as the head of a pin while the ovary will soon die and wither if none of its ovules are fertilized. The embryo of the seed is the resting or dormant stage of the small living plant within the seed. It consists of (1) a radicle pointing toward the micropyle, which emerges first in germination and develops into the root system of the plant; (2) one or more cotyledons; and (3) the plumule which is a group of folded leaves so small that it is seen with Fig. 3. Iris. A, B, surface view and cross section of the fruit. C, lengthwise section of a seed. (After Smith et al., A Textbook of General Botany. By permission of The Macrtiillan Company, publishers.) difficulty between the cotyledons just above (4) the epi- cotyl. The plumule produces the shoot when the seed germi- nates (Figs. 1 and 2, D). It will be noted that in many seedlings the first leaves above the cotyledons differ in size and shape from those appearing later. Many legumes have simple leaves above the cotyledons and all later ones are compound. The largest part of the seed is that which contains the supply of stored food for the growth of the seedling until it is large enough to make food. In albuminous seeds, such as 10 PLANT GROWTH castor-bean, Iris (Fig. 3) and corn (Fig. 1), the food is located in a region called the endosperm, which is the prod- uct of the polar bodies of the ovule and the second sperm. In exalbuminous seeds, such as the common bean, the food is stored in the two cotyledons ( Fig. 2 ) . These fleshy bodies, readily distinguishable in cooked beans, are the first struc- tures to appear above the ground in germination. Plants are classified according to the number of cotyledons or seed leaves of the embryo. Those growing from seeds with one cotyledon, such as corn, are called monocotyledons; those growing from seeds with two cotyledons are called dicotyledons. The cone-bearing trees, such as pines, bear seeds with more cotyledons, usually from five to eleven. Monocotyledonous seeds of the grass type have a small embryo and a large endosperm. The embryo is only about 8 per cent of the wheat grain, but it is a little larger in corn. The epithelium (shown as a double line in Fig. 1), is a single layer of cells between the embryo and the endosperm, which secretes enzymes to digest the stored food when conditions are favorable for germination. The iris type has a small embryo (Fig. 3, C) embedded in the endosperm. Such monocotyledonous seeds as the grasses and orchids are so small that their structures are difficult to see and their germi- nation is poor because the supply of stored food is so limited. The best seeds should always be used by the gardener, because the difference in cost is so small when compared with the difference in the plants grown from the best and from inferior seeds. Select well-bred seeds of good varieties from reliable seedsmen. Home-collected seeds of many species produce inferior plants, largely because of cross pollination. For those who are interested in developing their own seeds suggestions for artificial pollination are described in Chapter 18. The vitality or viability of a seed, which is its internal capacity to germinate and grow under favorable conditions, SEEDS AND SEEDLINGS 11 is affected by the conditions under which the seed is devel- oped, the age of the seed and the conditions of storage. Seeds developed under unfavorable conditions will produce inferior plants. Immature seeds lack vitality because of in- sufficient stored food, and seeds past their maturity lose vitality with increasing age. High moisture content and high temperatures in storage increase respiration and lower vitality. Longevity refers to the length of time a seed can retain its vitality in storage. The longest record of longevity in seeds is held by Indian lotus, which germinated after being buried in peat for two hundred years. Many seeds remain viable for ten or more years, but most of our garden seeds have a longevity of less than five years. The facts regarding the age and germination after thousands of years of seeds from Egyptian tombs are too fragmentary to be credited here. The seed uses the stored food while it develops into a seed- ling by first sending a radicle or primary root into the soil and later the plumule or shoot into the air (Figs. 4 and 5). The seed should be planted deep enough to anchor it while the primary root enters the soil, otherwise, the root will push the seed out of the soil instead of itself going deeper into the soil. If the seed is planted too deep, energy is wasted push- ing the shoot through the soil, so that in extreme cases it may be unable to penetrate. The early root growth is much more rapid than the shoot growth. This enables the root to absorb water and mineral salts for the rapid growth of the entire plant which follows. Since the seed must furnish all the food for the early growth of the seedling, it is not surprising to find a well- balanced combination of starches, proteins, minerals, en- zymes and vitamins in storage form. Frequently minerals are in larger amounts at the seed surfaces, but the enzymes and vitamins are more concentrated in the embryo. 12 PLANT GROWTH Fig. 4. Seedlings of hollyhock (left). The smallest seedling shows the early root growth, the next the cotyledons protecting the shoot as it pushed through the ground, and the largest shows the two cotyledons and the shoot between them. Oats (right). The pointed coleoptile pushes through the soil, after which it splits and the true shoot emerges (2/3 natural size). (By Antoinette K. Ketner.) SEEDS AND SEEDLINGS 13 The growth of the seedling is oriented by its response to gravity, therefore it is not necessary to give consideration to . * .* **-■-■ <^ •. ■'.*. /__ Fig. 5. Seedlings of bean (left). The cotyledons protect the shoot as it comes through the soil. The largest one shows a pronounced primary root. Pea (right). The cotyledons remain where they were planted, but the shoot pushes through the soil in a bent position protecting the growing point (i natural size). (By Antoinette K. Ketner.) the position of a planted seed. The radicle will even make a sharp bend to grow downward if the seed is so placed that it points upward. The same is true of the plumule. This 14 PLANT GROWTH direction of growth has recently been found to be due to the growth hormones, which are discussed in Chapter 20. Soil frequently forms a crust on the surface through which the shoot must force its way. Several methods of protecting the tender growing cells may be observed, as shown in figures 4 and 5. The common bean and hollyhock push the cotyledons through the soil by the elongation of the hypocotyl, with a sharp bend at the top, before the tender plumule emerges. The garden pea pushes the plumule up- ward in a shepherd's crook position protecting the growing tip, while the cotyledons remain where the seed was planted. In the grasses, as is illustrated by oats, the cotyledon remains in the endosperm while a tough sheath, called the coleoptile, fits over the end of the growing shoot until it emerges, after which the sheath splits to free the plumule. Most albumi- nous seeds other than the grasses germinate similarly to the common bean and bring the endosperm into the air with the cotyledons before the plumule appears. The primary root grows secondary roots and in most cases is the beginning of the root system of the mature plant. The tap root is a continuation of the radicle in most cases. In the grasses a permanent root system develops later at a uniform distance from the soil surface regardless of the depth of planting. When the cotyledons come above the ground the hypocotyl, meaning the part which is below the coty- ledon, has a structure unlike the older shoot. The cotyledons of some plants, such as the tomato, shown in the photograph (Plate I), become green, grow and make food, but many others lose their food to the growing plant and gradually shrivel and later fall off. The seedling stage is considered past when the plant no longer depends on food supplied from that stored in the seed. Chapter Three GERMINATION OF SEEDS The optimum conditions for germination vary for differ- ent species of seeds, but even for a given species of seeds the conditions may vary over a wide range, as described later under soil moisture and temperature. The gardener always strives to reach the optimum for each kind of seed. After selecting good seeds, the following should always be con- sidered: the best moisture condition, the best temperature, and adequate air for the oxygen supply. The seed must absorb a large amount of moisture, often double its dry weight. A soil which is so dry that it supplies water to the seeds more slowly than a moist soil, retards their germination. Since most seed coats are permeable to water on all surfaces they should have the moist soil pressed tightly against them at all points when they are planted. For this reason the gardener often walks on the planted row. Other methods of compacting the soil are: tamping with a hoe or board, or with a horse-drawn roller. Soaking seeds in water before planting is not only unnec- essary but actually may be harmful in that it reduces the germination vigor because of the loss of mineral salts to the water and the lack of oxygen which is essential for respira- tion. Water is absorbed by seeds with so great a force that it is readily taken from soil. If a thin-walled glass bottle is filled with dry peas and water is added, they will absorb the water and swell with a force great enough to break the bottle. Other experiments have shown that water enters dry seeds against a force of several hundred pounds per square inch when they are in soil with less than the optimum water con- tent. This accounts for the rapid and advantageous absorp- tion direct from the soil. 15 16 PLANT GROWTH The temperature of the soil at planting time plays the most important part in the speed of germination of many seeds because the rate of water absorption and of enzyme activity like all other molecular and chemical actions are regulated by temperature. Because of the difference in the optimum temperature for the enzyme activity, some seeds, such as peas, germinate better at low temperatures than other seeds, such as zinnias. At a temperature below 40° F. it requires six to ten times as long for red clover to germinate as at the optimum temperature of about 60° F. Since all the passages through the seed coat are extremely small and are filled with water, the oxygen for respiration must be dissolved in the soil water before it can enter the seed. When the moist soil is packed around the seed, as ex- plained above, the soil moisture comes in contact with the moisture in the passages of the seed coat and the oxygen can enter the seed through these water passageways. The air spaces in the soil supply the oxygen to the water as it is absorbed by the seed. Since the seed needs both oxygen and water, the importance of an optimum amount of each is evi- dent. However, excessive rain causes an oxygen deficiency because as the water content of the soil increases, the soil air decreases. Seeds may fail entirely to germinate in an exces- sively wet soil, also the growth of plants is retarded and their leaves turn yellow with excessive rain. A loose porous soil supplies air most effectively. Chapter 7 describes soil con- ditions more fully. Seeds planted nearest to the optimum conditions will germinate quickest. Planting time must be adjusted to the conditions of the soil and temperature, but early planted seeds frequently produce earlier plants than those planted under more favorable conditions even though their rate of germination and growth was slow. The minimum tempera- ture for germination of peas is under 40° F. while for cucum- ber it is above 60° F., hence peas may be planted two or GERMINATION OF SEEDS 17 three months before cucumbers and zinnias. Personal ex- perience and suggestions from seed catalogues are good guides for the observant gardener in determining the best planting conditions. The physiology within the seed resulting in germination is regulated by three external conditions: temperature, mois- ture, and oxygen. Since food is stored for early growth only water and oxygen must be absorbed. The following changes take place simultaneously during germination in a closely connected and related way. (1) Water diffusing through the seed causes it to swell and weaken the seed coat. (2) The enzymes become activated and dissolved in the water and diffused to the stored food. The enzymes, or the sub- stances which become enzymes, appear to be in the dry seeds and sometimes in localized areas; for example, the enzymes of the corn seed are in the epithelial layer (Fig. 1) . The rate of enzyme action increases rapidly with an increase in temperature, in fact in many cases an increase of 18° F. will double the action of the enzyme. With a favorable tem- perature, the enzyme actions in a germinating seedling ap- pear to supply soluble food materials, as indicated by their abundance, more rapidly than they can be used. (3) The stored food of the endosperm or cotyledons is made soluble and diffusible by the enzymes: large molecules of starch, insoluble in water, are broken into smaller molecules of water- soluble sugar; molecules of fats are converted into fatty acids; and molecules of proteins into amino acids. (4) The food diffuses to the growing parts of the embryo. (5) Part of the food is used in respiration or plant oxidation^ to supply the embryo with energy. (6) Part of the food is used to 1 The process of respiration illustrates the kind of oxidation taking place in plants and is the reason for their need of oxygen in the germination process. This can be tested by putting a quantity of soaked seeds in a thermos bottle and measuring the increased temperature at the end of one or two days. The experiment is more successful if a small bottle of sodium or potassium hydroxide solution is placed in the bottom of the thermos bottle to absorb the carbon dioxide from the respiring seeds. 18 PLANT GROWTH build more protoplasm and to make more cells, thus increas- ing the size of the embryo. The rate at which these numer- ous activities in a germinating seed proceed in a coordinated manner depends in several ways on the conditions of the en- vironment with regard to temperature, moisture, and oxygen, any one of which may limit the rate of development. Since the stored food is being used for respiration and growth by the young embryo, the seedling will lose in dry weight until it is able to carry on photosynthesis in the new leaves to make food at least as fast as it is being used. A young seedling several times the size of the seed may have a dry weight of less than 60 per cent of the original seed. This stage in the life of the plant is a critical one because it has not developed protective tissues against insects or loss of moisture and has a small reserve of food. Wheat has used more than 80 per cent of its stored food during the first nine days of germination, after which it is largely dependent on food from photosynthesis. If seeds are planted too deeply or if some other condition is unfavorable for rapid germina- tion, they may exhaust the stored food before they are able to synthesize it fast enough for vigorous growth. The fact that seedlings will grow slowly or may die be- cause of lack of food may be employed in a number of ways to control weeds. They are easily pulled at the seedling stage because of the small root system, or they can be covered and soon starve to death. Weed seedlings will be less likely to establish themselves in a lawn where the grass is cut two inches long than in one where it is cut less than an inch long, because the additional shade retards photosynthesis, further weakening the young weeds when they are deficient in food. If two sets of seeds are planted in soil and given compa- rable conditions except that one set is kept in the dark and the other in the light, the seedlings in the dark will "grow" at an astonishing rate. They appear to be growing faster than those in the light, but if they are examined more closely it GERMINATION OF SEEDS 19 will be seen that the stems alone are making rapid growth. They are becoming etiolated. The internodes will be long but perhaps there will be no more of them. The stem will be smaller in thickness and will be softer in texture. The leaves will be small, with long petioles. If the roots are examined, those of the plants grown in the dark will be found to be smaller and poorly branched in contrast with those of the plants grown in the light. This difference in the character of growth is believed to be due to the effect of light on a growth-regulating hormone. The plants grown in the dark are heavier due to more water in the larger cells, but if the dry weights of the plants are determined, it will be found that the larger plant, grown in the dark, has not produced as much plant material as the plant which was grown in the light. The photograph (Plate XV) showing pea and radish seed- lings, and the table on page 181 illustrate what happens under these conditions. Some mineral salts are absorbed by the very young seed- ling, but the amount is negligible in comparison with the weight of the stored food or later with the weight of the manufactured foods. A favorable supply of mineral salts in the soil, however, will stimulate growth very early in the seedling development. If a quickly available nitrogen fer- tilizer is applied, it may increase the early growth of the shoot, and cause the plant to be a little earlier in its flower- ing and fruiting. Most seeds should be planted as shallow as possible in order that they may get an abundance of oxygen, but deep enough to give them constant moisture. While it is easier to plant them too deep than too shallow, a good rule is to try to cover them with a depth of soil four to six times the diameter of the seed. A few seeds, such as some lawn grass seed, need a little light to germinate best, and must, therefore, be cov- ered very little if at all. If the soil around young seedlings becomes so dry that water cannot enter the seedling, it will 20 PLANT GROWTH soon die because of the destruction of the protoplasm. For this reason watering is essential until the root is deep enough in the soil to have a more uniform water supply. The gar- dener can expect many kinds of seeds to come through the ground in about four days if the moisture and temperature are optimum, but others may require two weeks or longer even under ideal conditions. Many seed catalogues give a table of dates to plant and the time required for germination. Seedsmen must have their seeds tested for purity and germination before selling them, but seeds that have been kept for some time should be tested before planting. A simple method consists of using a dinner plate with two pieces of cloth of coarse texture well soaked in water and wrung until moderately wet, between which the seeds are placed where they will absorb enough water. They should be covered with a piece of glass or a second plate to prevent the loss of moisture, and placed in a favorable temperature. The number of seeds should be large enough to give an accu- rate test. They should be examined at two-day intervals and the seedlings removed as they germinate. The cloths and plates should be put in boiling water in order to kill molds that accumulate before being used again for testing. Healthy seedlings that will develop into plants with strong stems and well-branched root systems may be ex- pected from carefully selected seed planted where there is plenty of light in well-prepared soil of relatively low moisture and medium nitrogen content. These conditions make strong seedlings which are a necessary foundation for a suc- cessful garden. Chapter Four ^* ^^ CELL STRUCTURE AND PROTOPLASM The unit of life is the cell, and one or more of these units constitute the structure of plants and animals. Although widely varied in size and shape, most cells are microscopic, somewhat elongated and box-like in structure, with each of their several sides attached to another cell some- what in the fashion of the cells of a honey-comb. The cells in most tissues are so small that in cross section one hundred to one thousand are found per linear inch or ten thousand to more than a million per square inch. Most of them fit together in a definitely organized pattern that gives great strength to a tissue while in some tissues of the plant large cavities occur between and at the corners of the cells. The complexity of the plant is the result of the many parts and of the many different cells of which it is made. The actual number of cells in a bean, petunia or grass plant is not so important if we remember they are so numerous that they have never been counted, but the various kinds have been very well studied and described. It has been said that the surface of the leaf has more than a million radial cell walls per square inch to support it. If we look at a leaf with a mental picture of its cellular structure as shown in the dia- grammatic view of the cells (Fig. 15), highly magnified, we get an idea of the nature of the cell arrangement; those in the blade at the right have large cavities between them while those at the left in the midrib fit closely. Further descrip- tions of cell arrangement and size will be found in the chap- ters on the root and stem, but the details of individual cells are too small to be shown here. As shown in the resting stage of Figure 6, a cell has three distinct parts: the cell wall, the protoplasm, and the non- 21 22 PLANT GROWTH 1. Resting stage '■' 'p ^'W ■o .2. Prophase 3. Prophase m 4. Pirophaae 1 '/■■.■ ■■"','■"'■'"■ -'■ "■'"'■'-' '•-■^^.^^'"^^'■■^'■■•-■■^^ :• AmJI 'V^^S^V^v ■^^ ^:;: ■■/-C'lV-'-xX-'*-- ■'»*-'*>.• -■■" ■"■. ,■'■'■■■' ""-• .■'"- ^■■>" >""-■ ■ 5. Metaphase ;'i?i?i^^^ ■ : V •■■:■'•■.■■,.■- '■''•'..•^'^'^'■(■ifc-j-fc'-f.,-^; ;s 6. Anaphase V. Telophase 8. Telophase 9- Telophase Fig. 6. Semidiagrammatic representation of cell division in meristematic cells of a plant, showing nuclear division by mitosis. 1, Cell before division begins, with the nucleus in the resting condition. The chromatin threads form a network. 2, 3, 4, Pro- phase of nuclear division. In 2 and 3 the double nature of the chromatin threads is apparent. In 4 the chromosomes are distinct and their doubleness still apparent. 5, Metaphase of nuclear division; nucleolus and nuclear membrane have disappeared, the spindle has been formed, the chromosomes lie in or near the equator of the spmdle, and some of the fibers are attached to the chromosomes. 6. Anaphase. Movement of Plate I. Three stages in the growth of tomato seedhngs to show the development of the cotyledons and plumule. Plate II. A branch of a tomato plant showing compound leaves with a small growing bud at the lowest leaf and the bud on the next leaf grown into a branch. CELL STRUCTURE AND PROTOPLASM 23 living contents. The cell wall, indicated by the border line, is a non-living secretion of the protoplasm, most of which is cellulose, a substance similar to starch in its composition. This stretches as the cell increases in size, and becomes thick- ened with the age of the cell by addition from the proto- plasm. Recent studies have shown the wall to be made of an extremely fine network of needle-like crystals. Most important is the strength and shape the cell walls give to a plant. The main portion of the cell, the protoplasm, is the living, nearly transparent material which may be further divided into cytoplasm and a spherical more dense portion with a net-like structure, the nucleus. The protoplasm is a complicated chemical compound made up of carbohydrates, proteins, water, fatty substances, and simple compounds of mineral salts so organized as to have the characteristics of life. The synthesis of the protoplasm is an important function of growth similar in all organisms, so far as is known at present. Next to water, protein is the most abun- dant, most variable, and therefore most studied constituent of protoplasm. In addition to the elements carbon, hydro- gen, and oxygen, found in carbohydrates and fats, proteins have nitrogen and sulphur, and usually other mineral ele- ments in very small amounts. The non-living often invisi- ble content of the cell, scattered through the protoplasm, is made up largely of solutions of salts, sugars and other food materials. This non-living food matter is usually either in one or more spaces, called vacuoles, near the center of the cell or in many vacuoles scattered throughout the protoplasm. The above is shown in outline form with additional sub- divisions. daughter chromosomes away from the equator of the spindle. 7, 8, 9, Telophase. In 7, daughter chromosomes form two compact masses. 8, Reorganization of resting nuclei begun, the double nuclear threads become apparent, nuclear membrane and nucleolus appear. In 9 the nuclear net is forming and the new cell wall separates the two new cells. (Reprinted by permission from Holman and Robbins' Textbook of General Botany, 4th edition, John Wiley and Sons, Inc., 1938.) The cell 24 PLANT GROWTH Cell wall (Cellulose) {Cytoplasm (water, proteins, carbohydrates, fats, and mineral Nucleus, which contains the chromatin (similar to cytoplasm, but the protein has a high phosphorus content) Non-livine 1 Water solutions of salts, sugars, and other foods content i Insoluble foods as starches and proteins L Wastes The cell wall varies in thickness with the function of the cell. In some strengthening cells such as those in wood, the cross-sectional area of the wall may be as great as the re- mainder of the cell. Even succulent vegetables such as asparagus may become woody as the cell walls thicken. Rapid continuous growth produces more desirable vegetables because in rapid growth the cell walls stretch thin as the cell increases in size. The function of a cell wall is comparable to an automobile tire that resists the expansion of the inner tube, since it re- sists the expansion of the protoplasm and gives shape to the cells and to the plant. The pressure of the protoplasm against the cell wall often exceeds one hundred pounds per square inch; the air pressure in a tire is from twenty-five to fifty pounds. When excessive loss of water from a plant re- duces this protoplasmic pressure against the cell walls to zero, the plant wilts, in other words water pressure prevents wilting. The portion of protoplasm exclusive of the nucleus is known as cytoplasm. In addition to its complex chemical nature it appears to have very definite structural nature or physical complex. Chemists can analyze protoplasm and tell us its elements, but the physicist has been unable to give us the details of their arrangement, which is generally be- lieved to be the "key" to life. The cytoplasm absorbs and holds water with an enormous force, and the withdrawal of large amounts of water may kill it. An exceedingly thin layer of cytoplasm, with different properties from the rest, lies next to the cell wall and seems to control the substances CELL STRUCTURE AND PROTOPLASM 25 that enter and leave the cell. For example, if an uninjured beet is soaked in cold water, the color and the mineral salts will be held in the beet by this layer, but if the cytoplasmic layer is injured, as by heating the water, the color and min- eral salts will diffuse from the beet into the water. The green color of plants and most of the other colors are located in special parts of the cytoplasm called plastids. A single cell of a leaf may have several hundred of the green plastids called chloroplasts, named from the pigment chlorophyll. The cells in the leaf diagram of Figure 15 show small numbers of chloroplasts located near the cell walls. The nucleus is often spoken of as the center of life be- cause cells cannot live without the nucleus. It is believed to control the activities of the cell by its secretions, but this needs further study because of our rapidly growing knowl- edge in the fields of hormones and of enzymes, and their regulatory power. Since the size of the nucleus of a young growing cell is about 10 per cent of the total cell but decreases with the age and diminishing activity of the cell, it suggests that the nucleus may secrete the above all-important sub- stances as one of its functions. The highest phosphorus , content of the cell is in the nucleus. The material of most interest in the nucleus is a protein called chromatin, which in very small granules is scattered, doubtless in an orderly manner, through the nucleus. At the time of cell division these chromatin granules are clearly seen to make larger pieces in an orderly fashion called chromosomes (see 4, 5, and 6 in Fig. 6) . Very recently a group of scientists claimed that by proper techniques it may be demonstrated that the chromatin retains its orderly identity at all times. The number of chromosomes in a cell is always the same for all the cells throughout a plant and is generally uniform for the same species or variety. Many plants, but not all, have had their number of chromosomes determined by actually counting them in highly magnified dividing cells. 26 PLANT GROWTH The counting of chromosomes is exceedingly difficult be- cause of their small size. A few plant cells have chromo- somes large enough to be seen clearly when magnified fifty times, but in many others they are so small that they appear as dust particles when magnified a thousand times. Most of those counted fall within a range of from twelve to twenty- four for each nucleus in the plant cell. In a few plants the phenomenon of polyploidy has occurred naturally, and recently artificial treatment has induced it in many others. Polyploid plants are those which have more chromosomes than is common for the species, often double the number, but always a multiple of the number found in the sex cells. How the identity of chromosomes, and the chromosome con- tent can be maintained and transmitted from generation to generation has been the subject of much study. Cell divi- sion gives an idea of the mechanics of chromosome conti- nuity but the physiology of control remains a secret. Chromosomes, believed to be that part of the sex cells which transmits and gives to new organisms their character- istics, are so important that their structure and behavior have been the subject of extensive study. From this study a body of facts has been deduced and certain hypothetical theories have been projected, some of which are supported by con- vincing evidence and are used in the study of inheritance. Chromosomes are found to consist of numerous small units of chromatin called genes which are arranged in a single row similar to an orderly row of blocks or of popped corn, each of which differs from the others in some way. This differ- ence may be one of chemical arrangement in the protein molecule. Since these units, thought to be the genes, have been seen in the salivary gland cells of the common fruit fly and, less clearly, in certain plant cells, they are believed to exist, and further evidence substantiates the theory that each becomes a determiner of, or a carrier for, a particular characteristic. In other cases a number of genes together CELL STRUCTURE AND PROTOPLASM 11 ■yh± ,\ lo.t •, 0.J i \Q6 '. Y '. 15 \\z.i \ 4.5 . \ B.5 +\ \6.9 75 137 ■ • \I6.± ■ • "lit 20. 21. 27.5 T 27.7 m lY yellow CB) Hairy wing (VJ) scute CH) lethal -7' broad (W) prune CE) whiteCE) face+CE) Notch CE) Abnormal (B) echinus (E) bifidCW) ruby(E) crossveinless CW) club CW) deltexfW) cut Cw> singed CH) +an CB) lozenge CE) f 0. 2. 3.± telegraph (VV) Star CE) aristalessCS) 0. roughoid CE) • 6.± expanded Cw) 12.t 13. 14.+ 16. Gull Cw) Truncate Cw) dachsous CB) Streak CB) 20. divergent CW) ■ ■ 3t. dachs Cb) ■35. Ski-H (W) 26. 26.5 sepia CE) hairy CB) 35. vermillionCE) 36.1 miniature Cw) • -41. Cammed Cw) 36.2 dusky CW) Sa* furrowed CE) 35. rose Ce) 36.2 cream-ffl CE) 45. sable Cb) 44.4 garnet Ce) 54.2 small wing 54.5 rudimerrtnryOiV) 56.6 forked CH) Bar CE) small eye fused (W) 57. 583 59. 59.6 62. 65. Beadex CW) Minute-n CH) Cleft Cw) 46.± Minute-eCH) 485 black Cb) , 48.7 Jaunty Cw) 54.5 purple CE) 57.5 cinnabar Ce) 60.± safraninCE) - ■ 64.± pink-wing CEW) 67. vestigial (w) 68.± telescope CW) - - 72. Lobe CE) ■ 74.± qap Cw) -»J. 70. bobbedCH) +755 curverd Cw) 40.1 40.2 40.4 42.2 Minute-h CH) tilt Cw) Dichaete CH) thread CB) 835 fringed Cw) ■ • 44. scarlet CE) 48. pink Ce) 49.7 maroon CE) I50.t dwarf CB) 150. curled CW) 545 Hairy wing supr 58.2 Stubble CH) 58.5 spinelessCH) , 587 bithoraxCB) '"59.5 bithorax-b ,62. stripe CB) ^65.1 gloissCE) 66.2 Delta Cw) 695 hairless Ch) 707 ebony Cb) 72. band CB) 75.7 cardinal CE) 76.2 white ocelli CE) -90. humpy Cb) 995 arcCw) fOO.5 plexus Cw) I02.± Tethal-Eoi 1105. i>rownCE) , ai05.± blistered (W) 106. purpleoid CE) |I07.± moruJaCE) 607. speck CB) , 107.5 balloon Cw) bent Cw) shaven CB) eyeless CE) rotated Cb) Minu+e-IY(H) male fiartility Long brifetleo( i 91.1 rough Ce) 93. crumpled Cw) ■ ': 93.8 Beaded Cw)^ 94.1 Painted Cw) ^ 1007 claret Ce) - - 101. Minute CH) mar© fertility i 106.2 Minute-9 fH) Fig. 7. Linkage map for Drosophila melanogaster, showing relative positions of many of the known genes in the chromosomes as determined genetically. The letters in parentheses indicate the portion of the fly in which the characters appear: B, body; E, eye; H, hairs; W, wings. (From Sharp's Introduction to Cytology, 3rd edition adapted from Morgan, Sturtevant, and Bridges (1925) and Stern (1929), McGraw-Hill Book Co., 1934.) 28 PLANT GROWTH are thought to be responsible for a single characteristic. The number of genes to a characteristic chromosome is variable, but usually there are more than one hundred and in some cases perhaps many hundreds. The exact location of genes in the chromosomes has been attempted as is shown in the study of the fruit fly (Fig. 7). It should be borne in mind that most of the evidence for genes and all the evidence that they carry hereditary characteristics is experimental and not something seen with a microscope. This will be de- scribed further under the title of "the hybridizing of plants." Cell division must be understood to appreciate plant growth and to deal with plant breeding. Cells divide only where growth is taking place. These growing places are the meristematic regions found at the tips of all roots and all stems, in the cambium region of dicotyledonous plants and at the base of the internodes of monocotyledonous plants. A few specialized regions of division might be added such as in very young growing leaves, in the developing flower parts, and in fruits and seeds. It should be remembered that every organism begins as a single fertilized egg cell and by the process of cell division, growth and specialization a single cell becomes a plant or animal with many millions of cells. Reduction division, in which the number of chromosomes is reduced in the formation of sex cells, is described in Chap- ter 18. The diagrams of Figure 6 show certain stages of divi- sion in a continuous process which can be better shown with motion pictures. It should be noted in numbers 2 and 3 that in the resting cell the chromatin of the nucleus forms a thread. This thread splits longitudinally in number 4, and in number 5 is broken to form the numerous chromosomes each of which contains a portion of every gene. These newly formed chromosomes grow to normal size and arrange themselves across the equator or center of the cell as shown in number 5 and then pull apart and begin to move toward CELL STRUCTURE AND PROTOPLASM 29 the opposite ends of the cell in numbers 6 and 7, so that each end will have exactly duplicate sets of chromosomes, like the cell from which they came. A wall forms in numbers 8 and 9, dividing the cytoplasm to make two new cells. The chromatin returns to the resting condition in number 10. This phenomenon is repeated hundreds and hundreds of times in the development of every plant, in fact it takes place during all the growing periods throughout the life of the plant. REFERENCE Sharp, L. W., Introduction to Cytology, McGraw-Hill Book Co., 1934. Chapter Five ROOTS The root will be used as a beginning point for a detailed study of the structure of the various parts of the plant. This will be followed in the two succeeding chapters by root functions as related to the soil. Roots are usually the least studied of any part of the plant because they are inconspicu- ous and difficult to free from the soil. When their important functions are appreciated it becomes clear that they are of the utmost importance for the best growth of the plant. The root systems of plants are divided into primary, secondary, and adventitious roots. The primary root of a plant develops directly from the seed when it germinates. The roots coming from the primary root and all their branches are known as secondary roots (Figs. 4 and 5) . All other roots, whether they arise from stems or leaves, are ad- ventitious roots. All secondary roots originate within the root while it is young, near the xylem region, and push through the outer tissues into the soil. Root systems are divided into two groups according to the nature of their growth. A tap root system results from the continuous and vigorous growth of the primary root, such as in carrots, alfalfa, and many of the trees. The gar- den carrot has a tap root, the top of which is modified for food storage but a thin part of it may penetrate the soil more than seven feet, according to Weaver and Bruner in their book on "Root Development of Vegetable Crops." They found the secondary roots extending in every direction from the tap root to a distance of two and a half feet or more. A longitudinal view of a carrot may be used to describe some of the structures common to young roots. The central 30 ROOTS 31 part, usually more than one-third the total diameter of the root, is xylem or stele through which conduction of water and other substances takes place. The cambium is at the outer edge of the xylem while the more tender outer part consists mostly of cortex with the pericycle and phloem, a narrow band, next to the cambium. The secondary roots can be seen as light-colored threads passing through the cortex region from their origin in the xylem, where they grow when the root is very small. While they push through some tis- sue, much of the cortex grows around the secondary roots after they have grown some distance into the soil. Seeds of plants with tap roots are usually started in permanent beds because tap roots broken in transplanting branch and may weaken the plant. It would be an important problem to determine the effect in several types of soil of transplanting on a wide variety of plants with a pronounced tap root. In sharp contrast to the tap root, the fibrous root system results from a slow-growing tap root and fast-growing secon- dary roots, such as in grasses (oats. Fig. 4), or, as in the ad- ventitious roots, from a more or less even rate of root growth, such as is common from the nodes of certain grasses. This type of root system is usually more shallow than a tap-root system and may suffer sooner from drought. Some of the horizontal roots of both systems may be so near the surface that they are easily destroyed in cultivation. These hori- zontal roots may be in dry soil where frequently no water is available. However, since the soil is always well aerated near the surface, these horizontal roots are very important to the plant in its absorption when the deeper soil is so wet that its oxygen content is seriously reduced. The root system of a maturing lima bean plant shown in Figure 8 represents a weak tap-root system, with extensive branching near the surface, and illustrates the type of system which suffers from drought because many of the roots are in the upper four inches of soil. This shoot was nineteen inches 32 PLANT GROWTH high and the leaf spread was thirty inches in diameter in comparison with the root depth of five feet and root spread of more than six feet. In such a plant the absorbing area of the root system is considerably greater than the surface of the shoot, through which the water is lost. Adventitious roots arise at any unusual place such as on a vine or tomato stem that comes in contact with the soil. Fig. 8. The great extent of a maturing lima bean plant is shown. Two hundred cubic feet of soil were ramified by the roots of a single plant. (From Weaver and Bruner's Root Development of Vegetable Crops, McGraw-Hill Book Co., 1927.) Cuttings depend entirely on the development of adventitious roots. In transplanting, should a large part of the normal absorbing root system be destroyed adventitious roots will develop. It is doubtful if adventitious roots are as success- ful for plants as the normal ones, when distribution in the soil and the union with the plant is considered in relation to the growth of the mature plant. It must be noted, however, that this question is not settled for all plants, and many ROOTS 33 plants are improved by the increased number of adventitious roots caused by transplanting. Probably the whole prob- lem of cuttings as compared with other means of propagation rests on this question. It is also possible that with the newer methods of chemically stimulating the rooting of cuttings they will be more successful. A number of simple experi- ments will suggest themselves to those interested. The structure of a root is complicated to fit it for the many functions of the root system. The chief functions are ab- sorption, conduction, storage, and anchorage; The younger, more active and interesting parts of roots can be seen to the best advantage if they are grown in a very loose, open soil, from which, if they are lifted gently, the root cap and grow- ing region, about one-fourth inch long, will protrude from a mass of soil held around the older root by a mat of root hairs. Or seeds may be germinated on moist paper where the in- dividual root hairs may be seen from the smallest to their normally mature size, as shown in the enlarged view of Figure 11. Absorption is so important to the growth of plants that the following chapter has been devoted to its more detailed study, but it should be noted that only the younger portions of roots, those without a cork layer, are permeable. Most of the water and mineral salts enter the roots through the root hairs shown much enlarged in Figures 9, 10, and 11. They are extensions of certain of the outer cells of the root. Since the root must supply these materials for the entire plant, it is not surprising to find cells adapted to conduction at the regions of absorption, shown best in Figure 10 as tubes, collectively called xylem in the cross section of a young root in Figure 9. The phloem cells, through which foods reach the roots from the leaves, are less prominently developed here. In older roots, the xylem is a large part of the root. In perennial plants the roots continue to add to the xylem, phloem, and cork areas, but the cortex may decay and dis- appear. 34 PLANT GROWTH Parenchyma [ Endodermis Root Hair Epidermis Parenchyma Fig. 9. Cross section of a young root. Root hairs and the small amount of xylem and phloem are characteristic. (From Smith et al, A Textbook of General Botany. By permission of The Macmillan Co., publishers, 1935.) ROOTS 35 hi Region of Root Hairs -I Epidermis Spiral Vessel j^Central Pitted Vessel Growing Point 1 Boot Annular Vessel Root Hair iff r - lEndodetmia Cortical EiParenchyina ir'-Pericycb Procambiuni Fig. 10. Longitudinal section of the root of barley {Hordeum sativum). (Re- printed by permission from Holman and Robbins' Elements of Botany, John Wiley and Sons, 1938.) 36 PLANT GROWTH The growth in length of the root and its pushing through the soil has been of interest to many. Darwin tried to figure the energy necessary for a root to enter the soil and in his investigations did find that it grows with a rotating motion, which enables it to go through the softer areas or Region of cell division' Region of elongation Root cap Fig. 11. Enlarged view of the end of a root, showing root cap, growing region and root hairs. (From Transeau, Sampson and Tiffany, Textbook of Botany, Harper and Bros., 1940.) Openings in the soil. The force, more recently, has been found to be derived from osmotic pressure. The growth in length, shown in the section of Figure 10, takes place within a quarter of an inch, just back of a group of tough cells called the root cap, often visible with the eye. The growth area may be divided into two regions; the grow- ing point where the cells divide, which is about one-thirty- ROOTS 37 second of an inch long and is found just back of and inside of the root cap, but merging into the second, where most growth in length occurs. Fully grown cells may reach fifty or more times their length at the time of cell division. When fully developed they begin to change their character in the region of the root hairs to become the various tissues of the mature root. These areas of development may be located in a general way on a young root similar to the one in Figure 10, when the root cap and dividing areas are slightly darker. The root hairs appear as a white covering. Food storage is an important function of the roots of biennials and perennials. The abundance of plant food stored may be seen by covering a cross section of a root with a solution of iodine^ for a few minutes, washing, and ob- serving the blue-stained starch in the tissues of the root. This is used during the rapid spring growth of root, stem, and leaves. For this reason the plants which have favorable con- ditions during the summer will have stored more food and therefore will make a better growth the following spring. Furthermore, most plants have rapid root growth before the buds open in the spring, for which they depend entirely on the stored food, therefore, it is likely that the successful grow- ing of plants depends largely on the storage of food. Late summer growth takes stored food which should be reserved for spring and in addition the growth is so tender that it often winter-kills. This can be largely regulated with fertilizers as explained in Chapter 22. A perennial will usually have more food stored in the root and the stem than will be used for the spring growth, as may have been noted when a thrifty plant has had enough reserve stored food to grow a new set of leaves if the first set has been destroyed by frost or insects in the early spring before storage had taken place. ^ A solution of iodine for this purpose may be made by dissolving potassium iodide in water at the rate of one-half ounce per quart of water and adding one-tenth of an ounce of iodine crystals. 38 PLANT GROWTH Anchorage may be incidental to the structure for con- duction of the root but its importance cannot be questioned, since it is necessary that a shoot be in a position to get an abundance of air and Hght. Recent work in plant breeding has stressed the character of a root system that provides good anchorage, notably in the breeding of corn. Root hairs are tubular projections of the epidermal or outer layer of cells of the young root (Figs. 9, 10, 11) . Root hairs are frequently less than a sixteenth of an inch long, but there may be in some cases a quarter of a million per square inch of root surface, which will increase the absorbing surface ten times or more. The figures show the normal position of root hairs, as they begin just back of the elongating region and extend some distance along the root. When the older root develops the corky layer the root hairs die. From the above it is clear that the absorbing region of a root system moves farther and farther from the plant, or in other words into new absorbing areas. With favorable conditions the root system grows sur- prisingly large; in fact, studies have shown the root spread to be much greater than the shoot spread, resulting, even in larger trees, in a relation similar to the one described for the lima bean. Studies of root growth all indicate that their spread is more than twice as great as the ordinary planting distances, resulting in severe competition in the soil in their growth and for the absorption of water and salts. This is true for most of our garden plants unless we supply excessive amounts of available fertilizers, and apply frequently an optimum amount of water, both of which reduce the growth of root systems. Perennial grass plants grown with unlimited space have been found to grow as much as a total length of one hundred fifty miles of roots each year, but never exceeding a total root length greater than about three hundred miles, since after reaching that length the older portions die at about the same ROOTS 39 rate that new growth occurs. Lawn grasses grow In severe competition with each other and also have the leaf surface reduced by frequent cutting, both of which restrict the growth of the root systems, but they doubtless grow new root areas while others die in a similar way. In the same way parts of the roots of trees die as other roots grow. These decaying roots are very beneficial to soils by adding humus and leaving pores for the exchange of air and the entrance of water, preventing run-off except in unusually heavy rainfall. Mycorrhiza is a symbiotic fungus growth on the young roots of many plants, especially trees, which prevents the growth of root hairs, but does their work in the absorption of water and salts from the soil for the plant and in return gets mycorrhiza food from the plant on which it grows. It is now considered to be so beneficial to the growth of forest trees that extensive experiments in the inoculation of plant- ings in new areas are in progress. The fungus covers the young roots with a dense compact mantle of fine hyphae like those of the common bread mold but much more extensive than the ordinary growth of root hairs, and may have func- tions in addition to those ordinarily attributed to the root hairs. In the woods, especially in the spring or fall, one may see many toadstools, some of which are the spore-bearing structures of the mycorrhiza on the roots of the nearby trees. Chapter 21 deals with the problems that the gardener must consider to improve the conditions that favor root growth. It has been shown that roots grow longer and branch more with a low or medium soil moisture, than with a high soil moisture; this may be associated with aeration. Since roots do not penetrate a hard compact soil so freely as a looser one, deep cultivation, even trenching, before planting will encourage the development of the root system in a larger volume of soil. Roots branch more in a soil with a favorable amount of mineral salts. Organic matter in- 40 PLANT GROWTH creases root growth, due to the products of its decomposition, and the aeration of the soil. The importance of a good root system cannot be ques- tioned. The root system must extend through a large vol- ume of soil in order to get an abundance of mineral salts and to get enough water even under conditions of drought. The root system stores food for spring growth of the shoot; there- fore, a good top can grow only on a good root. But the roots cannot be seen, neither can they be judged by the tops under all conditions, since high nitrogen may cause the tops to look vigorous and depress the root growth. The gardener must study his plants from year to year in order to learn the best methods to be used in growing them, therefore, the roots require thoughtful consideration, because the tops will surely disgrace the grower who neglects the roots. REFERENCES Transeau, E. N., H. C. Sampson, and L. H. Tiffany, Textbook of Botany, Harper and Brothers, 1940. Weaver, J. E., Root Development of Field Crops, McGraw-Hill Book Co., 1926. Weaver, J. E., and William E. Bruner, Root Development of Vegetable Crops, McGraw- Hill Book Co., 1927. Chapter Six ABSORPTION OF WATER AND MINERAL SALTS Animals, including man, depend on food made entirely by plants, and made, as all the foods found on the earth are, from simple materials, which animals cannot utilize as food. From water and carbon dioxide plants make the world's sup- ply of starch as is explained in Chapter 12, and the mineral salts they use in the manufacture of protein. Plants can get these materials only in the smallest units in which they exist, called molecules or in at least some cases, atoms or ions. A molecule is so small we can imagine nothing small enough to compare with it. Bacteria, visible only with very good microscopes, are very large in comparison. It would be necessary to have at least a billion water molecules in a group to be visible to the naked eye. In addition to being very small, every molecule is in motion independent of the motion of every other molecule. Food material for plants must always be in that condition when it is absorbed by a plant. Solids, as salts, or gases, as carbon dioxide, must be in the form of molecules dissolved in water when they enter a cell of a plant. Diffusion is the term applied to this movement of mole- cules and ions when they move from regions of higher con- centration toward regions of lower concentration. Each unit moves independently of all other units which may surround it, as for example, sugar molecules may move from a lump of sugar into water while the water molecules move among the sugar molecules. This movement results in a uniform distribution of the sugar and water molecules. Other com- mon illustrations are: the diffusion of ether molecules from the liquid condition as it evaporates among the molecules of 41 42 PLANT GROWTH air, or the diffusion of salt through meat. Diffusion from the soil into the plant will be discussed later. Osmosis is diffusion of molecules through a membrane. This is of great importance to an understanding of the plant's processes of absorption and excretion. The cell mem- brane of protoplasm acts as the membrane which is perme- able to water, salts, and some organic molecules, but it is not permeable to sugars and proteins, or at least only very slightly. A membrane of a root hair filled with cell sap, in contact with the soil water, is a good example. The cell sap consists of a sugar, soluble proteins, mineral salts, and plant acids in a water solution. Since so many molecules are dissolved in the water, the solution is concen- trated, or the amount of water is less than in a solution with fewer dissolved molecules. Various cells of the plant differ in the concentration of their cell sap. It is usually lowest in the water-conducting cells. It may vary from less than 1 per cent to 20 per cent but is usually close to the lower figure. The concentration becomes greater in dry soil and less in more moist soil. The water in the soil, or the soil solution, has mineral salts and products of decay, but seldom sugar, and the total concentration must always be less than the concentration of the cell sap of the root hair. When this is true, the water diffuses from more water in the soil solution toward less water into the root hair. If excessive amounts of salts, as fertilizers, are applied to the soil, it may increase the concen- tration of the soil solution above that of the cell, causing the water to go from the root hair to the soil. This would cause the plant to wilt and die. One method of killing weeds is to apply salt to the soil in sufficient quantity to prevent the diffusion of water into the root hairs. In large areas of soil there is little danger of killing plants with fertilizing salts, but in flower pots only small amounts may be used. The cellulose walls of the cells (Fig. 6, A) are in the form of very small needle-like crystals separated by exceedingly ABSORPTION OF WATER AND MINERAL SALTS 43 thin films of water. It is important to note that these films of water connect the water of the soil to the water of the protoplasm and cell sap of the cell. The root hairs are be- tween and around the soil particles and make the water con- tact between the plant and the soil. If the connection of the root hairs and the soil is broken, as happens in transplanting, the plant cannot get water by diffusion until new contacts are made. It is for this reason that the plant must be guarded against excessive transpiration after transplanting. Mineral salts must be in a water solution in order to be diffusive. They follow the same law of diffusion as explained for water molecules, that is, they tend to go from where there are more of a certain kind of molecules or ions to where there are fewer of them. Calcium ions may serve to illustrate in general the absorption of mineral ions. The calcium must first be in solution in the soil water. Oyster shells or lime applied to the soil must form a solution to be valuable. As the plant's content of calcium is used, perhaps to form a part of the cell wall or insoluble crystals of calcium oxalate, the concentration of calcium in the water in the cells becomes lower than the concentration in the soil solution around the root hairs. When calcium is more concentrated in the water outside the cell than in the cell it will diffuse into the cell. Since there is a constant use of calcium by a growing cell, it follows that it will be absorbed, that is, it will constantly diffuse into the cell. Poisonous ions, if they are present in the soil in solution, may diffuse into the plant in the same way that useful ions enter. It has been suggested that the residue of continuously spraying with copper and arsenic sprays might become dangerous to the plants and the plants might become poison- ous as food. Since these ions would be continuously removed by plant growth and by leaching from the soil, it is hardly possible that such a danger exists. Selenium is an element found in certain soils and is therefore absorbed by plants. It appears to be more toxic to animals than to plants, and 44 PLANT GROWTH for that reason plants grown on such soil have been found to be poisonous to animals including man. Such cases are very rare. Turgor pressure is the force of the cell contents against the cell wall. This pressure is produced by the osmotic action of the water through the cytoplasmic membranes. The phenomenon of turgor pressure is important in many activities of the plant. It, as has already been explained, is the force which pushes the root through the soil (see page 36) . It is the pressure which causes the stretching of the cell walls in growth. It keeps the leaf and stem cells from wilt- ing. It causes the seed and the pollen grain to germinate. Turgor pressure might be said to be a force in every living cell. Turgor pressure acts as a balance on the absorption of water by a plant. Since osmosis is the force by which the water tends to enter the cell, turgor pressure within the cell tends to balance or prevent the entrance of water. When a plant is wilted, it has no turgor pressure, hence there is no force to reduce the entrance of water. When a plant is turgid or has a high turgor pressure, more force must be overcome when water enters. Turgor pressure may vary from zero to several atmospheres, depending on the water and the soluble matter in the cells of the plant and in the soil. The turgor pressure of the cells varies through the day. The amount of water tends to decrease because transpiration is faster than absorption. The amount of sugar tends to increase in some plants during the day; in other plants the sugar of photosynthesis is changed to starch with no effect on the cell-sap solution. As the water of a cell decreases and/or the soluble substances increase, the turgor pressure of the cell increases. A few mineral salts are being found which do not obey the old commonly accepted laws of diffusion in their absorp- tion by plants. Potassium, for example, is absorbed in much ABSORPTION OF WATER AND MINERAL SALTS 45 larger amounts than can be explained by diffusion. Potas- sium goes from a more dilute soil solution to a cell with a higher concentration of the ion. It appears to be absorbed several times more rapidly when the roots have readily oxi- dizable food material and plenty of oxygen, and may be a case of actual plant energy used in absorption. If this is true, and the evidence is most convincing, salt absorption is the result of work by the plant and can be controlled by enzyme and hormone regulation. This research is in prog- ress and may suggest methods of stimulating these growth regulators to speed up the development of the plant. Other elements appear to be absorbed with difficulty and a wide difference in their concentration is always found. In some cases the cell sap has a very low concentration of the ion. This is spoken of as selective absorption. Much research work has been done on this problem, but we have many unanswered questions. The following table shows one of the cases of unequal diffusion of elements by a pondweed: Analysis of the sap of Nitella and of the fond water in which it was growing, by Hoagland and Davis Ion Concentration in sap Concentration in pond water Ca 13.0 1.3 Mg 10.8 3.0 Na 49.9 1.2 K 49.3 0.51 In the case above all the elements are more concentrated in the cell sap than in the pond water, but note that the Mg is three and one half times as concentrated, while the K is almost a hundred times as concentrated. These are not uncommon results. REFERENCES Hill, J. Ben, L. 0. Overholts, and H. W. Popp, Botany, McGraw-Hill Book Co., 1936. Meyer, B. S., "The Water Relations of Plant Cells," The Botanical Review, vol. 4, pp. 531-547, 1938. Miller, E. C, Plant Physiology, McGraw-Hill Book Co., 1938. Chapter Seven THE SOIL The soil, that complex substance forming the outer layer of the earth's surface, is a great reservoir of water and mineral salts essential for plant growth. Many different kinds of soils will furnish the necessary materials for plants, but a good soil has a proper balance of the following five constituents: mineral particles, water, air, organic matter, and living organisms. The basic material of most soils is the mineral particles of varying size formed by the decomposition of rock. Sandy soil has a high proportion of coarse particles; silt soil has a base of exceedingly fine particles. In the eastern United States most soils are mixtures of various sizes of particles. The size of the soil particles is important because the surfaces hold the water and mineral salts which the root hairs absorb. A single cubic foot of fine garden soil may have more than ten thousand square feet of soil particle surface, while a sandy soil has only 10 per cent as much. Since this differ- ence in soil surface results in a proportional difference in the water-holding capacity of soil, sandy soils require more fre- quent watering than fine soils for good plant growth. The kind of rock from which soil has been formed is very important because some soil particles are continuously dis- solving to furnish the mineral salts in solution for the plants. A soil formed by a combination of rocks is more productive than one from shale or slate alone. Some of the best soils are largely from a limestone origin. However, the degree of weathering usually is more important than the parent rock from which it is formed. 46 THE SOIL 47 Plants absorb their minerals from the capillary water solutions. A rich soil has a greater supply of soluble material than a poor soil. Thus many soils can be improved by in- creasing their acidity so that the rock particles will be dis- solved more rapidly, and by checking excessive drainage which loses fertility to the underground water. The amount of soluble matter may vary from time to time even though in general the insoluble portions continue to become soluble. It may be reduced by greater absorption at times of rapid plant growth, and during rainy periods when some of the soluble material may be carried from the soil in the underground drainage water. A soil with a greater water-holding capacity should be better able to supply plants with mineral salts. The soluble and insoluble compo- sition of fertilizer, as shown by analysis, should be studied in any effort to improve soil. It should be kept in mind that while the insoluble is unavailable to the plants, it slowly becomes soluble and available. Bonemeal and other organic materials are among our best and safest fertilizers because they become slowly available. The growth and development of all organisms is a response to environment, which for plants is air and soil. Man has been able by plant breeding (discussed in Chapters 18 and 19) to make plants that are better adapted to certain environments, and although he has little power to regulate the temperature, the sunlight, and the rain, he can use the basic mineral soil available to him and can alter the proper- ties of the soil complex most markedly in many ways. Soil science has made it possible to make a soil suitable for practi- cally any plant. The first steps of improvement might in- clude the increase or decrease of the organic matter, the number and size of air spaces, the numbers of living organ- isms, the relative acidity, and of the temperature and water- holding capacity of the soil. Soil improvement requires a careful study of the soil and the response of the plants to the 48 PLANT GROWTH soil, followed by careful experiments with various methods used to build better soils. Constructive work may be done each year, when a well-managed soil should continue to get more productive. By irrigation and sprinkling to improve the water con- tent of soil the growth of plants is probably improved most because growth is restricted more by unfavorable soil water content than by any other single factor. Soil water is often divided into three types: capillary, gravitational, and hygro- scopic. Capillary water is that water which is held to the soil particles by cohesion or molecular attraction. The excess of water that passes through the soil after a long rainy period is called gravitational water because the pull of gravity over- comes the force with which the water is held to the soil. The capillary water is in thin films covering every soil particle to a varying thickness depending on the moisture content of the soil. A sandy soil may hold as much as 20 per cent of its weight as capillary water which would be available to grow- ing plants, but a fine soil with a high colloidal and humus content may have a capacity five times as great as the sand. As the plants take water from the soil, or as it evaporates, the films become thinner. The force with which the water adheres to the soil is inversely proportional to the thickness of the film, which may become so thin that the plant absorbs it more slowly than it loses water by transpiration; therefore, it may wilt during the day but recover at night when the plant loses water very slowly. Finally, the reduction of water in the soil increases the force with which it adheres to the soil particles, until the force holding it to the soil is equal to the force with which it enters the plant and absorption ceases. To go back to the theory of diffusion, the concen- tration of the cell sap is not great enough to cause the water to diffuse from the soil particles into the root hair. This bal- ance of forces is reached while the films are thick enough to THE SOIL 49 give up a little water to a dry atmosphere by evaporation. When water no longer evaporates from a soil, it is said to be air dry, and the water remaining is called hygroscopic water, most of which can be removed by heating the soil. The following simple experiment tests the water-holding powers of soil. Pulverize a quantity of soil and dry it for several days in the hot summer sunshine or in an oven at about boiling temperature for a day. Gently tamp two pounds of the dried soil in a tin can, of about one quart capacity, with a few perforations in the bottom. Slowly add water, an ounce at a time, until it is well soaked and the ex- cess water drips from the can. The percentage of water held by the dried soil is known as the ''water-holding capacity" of the soil. A good soil should hold 50 to 100 per cent of its weight in water. The water that was not held by the capil- lary forces of the soil, but passed through the perforations, was the gravitational water. The movement of the water in the soil depends on the fact that it is in continuous films from soil particle to soil particle and is held to the soil particles with a force propor- tional to the thickness of the film. If water enters a root hair, the film at that point becomes thinner and the pulling force of the soil increases, which causes the water to move in that direction. This is the movement of capillary water and it is similar to capillary movement in a finely drawn glass tube or even the slight curve of the water at the surface on the sides of the tumbler at the dinner table. The reverse holds true; when water is added to the soil, the films become thicker and the water moves by capillarity to where they are thinner. This movement is very slow as can be seen by digging into the soil after a rain. Since some force is neces- sary to cause the water to move through the soil there is a tendency for it to accumulate near the surface after a rain, by increasing the thickness of the films, instead of becoming equally distributed on all the soil particles even at greater 50 PLANT GROWTH depths. The "field capacity" is a term used to express the amount of water held by a soil to a given depth. Only addi- tional water will pass to the deeper soil, and will increase the depth to which the field capacity has been reached. This will be considered later under the heading of artificial watering. The mineral salts in solution in the capillary water will move with the water, but they also move by diffusion. If a root hair is absorbing a particular ion (the smallest unit of an absorbed substance, as an ion of potassium) faster than it is absorbing the water in which it is in solution, the concen- tration of the ion will be decreased which will cause that kind of ion to move by diffusion toward the root hair at a greater speed than the solution. In other words, ions and water molecules enter the root hairs as individuals by diffusion, and they may move that way in the water films around the soil particles instead of moving as a group of mixed molecules, which make a solution such as a drop of a sugar solution. The water-supplying power of any soil refers to the amount and the rate at which the plant can absorb water from the soil. Since the water moves by capillarity in the soil, it is clear that a plant can get from the soil the water immediately around the root hairs and the water that will move to the root hairs. A soil of finer texture, other condi- tions being the same, will hold more water, in proportion to the increase of particle surface in contact with a root hair, than a coarser soil. It is evident for this reason that a fine soil will normally supply more water to plants than a sandy soil and that plants will wilt sooner in a sandy than in a fine soil. The air of the soil is necessary to supply oxygen to roots and to the organisms causing decay. Protoplasm must have oxygen to grow or in fact to live. A loose porous soil with a low water content will contain a large quantity of air. The oxygen can diffuse into this air as it becomes depleted of oxygen by the roots. However, a soil with too much water THE SOIL 51 will be deficient in air and growth will be retarded. This need of oxygen can easily be tested by trying to germinate seeds in boiled water which has cooled and has a layer of oil over it to keep the oxygen out and in other water that has been vigorously stirred to oxygenate it. Roots will seldom enter soil, because of slow growth, where oxygen is limited by lack of air spaces due to excess water or to the fact that the soil particles are packed. The organic matter of any soil can be increased by fre- quent applications of manure or other vegetable matter. It has such a profound influence on the soil that it should be given a great deal of consideration. Plant growth can be improved more by building up the organic matter of the soil through yearly applications than by any other method with the exception of the use of water in cases of drought. Organic matter not only gives a darker color to the soil which will cause it to absorb more heat and make it warmer in the spring, but it increases the water-holding capacity, since it may hold several times its weight in water. Organic matter decays slowly, forming acids which favor the dissolving of rock particles, and the decayed matter furnishes the food material for the growing plants. Recently it has been ascribed greater importance as food for the organisms that cause its decomposition. For example the bacteria that live in the soil and fix nitrogen must have large quantities of organic matter from which to get their food in order to reproduce, grow, and fix the maximum amount of nitrogen. Organic matter may some day be considered of most impor- tance because of the soil organisms it supports. The living organisms of the soil, most of which are bene- ficial, are so numerous that only a special student studying small amounts of soil with the thoroughness of the bacteri- ologist can appreciate their importance. Many of these fungi and bacteria are important in bringing about the decay of the organic matter of the soil, others as described in 52 PLANT GROWTH Chapter 23 add much of the nitrogen to the soil. Larger organisms as the worms of various kinds improve the soil by aeration and water absorption through their burrows. Plants will grow in a soil free of these living organisms but the soil will not remain fertile. REFERENCES Kellogg, Charles E., The Soils That Support Us, The Macmillan Co., 1941. Russell, E. J., Soil Conditions and Plant Growth, 6th edition, Longmans Green and Co., 1932. Salisbury, E. J., The Living Garden, The Macmillan Co., 1936. Plantcraft: Proper Conditions for Plants, Porter Chemical Co., Hagerstown, Md. Chapter Eight STEMS AND BUDS The stem is the structure, usually above the ground, which bears the leaves. It develops from the plumule part of the seed (Plate 1) . The tomato seedling at the left shows only a bud-like plumule which develops into the shoot in the other two stages. There are a few underground stems such as the white potato tuber and the rhizomes of quackgrass and ferns. The nodes of stems are usually swollen areas from which the leaves grow. The above plate shows a node with its com- pound leaf, and the buds which appear just above the leaves develop into branches (Plate II). The two growing buds may be seen just above the lower compound leaves; the upper one is a sizeable branch. The length of internode, which is the distance between two nodes, depends largely on the growth conditions at the time of development. If they are too long the plant probably lacks light, has too much water, or nitrogen. The cabbage plants shown in Plate III were grown with light and minerals limited to starve them. They show many leaf scars very close together because the internodes are short. Stems of garden plants are of two kinds, first monocoty- ledonous, such as corn and lily, which usually grow in length chiefly at the base of the internodes, branch sparingly, in- crease in diameter only a little, and have the woody fibers scattered more or less evenly through the pith. The second are dicotyledonous plants, such as tomato and all broad- leaved trees, which differ in all these particulars. They grow in length just back of the terminal bud, branch more freely, increase in diameter by adding cells at the cambium layer, 53 54 PLANT GROWTH and have the woody region in a more or less heavy cyHnder between the pith and the bark. The buds are usually more conspicuous on this type of stem than on the monocotyledo- nous stems. Transverse View Transverse View — epidermis — phloem ray xylem (wood) ray cambium phloem fibers — -primary xylem secondary ""xylem — cortex \cQrtex pi ^epidermis wood ray cells yessels spiral--''' _scalariform- Radial View phloem cortex xylem \ cambium Tangential Vietv Fig. 12. Semidiagrammatic representation of a one-year-old stem of tulip-tree, in transverse, radial, and tangential views to show the relative size and shape of the many kinds of cells. (From Hill, Overholts and Popp's Botany, McGraw-Hill Book Co., 1936.) The main tissues of the stem correspond with those of like functions in the roots. The central region is the pith, usually absent in roots. In young, and in herbaceous stems Plate III. Cabbage plants grown 26 months with low light and in small pots. The starved plants have lost many leaves as shown by the nodes. (1/3 natural size.) Plate IV. Detail of whip graft. (1) stock, (2) stock and scion, and (3) graft completed and tied. STEMS AND BUDS 55 which grow for only one year, food is stored in the pith. The wood or xylem tissue carries the water through the stem to the leaves and forms the chief part of a woody stem but only a small part of an annual plant, or even in a one-year-old woody stem, as illustrated (Fig. 12) by the main tissues in the tangential view of the first year's growth of the stem much enlarged to show the cells in detail. The xylem cells vary in shape and size both in longitudinal and cross section. Some people believe that all the mineral salts go up through the xylem, but others believe that a part of it goes through the phloem. In trees large quantities of food may be stored in the xylem. The cambium is a layer of cells around the xylem. It divides repeatedly during the life of the plant to add new xylem cells on the inside and new phloem on the outside, increasing in this way the diameter of the stem. The bark consists of the several types of cells found outside the cam- bium. The phloem which is next to the cambium carries food material through its sieve cells from the leaves to other parts of the plant. The cambium cells break when the bark is removed from a woody stem during active growth. Out- side of the phloem a stem usually has a cortex made up of thin-walled cells in which food is stored. Older stems have a band of varying thickness of corky cells on the outside to prevent excessive loss of moisture. Younger stems have a single layer of epidermal cells with cutin on the outside to retard the loss of moisture. Since the outer layer of bark sloughs off in pieces from older trees the cork becomes the outside layer. For a few weeks in the spring, from the time the buds begin to swell, the stems of most perennial plants grow in length after which the cells thicken and become permanent tissues. The leafy stems of twig C (Fig. 13) grew in the spring and have ceased to grow in length. Other stems, such as the tomato, geranium, petunia, and most of the other her- 56 PLANT GROWTH baceous plants, continue to elongate during most of their period of life. Growth in thickness of woody plants is most rapid also in the spring but it continues all summer at a slower and slower rate. The difference in the growth rate in late sum- mer and the following spring can be seen by the much larger spring cells, which make the yearly rings so conspicuous in cross sections of wood. They are equally clear as long streaks in oak flooring and in much of the wood of our furni- ture. The limited summer growth enables the plants to store food for the early growth of the following year. Buds appear on new wood at the axils of the leaves about the time the leaves appear. Buds which are clearly evident on the above twig C will continue to grow larger through the summer as is shown on twig D. Some buds, called dormant buds, do not grow at the nor- mal time after their formation (leaf buds 3 and 4, Fig. 13). Dormant buds act as reserve buds and they will grow if those higher on the branch are destroyed, as may happen by freez- ing or by pruning, even after a dormancy of one to several years. All buds that grow at their normal period are called active buds. Buds are of three kinds according to their contents: Fig- ure 13 shows two kinds, leaf buds and flower buds. Leaf buds contain several embryo leaves with very small unde- veloped buds, and a very small branch. The flower buds usually have only embryo flower parts. The third type, called mixed buds, contains both leaves and flowers. In many cases the kind of bud can be determined with a micro- scope as early as July before they open the following spring. Most flower buds of trees can be seen during the winter with- out magnification. The experienced fruit grower can recog- nize them by their shape. Since the flower buds begin their development so many months, almost a year before they open, it is clear that cultural treatment to regulate flowering STEMS AND BUDS 57 must be practiced even before the buds appear in order to condition the plant properly. This will be discussed in Chapter 14. Nearly all leaves are opposite as in lilac or alternate as in elm, but they may be spiral, i.e., on more than two sides. The buds at the base of the leaves have the same arrange- ment. The position of the buds determines largely the direc- tion of branching and should be considered in pruning and training as explained in the next chapter. Most plants have a bud on the end of the stem, called terminal bud, which is the most active in growth and tends to continue the growth of the stem in the same direction. Side or lateral buds cause branching when they grow. The elm twig has so weak a terminal bud that the last or upper lateral bud usually grows, but in Figure 13 the two last buds grew, making a branch on twig C. Adventitious buds are those which grow at places other than the axils of the leaves. They may appear on roots, stems, or leaves, and they frequently develop after injuries, such as the removal of a large branch or the topping of a tree. A bud can grow only with dividing cells, but how a plant can develop such cells at unusual places is not known. When the bud scales fall from a bud they leave individual scars. The terminal bud scale scars are so evident that a series of these marks are called a terminal bud scale scar, which marks the extent of the year's growth. Two are shown on twig D in the above figure, which indicates that the lower end of the stem is three years old. Plants, such as spirea, which bear flowers on the current growth of the stem may be heavily pruned in the early spring to stimulate the growth of flowering wood. Other plants, such as most fruits, which bear flowers on the previous year's wood will have their potential flowers removed by spring pruning. This may be desirable if they usually bear too much fruit. 58 PLANT GROWTH The growth of the buds depends on the stored food sup- ply. It is a common nursery practice to cut some woody plants off just at the ground level when they are two to four years old and grow a single shoot from the cut stump. The excess food in the root will cause this shoot to grow as tall as the plant had been, and it will be a straighter, more desira- ble plant. The great problem is to know when the plant is storing enough food and when it must be encouraged to store more. REFERENCES Gray, Asa, Elements of Botany, American Book Co., 1887. Smith, G. M., J. B. Overton, E. M. Gilbert, R. H. Denniston, G. S. Bryan, and C. E. Allen, A Textbook of General Botany, The Macmillan Co., 1935. Chapter Nine PRUNING AND TRAINING PLANTS Pruning includes the removal of any part of the plant in order to improve its growth for the use of man. There are many reasons for pruning but they will usually fall under three general headings: (1) removal of diseased or injured parts, (2) training for shape, and (3) the improvement of the productiveness of the plant. It should be kept in mind that all pruning is interfering with the natural development of the plant, and, therefore a definite aim should be clearly in mind before any part is removed. After diseases enter a plant as described in Chapter 15 it is often possible to remove the diseased tissue to save the plant, by making a smoothly cut surface for the new healing growth. Likewise, injured tissue should be removed. Most training can be done by pinching off the buds or young shoots when it is evident they are growing in an undesirable direction. In most plants the highest or terminal bud on each plant or on each branch will grow. If this bud is not desirable as a leader, the branch should be removed just above a bud that points in a desira- ble direction. Pruning for the improvement of productive- ness usually involves the removal of larger portions of the plant and will be discussed in more detail under "Balance of Root and Shoot" in Chapter 14. Training is usually less extreme than the pinching of all the young side shoots or buds, as when a tomato plant is trained to a single stem; or a chrysanthemum is trained in the same way to produce a single large flower. The tomato shoot (Plate II) has two compound leaves below the fruit, with a small undeveloped branch at the lower leaf, and a much larger branch from the next node extending above the 59 60 PLANT GROWTH tomato. During the past summer all the buds except one of the lowest were removed from some tomato plants, in order to train them to two shoots per plant. In soil of average fer- tility these plants averaged more than eight feet in height on their trellis. This method encourages good fruit and it is kept clean. The opposite condition of many branches is pro- duced when the terminal buds are pinched to increase the number and growth of the branches and the production of many small flowers, as on the chrysanthemum. Most plants will respond to training by pinching off the buds or young branches. A study of the elm twig D (Fig. 13) will be help- ful in seeing the possibilities of training in the growth of trees. Upright growth would be encouraged by removing the branch and the lower buds of the main stem. Lower, more spreading, growth would result from the removal of the top above the bud on the right and the removal of the branch above its lowest bud. A hedge is pruned to increase the number of branches, therefore the tips are removed, but a fruit tree is thinned by removing lateral branches to en- courage the growth of its main branches. Tall, slender, or open plants are formed by the removal of the lateral buds, and bushy plants by the removal of the terminal buds. By a careful study of the position of the buds, it is possible to train plants to grow into unusual shapes along a wall as the French do. Thinning is actually a kind of pruning except that fruits rather than flower buds are removed. Thus more food goes to the remaining fruits and they grow larger. We usually want the largest yield with the fewest fruits. The fruit is a method of reproduction for the plant and therefore the sur- vival of the species in the wild state depends on the number of seeds instead of the number of bushels of fruit. De-flower- ing or de-budding is very important in floriculture since an excessive number of flower buds will limit the food to each and result in small flowers. Roses are commonly de-budded. PRUNING AND TRAINING PLANTS 61 The removal of old flowers prevents the formation of seed and the waste of much food material. Suckering is the removal of young shoots that may de- velop around the base of the stem, as in corn. It was thought Fig. 13. The seasonal history of an elm twig and of its protected buds. A, winter condition. B, early spring; the floral buds have developed into floral branches. C, mid- summer; 2 axillary leaf buds have developed into branches and produced leaves; 2 have remained latent; the floral branches have developed mature fruits and have fallen, leaving scars. D, late autumn; the leaves have fallen; in their axils are protected leaf and floral buds. TB, minute undeveloped terminal bud; LBi-LBi, axillary leaf buds; FB, axillary floral buds; SLS, scale-leaf scars; FBS, floral branch scars. (From Smith et al., Textbook of General Botany. By permission of The Macmillan Co., publishers, 1935.) 62 PLANT GROWTH that they exhausted the plant but more recently it has been shown that for corn they make as much or more food than they use and that suckering is useless if yield is considered. Trimming or shearing is a special type of pruning to make a plant or a group of plants look like a wall or cone. In extreme cases they may be shaped like animals. This trans- fers the attraction from the natural beauty of the plant to that of form. It is difficult to keep such plants in perfect form and otherwise they may be very objectionable. Although Dr. L. H. Bailey, the great dean of the plant sciences said, "The time to prune is when the knife is sharp," the better plan is to see that the knife is sharp for the pruning time best suited to each kind of plant. The pinching of buds and small branches can be done at any time, and will, if done carefully, control plant growth so that the removal of large branches is rarely necessary. The removal of larger branches should always be done in the early spring toward the end of the dormant season, when the cut surface dries less and the new growth will soon begin the healing process. The time of flowering should govern the pruning of flowering shrubs. If the flower buds are on the old wood, pruning should be delayed until after they flower, otherwise many of the flower-bearing branches are removed, resulting in sparse flowering. This, of course, is desirable in fruit trees and they are pruned in the dormant season. If the flower buds grow on new wood, i.e., spring growth, a rather vigorous dormant pruning will increase the growth of the new shoots and improve the size of the flowers. The amount to prune depends on the plant and on the aims of the owner. It has been found by recent experiments that peach and apple trees make larger, quicker-bearing trees if the shaping is done by the removal of buds and small branches instead of the older method of cutting out a large portion of the top each spring. This is probably equally true for other perennials. We usually prune roses heavily be- PRUNING AND TRAINING PLANTS 63 cause we want to reduce the number of flowers in order to get larger flowers. Hybrid tea roses are cut back in the faU to lessen their exposure to winds, but the main pruning is given in the spring after the danger of heavy frost is past. The climbing roses are pruned after their spring bloom, which will allow new shoots to develop and store food through the summer. There are some rambler types which are such strong growers that the canes may be removed to within a few inches of the soil. The other climbers should be pruned less vigorously but the best roses grow on the wood of the previous year. The older canes should be completely removed in order to keep the whole top in a young vigorous condition. In all cases of rose pruning the aim is to have the plants out of balance in order to force vigorous shoot growth. It must be kept clearly in mind that two problems face the grower in pruning to improve the productiveness of plants. First is the initiation of flower buds by a proper balance of root and shoot (Chapter 14) , and by length of day (Chapter 12). Second is the growth from the supply of food for the flowers or the fruit after the buds are initiated. Both are practiced extensively by greenhouse men and are applicable to most plants in the garden even though conditions are more difficult to control than in the greenhouse. Food results in growth and as more food is sent to an organ more growth occurs. Our largest chrysanthemums are grown on well-fed plants trained to a single stem and allowed to develop a single flower. Fruits may be grown larger, if the same prin- ciple is followed. Experimentally, a larger fruit can be grown by ringing the stem just below the fruit. Ringing is the re- moval of a ring of bark about a half inch wide, by carefully cutting to the wood with a sharp knife. This prevents the movement of food from the branch into the main part of the plant. Pruning should always be done to leave a smooth surface, cut close to the branch or trunk, and parallel to it. Wounds 64 PLANT GROWTH of about an inch or more should be painted with a lead and oil paint to prevent infection by bacteria or fungi, which might enter the open pores of the wood and cause the center of the tree to rot. If limbs of two inches or more are to be removed it may be difficult to avoid splitting into the tree. It is best to cut first from the lower side of the branch about one third through, about eighteen inches from where it is to be removed, then about six inches farther out cut from above until the limb breaks off. Now the stub can be removed close to its base with no danger of splitting to injure the tree. Pruning stimulates the growth of buds below the cut as has been suggested above. All terminal buds appear to secrete a hormone which descends and inhibits the growth of the buds which receive it, causing them to remain dor- mant. When a twig is removed the first bud back of the cut will cease to get the growth-inhibiting hormone and will begin to grow. It is for this reason one can so easily deter- mine the shape of a plant by judicious pruning. Simply expect the topmost bud to grow in the direction it is pointed. Trees may be trained to fill in sparse areas of their branching by pruning a little more heavily on the strong side and leaving buds which point toward the weak sides. Pruning appears to increase the vigor of a plant. This is a balance of root and shoot problem, and therefore means that the reduction of the top decreases the supply of carbo- hydrates to the point where nitrogen is available in excess. High nitrogen favors the growth of the protoplasm for larger, thin-walled cells, resulting in more rapid growth. If more new growth is desirable it is safe to try pruning more heavily. It is clear from this and the preceding paragraphs that it is unwise to prune a plant heavily in late summer because the growth so stimulated would be tender and therefore in danger of winter-killing. Tree surgery is rapidly becoming a respected profession because more and more science is being applied. Tree "sur- PRUNING AND TRAINING PLANTS 65 geons" should be chosen with great care and should be impressed with the fact that plants should be treated for permanent vigor and natural growth, and not for quick results gained by the use of stimulants which might be harm- ful. Too high nitrogen may appear to be very beneficial for a few months, but it may cause an earlier death of the plant. To prolong the lives of trees, decays and injuries can usually be repaired by tree surgeons, who remove infected areas and treat the cavities to prevent further infection. REFERENCES Bailey, L. H., The Pruning Manual,"The Macmillan Co., 1925. Publications covering special plants from your State or Federal Department of Agri- culture. Chapter Ten PROPAGATION Seed-plants must be propagated, because they, Hke ani- mals, get old and die. Other plants, which never produce seeds, such as mosses and ferns, may be killed by animals, fire, or cold but do not die of old age. They continually make new growth at one end and die at the other end; however, they too have the power to produce new plants from sexual and asexual methods of reproduction. It is a simple matter to dig the underground stem of a fern, as the bracken, where the growing point is just ahead of the leaf, and several feet back of the leaf the stem is dead, while farther back it is decayed. In fact some of our living moss and fern plants may be much older than our oldest trees. Propagation may be sexual, that is by seeds produced through flowers, or asexual by a number of methods which promote the development of new plants from root, stem, or leaf sprouts. Other natural methods will suggest themselves as we discuss artificial propagation. Sexual reproduction is the production through the seed of a new plant which began when the male sex cell fused with the female sex cell. Chapter 18 will make clear how a single plant produces many combinations of its characteristics in its sex cells. Furthermore, it is not necessary that the pollen grain come from the same plant that bears the female part of the flower but only from one of the same kind of plant (cross- pollinated). Thus, plants grown from seeds will resemble their parents but will vary widely in smaller details because of the many combinations of their parents' characteristics, just as sisters and brothers are all different. In many cases, where quality is the result of an exact combination of char- 66 PROPAGATION (:>! acteristics as in most of our tree fruits, plants produced by- sexual methods yield a very inferior quality of fruit. Asexual methods of propagation all differ from sexual, since the offspring is grown from a group of cells from one plant and, therefore, has always exactly the same protoplasm or germ plasm as that of the parent. This is possible because a small portion of a plant can regenerate a complete plant. A bud always arises by the division of cells as described in Chapter 4, and therefore has exactly the same gene combi- nation. Thus any number of cuttings or grafts made from the same plant will produce plants that are the same in all their general characteristics. For this reason fruits, roses, and many trees are usually propagated by some asexual method. Such a group of plants produced asexually from a plant is known as a clone. It is clear from the above that for plant breeding the sexual methods of propagation will give a wide variety of plants. This is covered more fully in Chapters 18 and 19. If uniformity is desired some asexual method of propagation, such as cutting, grafting, budding, or layering, must be utilized. A cutting is a small piece cut from a stem, root, or leaf, which will produce a complete plant like the one from which it came. Cuttings must form a root-promoting hormone, before roots are initiated, which may require only a week or, for some plants, it may take as long as several months. Dur- ing the last two or three years a number of chemicals have been found which appear to stimulate the hormone forma- tion or perhaps to act like the hormone (Plate X). This is discussed more fully in Chapter 20. Stem cuttings are of two kinds, dormant wood and green or growing wood cuttings. They must be from healthy tis- sue, usually four or five inches long, with one or more buds and a good supply of stored food. Dormant cuttings are made in the fall and stored in a cool moist place until early 68 PLANT GROWTH spring, when they are planted, but kept cold enough to retard bud growth until the roots are well developed. Some- times they are buried in the garden deep enough to be below the frost line. Green wood cuttings may be made during the summer, but since transpiration is Hkely to be excessive, part of the leaf surface should be removed and the cuttings should be shaded and should be sprinkled gently often enough to keep the surface moist. In greenhouse propagation cuttings are shaded and sprinkled once to several times a day. Cuttings must have a good set of roots to produce good plants. The same four closely related conditions must be considered for cuttings as was described for growing seed- lings, namely: plenty of stored food, adequate oxygen, mois- ture, and temperature. It has been found that cuttings made from tissue with a good supply of stored food will use it in better rooting and in growing a stronger plant. They need extra oxygen to oxidize the food and so are usually set in sand. This can be kept well moistened but still remain well aerated. Bottom heat is beneficial to hasten root growth when started on a greenhouse bench. They should be restricted from excessive transpiration, but some air circu- lation may be necessary to avoid the growth of damping off fungus. Most cuttings root better if they are made at right angles instead of long tapering cuts. Root cuttings may be used from plants that sucker natu- rally from the root. Raspberry and blackberry root cuttings are frequently made about two or three inches long from roots about as thick as a pencil. The cuttings may be made by taking a piece of root with a small sucker. Root cuttings should be planted very shallow to allow the bud to reach the surface quickly and begin to make food. Grafting is the placing of a cutting of last year's growth called the scion into another plant called the stock in such a manner that like tissue will grow together and become a single plant. There are several kinds of grafting, but they PROPAGATION 69 all depend on getting the cambium layer and a small portion, at least, of the phloem and the xylem in such close contact that the new growth will form a union through which food and water will be translocated. For this reason, only plants with a well-developed cambium are grafted, usually woody plants. The scion carries the variety of plant, or we may say the protoplasm which we want in the new plant, and there- fore always remains like the original plant, while the stock is usually chosen because of its vigorous or disease-resisting root system. Grafting is usually done near the end of the dormant season. Care must be taken to avoid the loss of moisture from the scion before or after grafting. Grafting wax is often used to cover the wounds of the scion and stock to prevent drying. Grafting is used for many reasons: (1) it is a very com- mon nursery practice because it offers a simple means of enormously rapid propagation of a good variety. Many of our best fruits have come from a single tree and in some cases from a single bud sport of a tree. (2) A plant breeder work- ing with tree fruits can cross the flowers; then he can plant the seeds and after one year's growth use the whips as scions and graft them on an older tree. In this way they will bear fruit in less than half the time necessary for the growth of a tree and much space is saved. (3) It enables people with limited space to have as many varieties of apples, pears, or cherries on a single tree as they wish. It has been reported that Burbank had more than six hundred of such grafts on a single plum tree. (4) The fruit of an older tree may be changed to a more desirable variety by grafting on a number of its branches. Root, tongue, or whip grafting is done by selecting the stock and the scion of nearly equal diameter and cutting with a sloping cut, about an inch long, then splitting each longi- tudinally so that they fit together, after which they should be bound snugly with waxed thread to hold them in place until 70 PLANT GROWTH a union is made (Plate IV). The greatest care must be taken to have a long union of the cambium and other like tissues. Seedling roots which are often used permit two or three stocks to be made from each. The scion should have two or three buds. Cleft grafting is usually done on older branches where the scion is inserted in the one side of the split end of a stock cut at right angles. Here again the cambium of one side of the scion must be in line with the stock cambium (Fig. 14). ABC D E Fig. 14. Methods of grafting. A, B, between stock and scion of similar size. C, D, E, between a large stock and small scions. F, G, H, bud-grafting. (From Smith et al., Textbook of General Botany. By permission of The Macmillan Co., publishers, 1935.) One, two, or more scions may be put in the same stock. The best-shaped plant will result later if only one is allowed to grow on a stock a little larger than the scion. Budding might be considered a form of grafting since a single bud with some bark and a little wood (in some cases the wood is removed) is placed in a slit of the bark of the stock plant so that the cambium will unite and grow phloem and xylem in the union (Fig. 14) . This method is used com- monly with peaches and roses. It is very important to keep the buds in a natural fresh condition until used. Budding is generally done low on the plant, that is about the ground level or just below, in early summer while the bark can be PROPAGATION 71 slit back to enter the bud against the wood. Frequently buds develop several years later from the stock and unless they are destroyed may rob the budded portion of food and kill it. In all cases of grafting and budding wound hormones are probably formed to stimulate rapid cell division. Only varieties that will readily form a strong union of the new tissue should be used for scion and stock. We do not know why all woody plants cannot be grafted on each other, as for example cherry on apple, but we know the tissues will not unite. Layering is a method of propagation similar to cuttings except that a shoot or branch is held against the ground or even covered at places with soil until roots grow, before sever- ing it from the parent plant. Cutting through the bark often hastens rooting. Roots will usually grow opposite each bud as it develops a shoot, after which the cuttings may be made. This method is more successful with plants that root poorly. Runners may be formed naturally, as in strawberries, and the new plants transplanted when they are well rooted. This is a form of layering. There are several forms of propagation among the plants that have bulbs, corms, rhizomes, tubers, etc., but for these, special reports should be consulted. REFERENCES Adriance, G. W., and F. R. Brison, Propagation of Horticultural Plants, McGraw-Hill Book Co., 1939. Bailey, L. H., Manual of Gardening, The Macmillan Co., 1925. Hitchcock, A. E., and P. W. Zimmerman, "Effect of Growth Substances on the Rooting Response of Cuttings," Contributions from the Boyce Thompson Institute, vol. 8, pp. 63-80, 1936. Zimmerman, P. W., and A. E. Hitchcock, "Response of Roots to 'Root-Forming' Sub- stances," Contributions from the Boyce Thompson Institute, vol. 7, pp. 439^145, 1935. Chapter Eleven LEAF STRUCTURE The leaves are outgrowths of the stem, consisting, usu- ally, of a slender stalk, the petiole, frequently having at its base small appendages called stipules, and the broadened conspicuous portion called the blade. The leaves begin to develop in the buds by the protrusion of a small portion of the dividing tissue of the stem tip. All the leaf cells retain this ability to divide for some time, so that a leaf grows in every region. For this reason repeated spraying is necessary to keep the growing leaves covered. Leaves are of many shapes or forms, but two general classes should be considered. Simple leaves, such as the zinnia, have the blade in one piece. In compound leaves the blade is completely divided into leaflets, either pinnately as in the tomato (Plates I and II) or palmately as in white clover. Thus, to determine which are simple leaves on a branch and which are compound leaves you must distinguish between petiole and stem. A stem ends in a more or less conspicuous bud, which never occurs at the tip of a com- pound leaf. The veins of leaves consist of strengthening tissues and conducting tissues which are continuations of the conduct- ing tissues of the root and stem, and they function in the same way. They branch and rebranch until the entire leaf is penetrated with a microscopic network of the cells of con- duction. They are divided into two classes: parallel vena- tion when they run in nearly parallel lines, as in grasses; and net venation when they branch in many directions, as in most broad leaves. The epidermis is the outer covering of the leaf, consisting of a single layer of flattened cells on both sides of the leaf. 72 LEAF STRUCTURE 73 o "^ ''^^ ^.£P . ^ 1^ CO O O CO <-> s,. Fig. 20. Zinnia. A, general view. B, section to show the position of the two kinds of flowers. C, a ray flower. D, a disc flower with a bract. E, a disc flower split to show the united stamens and the two-parted stigma. A and B, natural size. C, D, and E, 2 x. hibiscus, when they are called racemes. A bloom with outer flowers on longer pedicels than the younger ones and the main axils a little elongated so that the top is flattened as it is in cherry blossoms is called a corymb. In the wild carrot flower the main axis is an umbel. The head is an inflores- cence with a very short axis and sessile flowers as in the 114 PLANT GROWTH zinnia (Fig. 20, A and B). Inflorescences with branches from the main axis and a loose flower cluster such as the larkspur are called panicles. D iJie Fig. 21. Petunia. A, general view. B, split to show the relation of the flower parts. C, cross section of the ovary with the ovules. D, longitudinal section of the lower part of the pistil and a side view of the upper part. E, anther with cross sections to show the pollen cavity. A and B, natural size. C, D, and E, 9 x. FLOWERS 115 The complete flower consists of four types of parts. Further study of flower development, seed formation and plant breeding require a familiarity with these parts. The most fascinating way to become acquainted with flower structure is to collect several kinds to examine and compare their various parts. Several illustrations will be helpful in the terminology. •PETAL PISTIU ANTHER OVULE RECEPTACLE OVARY X Ik B C" Fig. 22. Rose. A, general view of a single rose. B, section to show the relation of stamens and pistils. C, cross section of the receptacle to show the ovaries. D, a single pistil. A, natural size. B, 2 x. C and D, 4 x. The sepals make up the calyx, which is the outer and lower whorl of flower parts (Figs. 21 and 22). These are usually small green leaf-like structures, which in herbaceous plants often enclose and protect the more delicate inner part of the bud. In a few flowers such as the tulip they are similar to the petals in color and shape. The calyx may fall off when the petals fall or it may remain, as it does at the base of a pea pod, on the apple opposite the stem, and around the capsule of petunia. 116 PLANT GROWTH The petals, which make the corolla or the showy part of most flowers, are either united, as in petunia (Fig. 21, A) and the zinnia (Fig. 20, B, C, D, and E), or separate as in the rose (Fig. 22, A). Such flowers as the sweet pea and snapdragon are called irregular flowers because of the irregu- larity in the shape of their petals. The nectar glands are usually attached at the base of the petals. Flower colors and odors are known, at least in some cases, to attract insects. The stamens, inside the petals, vary in number in accord- ance with the kind of flower. Some flowers have fewer sta- mens than petals, some have the same number, and some have many more (Figs. 20, E; 20, B; and 22, A and B) . The enlarged top of the stamen is the anther, which bears the pollen (Fig. 21, E) from which the male sex-cells develop. The anthers may open to shed the pollen before the stigma is "ripe," at the same time, or after, depending on the habit of the plant. When the pollen and stigma are "ripe" at the same time pollination is likely to take place between them, but early- or late-maturing pollen favors cross-pollination. The stamens are attached to the base of the flower through which they get food, but often they adhere to the corolla as in petunia (Fig. 21, B) . In a few flowers, such as the zinnia (Fig. 20, E), they form a tube surrounding the style. The pistil is in the center of the flower. In the sedums there are five pistils; the rose has several separate one-seeded pistils in an urn-hke receptacle (Fig. 22, B and C), but in iris there is a single pistil of three united modified leaves (called carpels) as is shown in Figure 3, B, by the three cavities for seeds. The petunia has two united carpels as shown by the division in ovary (Fig. 21, C) and the divided stigma in D. The top of the pistil is the stigma on which pollen must fall in pollination. It usually has a finely irregular surface or fine hairy covering which catches the pollen and which con- tains a substance that stimulates its germination. The style is a solid structure connecting the stigma with the base of FLOWERS 117 the pistil, called the ovary, which contains the one to many ovules (Fig. 21, C and D) in each of which a single egg cell develops. The pollen tubes must grow through the style, which may be quite long as is the corn silk which extends to the kernel. At least one pollen tube must grow through the style into the ovary to enter the micropyle of each ovule and discharge a pair of sperms for fertilization. Careful studies of the petunia were made in which it was learned by counting the seeds of several mature ovaries that each ovary contains at least five hundred seeds (Fig. 21, C and D). Since each of these seeds requires a pollen tube, the slim style (Fig. 21, D) about the size of a large pin, must contain at least five hundred pollen tubes. Other workers have found that usually more than twice as many pollen grains begin to grow pollen tubes as are needed to fertilize all the ovules. Although the diameter of the petunia stigma as shown in Figure 21, D, is about one-tenth of an inch, it is large enough to hold a thousand pollen grains, each of which is only about one-sixth of one-hundredth of an inch in diame- ter. A single layer over the entire stigma would require more than two thousand pollen grains. They are so small that only masses are visible and for that reason the cavity of the anther is shown in Figure 21, E, without pollen. Flowers differ in many respects; in fact, they are so differ- ent that most plants can be identified by their flowers. Their differences may be marked, and constitute a group, as the following three which will be described in some detail: the sweet-pea or clover type, the sunflower type, and the grass type. The sweet-pea type has two petals forming a keel over the stamens and pistil, two wing-like petals, and a broad petal opposite the keel, called the standard. Nine of the stamens have the lower parts of their filaments united to form a tube around the pistil, the tenth one remains separate. The pistil has the stigma at one side instead of on the end, and the ovary with a single cavity containing several ovules. 118 PLANT GROWTH Composite flowers, known as the sunflower type, such as the sunflower, zinnia (Fig. 20, A), and dandelion, grow in heads. Each part is a true flower with a smaU inconspicuous calyx of hairs, bristles, or bracts, called pappus. In the dandelion it becomes the group of hairs attached to the ripe seed. The corolla is of two kinds; as illustrated in the zinnia, each broad petal-like structure is a group of five united petals, called a ray flower (Fig. 20, B and C). The center has numerous flowers with the petals united to form small tubular disc flowers (Figs. 20, B, D, and E). D shows the bract common on many composite flowers. Dandelions have only ligulate flowers. The five stamens have their anthers united to form a tube around the style (Fig. 20, E, shows the flower opened to display the stamens and pistil) . The pistil has a two-parted stigma and an ovary which bears a single seed. The seed remains in the ovary which is planted as a seed. The grass flowers grow in groups called spikelets. They are so small that they are not recognized as being common to all our grasses including corn and other cereals. Each spikelet has a pair of scale-like parts called glumes, between which the one or more flowers are located. Each flower has a pair of scales which enclose the three stamens and the pistil. The pistil has a two-parted plumose stigma and an ovary with a single seed. A single corn kernel with its silk ending in the divided stigma is the pistil. At the base of the flower are two small bodies called lodicules which swell and push the pair of scales apart at the time of pollination. The flower structure is easily visible and delicately attractive when shedding pollen. The pollen is borne in great abundance in the stamens, several millions of the cells or pollen grains being produced by a single plant. In a few plants, such as the orchids, it sticks in masses, but in most plants the individual grains form a powder or dust. Each pollen grain is a single cell with C O On re C o (U _G -4-) C re C 'im" Plate XI. Buckwheat grown in sand with added mineral salts compared with soil check, No. 5. No. 2, tap water; No. 3, N omitted; No. 4, all essential salts; No. 6, B omitted; No. 7, K omitted. Plate XII. Cowpeas grown in sand with all the essential mineral salts in No. 1, compared with soil in No. 2. No. 3, S omitted; No. 4, Mg omitted; No. S, K omitted; No. 6, Ca omitted; No. 7, distilled water; No. 8, tap water; No. 9, N omitted; No. 10, P omitted. Plate XIII. Effect of low (left), medium, and high (right) concentrations of phosphorus (top), nitrogen (bottom) on cut creeping bent grass. (From United States Golf Association Green Section.) Plate XIV. "Hydroponics" in the greenhouse. Foreground plants in sand. Middle tank with plants in rack. Last, gravel with siphon to remove the solution when it reaches the top of the siphon. Supply bottle in the rear. Chapter Twenty PLANT HORMONES The nature of growth has been as eagerly sought as has been the explanation of life itself. They are certainly closely related. But how are roots, shoots, and flowers formed.? Why do most shoots grow upward and most roots down- ward.? During the past fifty years the discoveries of hor- mones, and their effect on growth, have explained some things about growth but have stimulated even greater inter- est in the problem. Hormones are substances which are produced in one part of an organism, and are transported to another part where they influence a specific physiological process. Many hor- mones are known in animals, such as those secreted by the thyroid gland, sex organs, etc. Plant hormones have been called growth regulators, growth substances, auxins, growth hormones, and phytohor- mones. It should be noted that they are used to condition the growth of the organism rather than to build its structure, as mineral salts, sugars, and proteins are used. Hormones are used in extremely small quantities; for example, if a tril- lion oat plants were treated with a dilute solution made from an ounce of auxin, a pronounced growth curve would occur on each of them. Auxin-a has been shown to be active in water solution in one part to 110,000,000 parts of water. Besides hormones, other substances which influence growth are common in plants in small quantities, as enzymes, vitamins, and bios. These differ from hormones in that they usually remain where they are formed to influence a special process. Enzymes increase the rate of an action already taking place. They are widely distributed through the 151 152 PLANT GROWTH plant, in fact, some kinds may be in every living cell. The action of many plant enzymes is similar to the action of ani- mal enzymes, as is the case of the digestion or oxidation of food. Starches, proteins, and fats are common in plants and in animals, and are broken into simpler forms by enzymes in both groups. Many of the vital activities of all organisms are regulated by enzymes but the details of the regulation are not understood. Vitamins are probably as essential to plant as to animal life, but plants are able to synthesize them. It is common knowledge that most vitamins for the human diet must come from plants directly or indirectly. Some recent research work suggests that the bacterial life (or its products) of the soil aids the plant in this synthesis. Plants grown on a soil rich in organic matter were higher in vitamin content than those grown on a poorer soil. A supply of stored food in the cells but not light is essential for their production. Some plants grown in nutrient solutions have shown a favorable response to added vitamin Bi. Since plants can make vitamins and therefore have them at all times, it has been difficult to study their functions in the plant. Recently certain plants {Camellia, Eucalyptus, and others) have been found, at least under certain conditions, to be able to synthe- size less than the optimum amount of vitamin Bi for their best development. This has been highly popularized and advertised, but it should be kept in mind that most plants in rich soil produce enough for themselves. In such cases soil or plant applications would be of no value. When vita- min Bi is added to test its need in a particular plant, check plants should be left for comparison. Bios is a growth substance found in relatively large amounts in yeast and in many other of its relatives, the fungi. When applied to higher plants it stimulates rooting. Bios has been found in the dividing cells of flowering plants. It appears to be important in embryonic cells, rather than PLANT HORMONES 153 in the cells which are elongating or those that are fully grown. These regions are described in Chapter 5. Embryonic cells grow by increasing the amount of protoplasm, but elongation is largely the result of swelling by increasing the amount of water which forms large vacuoles. A A A 2 3 4 12 3 4 12 3 4 Demonstration B C 0 E Detection A Collection LJmM L M K V □ LJ 0 P N Polar iransporf Fig. 24. Auxin (its concentration in the agar is shown by various shades of dotting) and growth of Avena coleoptiles. Ufper left. Demonstration of auxin formation by coleoptile tip. Plant No. 1 is left intact; Nos. 2, 3 and 4 are decapitated. On No. 2 the cut tip is replaced; on No. 3 auxin (in agar) is stuck. The right hand set of four plants shows the effect of this treatment on growth after three hours. Lower left. Collection of auxin from cut coleoptile tips. For a period of two hours 6 coleoptile tips are placed (H) on a layer of agar, 6x8x1 mm. (G). After removal of the tips the agar contains auxin diffused out of the tips, and is cut into 12 blocks (I). Upper right. Scheme or test method for auxin. Coleoptile (A) is decapitated leaving the primary leaflet (B). The latter is partly pulled out (C and D) and an agar block with auxin is placed on one side of the cut surface of the coleoptile (E). Two hours after application of the agar the resulting curvature (F) is measured. Lower right. Demonstration of polar transport of auxin. On the apical surface of a coleoptile cylinder cut from the seedling (K) a block of agar with known auxin concen- tration is placed and the basal surface is placed on a block of pure agar (L). A few hours later the greater part of the auxin will have been transported towards the lower block (M). If the coleoptile cylinder has been reversed (N and 0) no transport what- soever is detectable; all auxin remains in the upper block, in contact with basal cut surface (P). (From The Botanical Review, vol. 1, pp. 162-182. By F. W. Went, 1935.) 154 PLANT GROWTH The exact chemical composition of the auxin-a and -b hormones of plants is known, even though they occur in dilute amounts. However, it is possible that many different hormones exist. Organic chemicals have been found which cause reactions so similar to those caused by the plant ex- tracts, that it is believed they have the same or a similar basic chemical complex. Chemicals that are easily procur- able and easy to use are indole-3-acetic acid and phenyl- acetic acid. These may be purchased also in prepared dilutions from many advertisers in newspapers and garden magazines. More than fifty organic substances have been found to cause bending similar to that of the auxins and the above chemicals. Auxin-a has the formula C18H32O5 and auxin-b has the equivalent of one molecule of water less, that is, C18H30O4. The hormones or chemicals from which they are formed are probably in the seeds. The hormone is abundant in the coleoptile (Figs. 4 and 24) of the oat seedling, which has been used commonly for its study. Hormones are synthe- sized also in leaves, perhaps in root tips, and in fungi. To demonstrate the influence of auxin, oat seedhngs are grown in darkness, at a constant temperature of about 22° C. until the shoots are about U inches long. The tips of some of the coleoptiles may be removed in red or orange light and placed on small sheets of agar (Fig. 24). The tip of a cole- optile is shown pushing through the soil in Figure 4, and the older seedlings show the coleoptile after the plumule emerges from it. After the auxin has moved into the agar, small pieces may be placed on the cut surface of other coleoptiles, to which the auxin will pass from the agar, and cause in- creased growth on the side receiving the auxin. This is the method used to demonstrate the small amount of auxin nec- essary to cause bending, described at the beginning of this chapter. The same figure shows an important characteristic of PLANT HORMONES 155 growth hormones, namely, they move in only one direction through plant tissue. The only known difference in pieces of tissue similar to the coleoptile in L and in O is a very slight electrical charge, but the auxin does not flow in a root to shoot direction. The rate of movement of auxin is much too rapid to be due to the diffusion of its molecules. It may move at the rate of an inch in 2i hours, with little influence of tempera- ture so long as it is favorable to growth. The amount trans- located in a given time is influenced much more by tempera- ture. The maximum is between 30° C. and 35° C. Many experiments like the above and others indicate that the movement is related to the movement of the cytoplasm inside the cells. Auxin influences growth in various ways in different parts of the plant. Stems are elongated by its presence. The cell walls lengthen, perhaps by stretching more freely. If the auxin is equally distributed on all sides of the stem it will grow straight, but if more accumulates on one side, growth will be greater there, resulting in bending toward the side of lower concentration. Two types of bending have been ex- tensively studied. Stems grow upright because auxin moves downward, and, therefore, becomes more concentrated on the lower side which gives the lower side increased growth and forces the stem to grow upright. Auxin tends to move away from the lighted side of a plant. If a plant is placed in a window it can be shown that the auxin is more concentrated on the side away from the window which is the side that grew more rapidly. The role of auxin in root growth is not so clear. Auxin is present in the root in a more dilute condition than in the growing stem. Authorities are not agreed on whether the auxin moves from the shoot to the root or whether it is syn- thesized in the root. It appears to stimulate root growth in very minute amounts and to retard growth in larger amounts. 156 PLANT GROWTH In this way horizontal roots accumulate auxin on the lower side, which results in the optimum condition for growth on the upper side. This explains the downward growth of roots. Even more interesting is the influence of the synthetic hormones on the rooting of cuttings. The initiation of roots requires stimulation supplied by the growth hormones and a few other chemicals which cause an accumulation of the second hormone, called rhizocauline, at the cut surface from which the new roots start. Roots present on the treated material would be killed by the treatment. Plate X shows the conclusive results of rooting after treatment of holly cuttings. Cuttings should be made normally, then placed in the chemical solution (2 to 10 parts in 100,000 parts of water) for 10 to 72 hours. The strength and time should be varied to find the one giving the best results for the particular plants at the particular time of year. A low rate of transpiration from the cuttings gives best results. Lanolin paste has been used as a base to carry the rooting hormone, when it can be applied on the cut surface. Rooting compounds can also be purchased mixed with an inert powder, which can be applied to the fresh cuttings, after which it can be placed in the cutting bench at once. The values of stimulating the initiation of roots on cut- tings are most important where the untreated ones root with difficulty. Here a higher percentage of cuttings root, and a greater number root in a shorter time. Roots, stems, and leaves have been induced to grow roots by this treatment. Hormone action on buds was mentioned in connection with pruning. In most plants the terminal bud secretes the most hormone. It moves downward stimu- lating the elongation of the stem below, but it also inhibits the growth of all lower buds. This has been studied by many people but the details of this growth control are not under- stood. It appears to be an interaction of more than one sub- PLANT HORMONES 157 Stance. If the terminal bud is removed, the inhibition of the remaining top bud will disappear. The same type of inhibi- tion can be shown for sprouting potatoes. If a tuber is cut into cross section pieces with a bud on each piece, they will all grow if placed in a favorable condition for sprouting. Hormones are being used for commercial purposes in such cases as the spraying of fruit trees to delay the falling of fruit. It is not known whether the falling is due to the presence or to the absence of a natural plant hormone. At present it appears that a very small amount of spray solution of naph- thalene acetic acid or naphthalene acetamide on the stem of an apple will prevent the formation of the plate of cork cells across the stem (called the abscission layer) , the weak point at which the fruit breaks from the tree. The whole tree may be sprayed but the stems of the fruit must be covered. It retards the fall of the leaves for a shorter time than the fall of the fruit. Other applications have been attempted with success. It is not too much to expect that we may learn to use hor- mones in many ways, among which might be, to grow short stems longer, to prevent long stems from growing so long, to prevent the drop of flower buds, to inhibit the growth of certain buds, and to stimulate the growth of others. REFERENCES Boysen- Jensen, P., Growth Hormones in Plants, Translated by G. S. Avery and P. R. Burkholder, McGraw-Hill Book Co., 1936. Hitchcock, A. E., and P. W. Zimmerman, "Effect of Growth Substances on the Rooting Response of Cuttings," Contributions from the Boyce Thompson Institute, vol. 8, pp. 63-80, 1936. Nicol, Hugh, Plant Growth Substances, Leonard Hill, London, 1938. Went, F. W., "Auxin, the Plant Growth Hormone," The Botanical Review, vol. 1, pp. 162-182, 1935. Went, F. W. and K. W. Thimann, Phytohormones, Macmillan Co., 1937. Chapter Twenty-One SOIL IMPROVEMENT Good root growth is necessary for healthy plant growth. It is easy to see how important the soil becomes in root de- velopment, since the roots must obtain water and mineral salts, and grow in the soil, pushing between the soil particles, and finally, as they grow in diameter, pushing larger masses of soil aside. The soil has been explained as a complex mass in Chapter 7, and it is safe to say, that any kind of improve- ment will be beneficial in more than one way. Most soils can be improved at a small expense if the effort is continued over a period of time. Soil with a good texture is of a loose, crumbly consistency, which allows proper aeration for root growth and the in- crease in the growth and the number of soil organisms. It must be remembered that as the activity of the organisms increases the humus is destroyed more rapidly. With its in- creased growth, however, the root adds more humus as it decays. Pavlychenko of the University of Saskatchewan, found that single plants of grass had a root system with a total length of 300 miles, but no root was more than 7 feet long. A total length of 150 miles of root system grew each season and about the same amount died each year, leaving the soil porous and adding the decaying roots as humus. The roots for this study were washed free from the soil with extreme care. An example of this important work made a most attractive educational exhibit at the Atlantic City (1936) meeting of the American Association for the Ad- vancement of Science. Plowing and cultivation keep the soil loose. Worms greatly aid cultivation by making burrows through which 158 SOIL IMPROVEMENT 159 air and water can enter the soil. Darwin believed that the common earthworms brought soil up from their burrows and deposited it on the surface in such large quantities that side- walks have been buried. The degree of acidity may influence the texture of the soil, but acidity is more commonly considered in relation to the requirements of various plants. It is spoken of as the hydrogen (H) ion or pH of the soil and refers to the propor- tional number of free H-ions (which are responsible for the acid reaction). For general purposes we might make three groups of plants: those requiring strongly acid soil (azalea) ; those requiring nearly neutral soil (legumes); and the third group, between the extremes, but usually less exacting, with a slightly acid requirement (grass and most garden plants) . Soil can be readily tested for these three degrees of acidity by inexpensive devices to indicate acidity by color. With some study and experience it is quite possible to judge the acidity by the kinds of plants that grow best; for example where moss and sheep sorrel grow in the lawn, lime should be applied. Iron is insoluble in an alkaline soil, which may restrict the plant's absorption of that element. Liming an acid soil may set free a certain amount of potassium, ex- changing it for the calcium, so that, liming may supply the 37lant indirectly with potassium as well as calcium. Soil texture might include the dust mulch on the surface. The value of any mulch (dust, straw, or sand) is largely the retardation of evaporation and retention of a loose soil sur- face, as an aid to absorption of air and rainfall. A good mulch will prevent at least half the water loss from the soil by direct evaporation, which is often enough to save a crop. A dust mulch is valuable because it breaks the films of capil- lary water which tend to move to the soil surface. The more nearly all the films are broken the better the mulch. Shallow frequent cultivation both maintains a good mulch and de- stroys weeds. A black paper has been put on the market to 160 PLANT GROWTH take the place of a mulch. It increases the temperature of the soil, prevents the growth of weeds, and retards to a high degree the evaporation of soil moisture. Hoeing or cultivating soil is a common practice, but many experiments have failed to give evidence that it is valuable. It appears to have no merit in a direct way, but when done properly increases the absorption of rainfall, increases the aeration of the soil, destroys weeds, and probably increases the bacterial action in the soil. All this can be accomplished with a good mulch of grass, leaves, or other materials applied over the loose soil, in which case there is no need to hoe. If cultivating or hoeing is deep enough to destroy many roots the objection is obvious. The water-holding power of the soil can be improved, even though it depends largely on the size of the soil particles. In a small flower bed it may sometimes be desirable to add sand to a heavy clay soil or to add clay to a sandy soil. It is usually easier, however, to improve a heavy or a light sandy soil by adding humus. The humus has a high water-holding capacity and it will increase the bacteria and animal popula- tion, both of which will aid in increasing the water-holding capacity. There are probably few cases where the water- holding capacity is too great and in such cases good aeration should overcome any difficulty. Most plants require a well-drained soil; one in which the roots will not be immersed in water. Areas which are not drained naturally should have underground drainage in the form of tile about three feet below the surface. Drainage often increases the available soil water for plants by increas- ing the root depth, and makes the soil more porous. Roses, for example, which failed before drainage, may grow perfectly after drainage. Humus includes all stages of decaying organisms (plant and animal) in the soil. In most soils the humus is largely of plant origin because the lignified cell walls decompose very SOIL IMPROVEMENT 161 slowly. The value of humus to a soil can scarcely be over- estimated because it improves the soil in so many ways. The addition of humus to a soil is one of the surest ways to im- prove it. Decaying humus adds mineral salts and nitrogen- carrying amino acids. It improves the physical condition by making the soil looser and increasing the water-holding capacity and the aeration. Plants respond to applications of animal manures which cannot be explained by the above, and, in fact, has never been clearly determined. It has been suggested that the micro-organisms of the soil may be aided by manure and that the free-living nitrogen-fixing bacteria (Azotobacter) may increase the nitrogen supply to the plants. Recently it has been found to supply certain hormones which may be very important. Humus is a constituent of all soils but it accumulates very slowly under favorable conditions. Heavy annual applica- tions of manure adds humus only slowly. Best results are had by its slow natural addition to the soil. It is very easy to destroy the humus by excessive cultivation or by the loss of the top soil through erosion. Humus is one of the most important factors in soil conser- vation. Cultivation increases the rate of decomposition of humus, which decreases the absorption of rainfall, thereby increasing the amount of the soil carried away. Both types of humus loss have taken place from many farm soils, where cultivated crops predominated, with the loss in productivity becoming so great that the farms have been abandoned. The rotation of crops in which humus-adding crops are used in place of cultivated ones has been helpful for their influence on erosion. Compost, an artificial humus, is made, usually, by mixing plant material, sand, a high phosphate fertilizer, and some- times manure, in a pile to decompose for a year or more. It is a means for a gardener who has the space, to make use of 162 PLANT GROWTH refuse plant material and have an almost perfect soil for all kinds of garden planting. It is an especially desirable soil for planting seeds, if the seedlings must be transplanted. The living organisms in the soil are numerous, but, fortu- nately most of them are beneficial. The harmful ones, in most cases, belong to the group of insects that live in the atmosphere and lay eggs in the soil. These develop into larvae of various sorts in the soil. The Japanese beetle be- longs in this class and damages roots of grass or other plants. These can be destroyed by poison sprays on the leaves they eat, or by fumigating, or by poisoning the soil. If the soil is poorly aerated, bacteria which can change nitrates and set the nitrogen free may increase in number. This is discussed in more detail in Chapter 23. The beneficial organisms should always be encouraged by the proper cultural methods. Nature will maintain a bal- ance of organisms and the food for them. Therefore if we add organic matter and keep the soil aerated the organisms will increase in numbers. Many of the bacteria play a part in making nitrogen available to the plants, as explained under the topic of the nitrogen cycle in Chapter 23. Soil erosion has destroyed many millions of acres of farm land and has reduced the yields of millions more. The prob- lem is of less importance to gardeners than to farmers but if we are soil-erosion conscious we shall see many soil-covered sidewalks and many little gullies at the edges. Such un- sightly conditions indicate the loss of at least three valuable constituents: water, soil, and mineral salts. The loss may not be great but in most cases the remedy is equally simple: prevent the run-off. If the garden has a slope of more than 4 per cent. Government soil erosion literature should be consulted. The National Soil Conservation Service has found a num- ber of ways to prevent run-off. Most astonishing is the fact that the rate, and therefore the amount of percolation is SOIL IMPROVEMENT 163 several times as fast for clear water as for muddy water. Since the soil is porous clear water will enter rapidly but the silt of muddy water clogs the pores and makes the soil im- pervious. The rain water becomes muddy when the drops hit the bare soil, but if they hit vegetation which breaks them into a fine spray they will not disturb the soil. A good lawn is almost perfect in this respect because of the porous soil and the covering of both growing grass and a mulch of the lawn clippings. This should prevent run-off except in the winter and when the rainfall reaches cloudburst proportions. Soil erosion experiments have shown that a pasture may retain 90 per cent of the rainfall with no loss of soil while a like area in cultivation may retain only 50 per cent of the rainfall, and many tons of soil may be carried from an acre each year. Water run-off and the loss of soil depend on the amount of vegetative cover, on the rate and amount of rainfall, on the kind of soil, and on the slope. The slope is important because it influences the length of time the water will stand on the soil in order to allow for percolation. If the slope is greater than 6 per cent it should be terraced, if that is practical, in order to have a high per- centage of the area nearly level. If the terraces can be built as contours the water may be carried from one terrace to the next at the ends with a minimum of fall. If a part of the area is cultivated, it should be done in strips with lawn between the strips. If this practice is fol- lowed the muddy water of the cultivated area will flow into the grass and lose its mud. This practice is recommended to farmers under the term strip-cropping. REFERENCES Hall, A. D., The Soil, 4th Edition, Murry, London, 1931. Soil Conservation Literature of the United States Department of Agriculture. Soils and Men, Yearbook of Agriculture, United States Department of Agriculture, 1938. Chapter Twenty-Two FERTILIZERS Plants absorb some of each of the elements found in the soil solution, without regard to their need; in fact, poisonous ones may be absorbed in quantities sufficient to kill the plant. This subject will be discussed first from the point of view of soil culture and later in the more specialized water culture. Boron (an essential element) has been absorbed in amounts injurious to the plant. For example, at one time it became a serious problem when a small amount as an im- purity in one of the fertilizer constituents caused tomato flowers to drop, with a considerable loss to the growers. Selenium (an unessential element available in certain soils) may be absorbed not only in large enough amounts to be toxic to some plants, but also, which is more important, in quantities which are not toxic to the plants but are sufficient to cause serious injury to animals feeding on selenium-con- taminated plants. Of the more than forty elements found by analyzing plants only fourteen are known to be essential for their growth. Some of these are needed in quantities of less than one-tenth of an ounce in a ton of dry matter, yet without this small amount the plants can not grow and produce seeds. The fourteen elements may be divided into two groups. Three of the elements, carbon, hydrogen, and oxygen make up most of the plant (more than 98 per cent in corn), and they are obtained from the air and water, as described in Chapter 12. The other eleven elements, nitrogen, phos- phorus, potassium, calcium, magnesium, iron, sulphur, man- ganese, boron, zinc, and copper are obtained from the supply in the soil. Except for nitrogen these eleven elements may be recovered from the plant ash. 164 FERTILIZERS 165 Since such small amounts of the mineral elements are used by growing plants, the research in this field has been long and exacting. Most of these studies have been made by growing plants in water to which the various elements are added. Only recently manganese, boron, zinc, and copper have been added to the Hst of essential elements and other studies are in progress with other elements. This prob- lem has been studied for several centuries as illustrated by the work of Van Helmont (1577-1644), who grew a sixty- nine-pound willow tree from a sprout in a weighed tub of soil, and when he found a loss of only two ounces in weight of the soil, he concluded that plants were made entirely of air and water, a mistake for which we must not censure him too severely. Of the eleven elements supplied by the soil, only the first four, nitrogen, phosphorus, potassium, and calcium, are commonly added as fertilizer, since the last seven are needed in such small amounts by the plant, that enough is available in most soils to supply the needs of the plant. Probably no one has analyzed the chemical composition of strictly garden plants so completely as Latshaw and Miller have analyzed corn, but the results would be comparable. They found about 1 pound of nitrogen in 225 pounds of corn plant and much smaller amounts of each of the other elements; in fact they found nitrogen was almost equal in amount to the total of the other ten necessary elements. Since nitrogen is used in such large amounts in plant growth and is the one element that can be added by certain bacteria found in the soil and by leguminous plants, the next chapter has been devoted to this one element. The chemical composition of plants varies widely depend- ing on several factors, four of which will be described: (1) Newton found that the characteristics of plants vary in their absorption of elements when grown under the same condi- tions as shown by the composition of some common plants, as follows : 166 PLANT GROWTH Percentage of the dry weight of Plant Ca K Mg N P Sunflower 2.2 5.0 0.64 3.6 0.56 Beans 2.1 4.0 0.59 3.6 0.55 Wheat 0.8 6.7 0.41 4.5 0.49 Barley 1.9 6.9 0.54 4.7 0.52 Peas 1.6 5.3 0.50 4.5 0.19 Corn 0.5 3.9 0.40 2.9 0.39 Note that the peas have three times as much calcium as corn, more potassium and magnesium, nearly twice as much nitrogen but only one-half as much phosphorus. Other comparisons in the table are equally striking. (2) Carolus showed that the absorption depends on the availability of the elements. He found that when nitrogen and phos- phorus were applied to the soil, the plants absorbed not only more of these elements, but also more calcium and mag- nesium. (3) Absorption of salts is increased by an optimum soil moisture content. (4) A reaction of the soil with a pH (see p. 171) of about 6 produces the best growth for most garden vegetables with a minimum percentage of mineral salts. These facts indicate that more consideration should be given to the conditions of growth in the selection of our vegetables as sources of mineral foods. Fertilizers are applied to soils to supplement the deficient soluble elements of the soil. It must be remembered that the soil contains an enormous store of insoluble elements which are slowly becoming soluble. The fifty-year fertilizer experiment at the Pennsylvania State College Experiment Station shows clearly the stability of the soil. After forty years of cropping without fertilizer the yields had fallen to half the original, but if only phosphorus was added, in addi- tion to lime to reduce the acidity, the yield was maintained for fifty years, showing that all the other elements were sup- plied by the soil. With the best treatment, namely, added phosphorus, lime, potassium, and nitrogen, soil described as in good state of fertility at the beginning of the experiment FERTILIZERS 167 showed an increase in yield of more than 30 per cent at the end of fifty years. The stabiHty of the soil was demonstrated in a different way at the EngHsh Rothamsted Experiment Station. Wheat was grown without fertiHzer appHcation for many years on the same soil, with a gradual reduction of yield, but after the field was fallow for two years the yield was restored. It may be assumed that the elements were gradually becoming avail- able during the two years and were sufficient to produce a good crop. The one good crop, however, reduced the soil to its former infertile condition. It has been said that the aver- age soil has enough of the essential elements to supply crops for hundreds of years if they were all available. Since a chemical analysis of a soil will show the soluble and varying amounts of the insoluble elements it is clear that the result would not indicate the fertilizer needs. The best quick method is one in which a color reaction is used to indi- cate in a general way the available amount of each element. With this information and the knowledge of the previous treatment of the soil, fertilizers can be applied most effi- ciently. Several such "Soil Testing Sets" are on the market, but considerable practice is necessary before they can be used successfully. It is often better to write your State Ex- periment Station for directions for collecting a soil sample and have them make the tests. Recent methods have been devised to make tests of the growing plant tissues for the elements most likely to be deficient. This method is best where a continuous check on the needs of the plant is made. So many different conditions of the soil and environment exist that it is often wise to experiment with various fertilizer applications on small areas of the lawn and on several differ- ent garden plants. Valuable information regarding the amount and the particular kind of fertilizer best suited to a local situation can be determined. If one, three, and five pounds of fertilizer per hundred square feet of lawn are ap- 168 PLANT GROWTH plied on small adjoining areas in the fall you are almost sure to see differences during the following summer. Other ex- periments will suggest themselves. When crops are grown in rows, the treatment can be varied in much the same way, as well as in varying the relative amounts of the different elements. One can even omit nitrogen, potassium, or phos- phorus from some of the plants. The results should be judged by the yield and by the characteristics of color, leaf size, and stem growth. If possible certain areas should have normal or no treatment for comparison as checks, with any type of experiment. The tabulation on page 169 should be consulted. Garden soils are seldom so deficient in any ele- ment as to show clearly the deficiencies described in the table, but the contents of artificial solutions made by adding salts to water can be accurately controlled. This experimeiit may be carried out by growing the plants directly in the water solution or by growing them in washed sand to which the water solution is added. The photographs of buckwheat and cowpeas (Plates XI and XII) show the results of simple experiments in defi- ciency symptoms. Plants grown in clean sand with solutions added are compared with those grown in soil as checks. Color photographs of deficiencies are often shown in maga- zine advertisements for fertilizers. In performing either of these experiments a solution is made with ten of the eleven essential elements. The plant will be able to grow normally until it has used the supply of the omitted element from the seed and from the impurities of the other salts. Because of the difficulty of getting pure salts, plants grown in solutions lacking boron, zinc, and other elements used in very small quantities show deficiencies barely detectable. Those grown in solutions lacking nitrogen and other elements required in larger amounts show a growth proportional to the amount used in the plant and the amount stored in the seed. The cowpeas lost their leaves sooner without nitrogen. A small FERTILIZERS 169 difference in the purity of the salts will show in the results of certain experiments. The soil grown check plants had a dry weight equivalent to the full nutrient plants in these experiments. It is of interest to note that the plants grew appreciably better with tap water than with distilled. This shows the unusual dilution, as found in tap water, from which salts can be absorbed. A TABULATION OF THE CHIEF FUNCTIONS AND GENERAL SYMPTOMS OF DEFICIENCIES OF THE ELEMENTS TAKEN FROM THE SOIL Element Functions in the Plant Symptoms of a Deficiency Nitrogen Phosphorus Potassium Calcium Magnesium Iron Sulphur Manganese Boron Zinc Copper Constituent of protein, protoplasm, and chlorophyll molecules. Excess produces large plants. Constituent of nuclear protein. Im- proves root growth and seed for- mation. Aids in enzyme actions. Essential for photosynthesis and sugar translocation. Improves quality of fruit. Aids in absorption of other elements and in root growth. Constituent of chlorophyll. Neces- sary for formation and use of oils. Essential for synthesis of chloro- phyll. Constituent of protein. Aids nodule formation in clovers. Used in oxidation process and chlorophyll synthesis. Necessary for cell division. Not clear. Not clear. Small plant, of leaves. Yellow-green color Poor root growth. Bluish-green leaf color. Shorter internodes. Stunted growth. Premature death. Small chlorotic leaf spots. Poor root growth. Buds and young stems stunted. Chlorotic rusty spotting of older leaves. Lack of chlorophyll and poor root growth. Poor root system and vigor. Re- sembles plants low in nitrogen. Chlorotic leaves and death of the youngest ones. Young buds and leaves become brittle and break off easily. May cause "little leaf" and "white bud" diseases. At least some plants fail to grow seed. Fertilizers are usually purchased with a guaranteed per- centage analysis of available nitrogen, phosphorus, and po- tassium. These three elements together are called a com- plete fertilizer, and are expressed as a 4-12^ fertilizer, when 170 PLANT GROWTH 4 per cent of nitrogen, 12 per cent of phosphoric acid, and 4 per cent of potash are present. This is a good general pur- pose proportion of elements for much of the eastern United States, but many other combinations are available as well as incomplete fertilizers, containing one or two elements. If a large leafy plant is desired a much higher proportion of nitro- gen may be used. The analysis above shows only twenty pounds per hundred of available plant food, but it must be remembered that part of the other eighty pounds will become available gradually. The most successful method of fertilization over a long period is the application of manure and a phosphorus fertil- izer. The photograph (Plate XIII) shows the importance of higher amounts of phosphorus on root growth and deep color of the tops, since they improve from the smaller to the larger amounts. In the lower series the influence of varying amounts of nitrogen is shown. The reverse response in root growth is seen; but the tops are larger with higher nitrogen. The phosphorus and the manure make the most desirable combination of elements for most soils. The manure is often difficult to procure and to apply on the lawn, and besides it has many weed seeds. The use of three pounds per one hundred square feet of a complete fertilizer, in the late fall and again a month before active growth begins in the spring, on the lawn and the perennials will make a wonderful im- provement in growth. The garden areas that are dug and cultivated should have a like application before planting and two more at about twenty-day intervals when hoeing. It has been found that plants require large amounts of fertilizers when they are making their early growth and relatively small amounts as they mature. It is possible to use too much fertilizer, but this happens only rarely. It must be mixed with the soil, or, if put on the lawn, followed by a copious watering to carry it to the soil. In this way it becomes in- corporated with the soil and can be absorbed by the plant. FERTILIZERS 171 Soils slowly become acid because plants remove a little more of the alkaline than of the acid elements, and the alka- line elements leach from the soil more rapidly, leaving acid residues in both cases. The balance of alkaline to acid ele- ments is referred to as pH which means the ratio of hydrogen ions (acid) to the alkaline or basic ones. A pH of 7 means that they are in equal amount and neutralize each other. When acid ions increase to ten times as many in proportion to the basic ones the pH is 6 and when alkaline ions increase to ten times as many in proportion to acid ones, the pH is 8. A few plants, such as the azaleas, rhododendrons, most ferns, and orchids, are adapted to an acid soil of a pH ion concentration of 5.0 or lower. Those plants that naturally grow in an oak or evergreen forest are likely to grow better when the soil is kept acid. Soils may be made more acid by aluminum or ammonium sulphate. In the former the alumi- num becomes insoluble and in the latter the ammonia is absorbed by the plant, leaving the sulphate part as an acid in the soil. Most plants grow best when the soil is only slightly acid, as explained in Chapter 21. Lime, in any one of three forms, may be used to neutralize the soil acids; as finely ground limestone, calcium carbonate (CaCOs), as the burned lime- stone or quick lime called calcium oxide (CaO), or as the hydrated lime (Ca(0H)2) where one molecule of water unites with a molecule of the calcium oxide. The ground limestone although slow in its reaction is a little more con- venient to handle. The hydrated form is commonly used. Most lime contains some magnesium, but it should not exceed 14 per cent, since it is toxic in excessive amounts. Calcium, Hke phosphorus, has been found recently to diffuse very slowly through the soil while the other elements diffuse more rapidly. In order to have accessible to a large part of the root system a favorable supply of the calcium and phosphorus it is best to mix large amounts of it with at least 172 PLANT GROWTH the top foot of soil when preparing permanent areas for lawn or perennials. A ton of ground limestone and a ton of finely ground phosphate rock per acre may be used. Ground bone meal may be used to supply the phosphorus and to add a part of the calcium. Opinion differs regarding the frequency of applying lime, but it is safe to be guided by the change in the pH. Plants thrive in a range of one pH or more, that is, for most garden plants a favorable range goes from 5.6 to above 7 pH. In the more specialized technique mineral salts have been used in water solutions for many years for growing plants in experiments like the one explained in this chapter, but recently the method has been modified for use in commercial greenhouse culture, where it is believed that, at least under some circumstances, it is more economical than the use of soil. Flowers and vegetables grown by this method, called "hydroponics," are available now in many markets. Many people are using this method of growing plants as a hobby, but to be successful it requires a close study of the technique and should be attempted only by those who are willing to be disappointed at first. When the techniques have been mas- tered it is a most efficient and satisfactory way to grow large yields of excellent plants. Several water-culture systems have been devised, and often named for the man or institution making them popu- lar. "Soilless Gardening" by Gericke uses the water solu- tion, in shallow tanks or troughs, with the plants supported in a seedbed of vegetable material on a wire support. The seedbed and an air space between it and the solution are the distinctive features of the method, which provides for root aeration, the most difficult problem. The seedbed has a layer of straw or excelsior on the wire, then a layer of finer vegetative material, which will support the plant, hold mois- ture to be absorbed by the roots growing in it, and allow air to pass through it freely to the air-space below. The air- FERTILIZERS 173 space may be only an inch for young plants, but is allowed to increase as the young plants grow. The book "Soilless Gardening" must be consulted for the formula and the many details of the system. Dr. J. W. Shive, at the New Jersey Agricultural Experi- ment Station, devised various ways of bubbling air into the solution without the use of the seedbed. The bubbles are best if they are very small. Ordinary fish aquaria aerators seem to be satisfactory. A third method, which is very simple to operate, employs sand as a medium for the roots, and the nutrient solution is dripped on the top and allowed to drain away at the bottom. This insures good aeration if the sand is of the proper coarse- ness to allow the water to drain through freely. Simultaneously workers at the Agricultural Experiment Stations of Purdue University and New Jersey devised a fourth method, used commercially quite frequently, employ- ing a fine gravel or cinders in place of the sand. The gravel is too coarse for water to move rapidly by capillarity, there- fore the gravel is flooded two or more times each day to keep it wet and allowed to drain back to a storage tank. The solu- tion is kept in a tank beneath the gravel plant beds, and electric pumps, controlled by time clock switches, work auto- matically. This method gives the maximum of aeration and for large installations a minimum of labor. The solutions are tested two or three times each week and supplied with needed elements. The writer made use of methods one, three, and four in the greenhouse with very simple, inexpensive equipment, as shown in the photograph (Plate XIV). Tomatoes, buck- wheat, and corn all grew to maturity, the latter with a height of more than ten feet. Two problems — the nitrogen was too abundant and the pH too high — offered some difficulty, but both were later regulated satisfactorily. The pH must be kept below 6.5 if the solution is warm, to keep the iron in 174 PLANT GROWTH solution. The gravel was flooded by a simple automatic device requiring attention about every day. It depends on slow drip from a supply bottle, and an automatic siphon to carry the solution to a bottle below when it reaches the flooded stage. The siphon is shown at the end of the gravel tank. REFERENCES Bear, Firman E., Soils and Fertilizers, John Wiley & Sons, Inc., 1942. Gericke, W. F., Soilless Gardening, Prentice-Hall, 1940. Laurie, Alex., Soilless Culture Simplified, McGraw-Hill Book Co., 1940. Miller, E. C, Plant Physiology, McGraw-Hill Book Co., 1938. Turner, W. J., and V. M. Henry, Growing Plants in Nutrient Solutions, John Wiley and Sons, 1939. Chapter Twenty-Three NITROGEN Nitrogen is so important in the growth of plants that a complete chapter is given to its discussion. It is a large im- portant part of the living material of the plant protoplasm and of the storage protein. As shown by the photographs (Plates XI and XII) its complete absence from the soil solu- tion retards growth as much as though the plant were de- prived of all salts; in fact, it might be called the limiting element. Most soils are deficient in nitrogen and, therefore, its application brings a quick response in growth and improved color of the leaves. These pleasing results tempt the gar- dener to overstimulate with nitrogen and so to produce a plant with larger cells, longer internodes, larger leaves and a smaller root system (Plate XIII). Nitrogen should be applied most heavily in the early stages of germination of a plant to encourage rapid growth, and in decreasing amounts later in the season in order that as it ages the plant will develop sturdy structure and will store larger amounts of food material. Plants so treated are less likely to winter kill than others and are able to use the stored food for rapid growth the next season. The carbon to nitrogen ratio was discussed at some length in Chapter 14. The idea of ratio should be kept in mind in dealing with any phase of the nitrogen problem. The plant can be kept considerably off a balanced carbon and nitrogen ratio to produce a desired type of growth. All the methods for controlling the ratio may be used, but certainly it is influ- enced most by the pruning, the amount of sunlight, the soil moisture, and the applications of nitrogen. . 175 176 PLANT GROWTH The description of the relation of growth to nitrogen sup- ply should be read in Chapter 14. It has been found that, regardless of light, plants can build proteins in any actively growing cell that has an adequate supply of carbohydrates and nitrogen. Normally the nitrogen content of the soil decreases during the summer, which causes a deficiency in the plant resulting in carbohydrate storage. If a high ni- trogenous fertilizer is applied in mid-summer or early fall the plants may respond with a rapid growth which will not harden or mature enough to avoid winter-killing. The root systems of plants with high nitrogen are smaller in proportion to the tops when compared with those from better-balanced nitrogen-level plants (Plate XIII). For this reason high nitrogen plants will suffer more quickly from drought. A simple experiment to study the root effect of nitrogen may be made by using plants in pots and treating with varying amounts of nitrogen as described above. A garden experiment would be better if the removal of the roots is not too great a task. A study of the influence of growth and storage of carbo- hydrates may be made by keeping a high nitrogen level with roses by heavy pruning through the summer or by adding nitrogen to the soil at frequent intervals. Normally high nitrogen will stimulate excessive new growth until late fall. The new growth will not be hardened resulting in more winter-killing, and the following summer the growth may be poorer because of the lack of stored food. Nitrogen is commonly applied as nitrate of soda or as ammonium sulphate. The sodium leaves a more alkaline medium and the sulphur a more acid one. Acid-loving plants should have the latter, but if it is used on other plants it will be necessary to use lime more frequently. Ammonium nitrate may be used, in which case the nitrogen is available from both the acid and alkaline radicle. The nitrogen cycle diagram shows that plants are able to use either the am- NITROGEN 177 monium nitrogen or the nitrate nitrogen. Some plants appear to have a preference, but in many cases it depends on the conditions of the soil. Most garden plants can use the nitrate form better with a pH of 6.5 or lower, but above this pH in some cases they use the ammonia form more effec- tively. Nitrogen is often applied in an organic form as bone meal, tankage, cotton seed meal, or raw bone meal. These forms act slowly because they are not available until they are decomposed by bacteria as shown in the chart (Fig. 25) . Manure carries some nitrogen and, as explained above, it appears to stimulate the nitrogen-fixing bacteria. A lawn will need very small applications of nitrogen, since the high organic content left by decaying roots furnishes food for the nitrogen-fixing bacteria, and also the lawn holds moisture so well that the leaching effect is reduced. If nitro- gen is applied it should be done in the fall or in the early spring before the shoots begin their active growth. At this time the soil is low in nitrogen and growth may be limited. The size of flowers can be increased with nitrogen, since it reacts on flowers in much the same way as on leaves. Leafy vegetables can be increased in size with nitrogen. The nitrogen content of the soil is usually lowest after protracted heavy rains in mid-summer or later, when it may become so low that the plant reacts as indicated by lack of good green color. The problem of an adequate supply by nature of nitrogen salts for the soil is not understood. The atmosphere contains 79 per cent of free nitrogen but plants cannot use it until it is fixed in a molecule. The salts of nitrogen are very solu- ble and are readily leached from the soil. In fact under some conditions as much may leach from the soil as is used by the plants. A crop of corn may use more than a hundred pounds of nitrogen per acre, whereas the farmer seldom applies more than fifteen pounds per acre as fertilizer. Each crop except the legumes, which bear nodules on their roots containing 178 PLANT GROWTH nitrogen-fixing bacteria, removes more nitrogen from the soil than is appHed as fertihzer. The legumes include the clovers, vetches, beans, peas, lupins, etc., all of which de- velop small ball-like nodules on their roots if the proper bacteria are in the soil. A single strain of bacteria will inocu- late several but not all species of clovers. Another strain is necessary for lupins and a third for alfalfa. If the proper bacteria are not in the soil they must be added to the seed or the soil. These symbiotic bacteria can AIR 79% NITROGEN ANIMALS MINERAL FERTILIZERS -4 '' SOIL LEGUME LEGUME EXCREMENT PLANTS ^ NITRATES BACTERIA *"*" PLANTS / FREE LIVING BACTERIA AMMONIUM -♦NITRITES SOIL BACTERIA — > DECOMPOSE -> COMPOUNDS ORGANIC MATTER "*-"-^ LEACHES FROM SOIL Fig. 25. The nitrogen cycle. Nitrogen from the air may be traced by the arrows as it is acted upon by various bacteria until it can be used by plants. The plants may be eaten by animals or they may be decomposed by bacteria. convert free nitrogen from the air into a molecule which can be used by the legume, and when the legume is decomposed by other bacteria the molecular nitrogen remains in the soil. This and the later described bacterial action are shown in the nitrogen cycle (Fig. 25) which should be consulted as a guide to the whole problem. A legume crop removed for hay will not add so much nitrogen to the soil as the corn crop uses. Thus crops remove more nitrogen from the soil than is added by man, and leach- ing may lose a like amount. A small amount is washed from the atmosphere as ammonia by rains. Two sources may explain the problem of the additional supply: first, the work of the free-living nitrogen-fixing bac- NITROGEN 179 teria, and second, the bacterial action which Hberates the nitrogen of dead plants and animals. The various species of bacteria and their numbers, far beyond the concept of man, must remain in a kind of balance because of their dependence on each other. Without this great group of microscopic organisms, agriculture, as we know it, would be impossible. The fact is more startling when we realize that so little is known about the various bac- teria involved and that no general attempt is made to main- tain the most desirable balance. The nitrogen-fixing bacteria that live in the soil have been difficult to study but several groups are known to use humus as a source of energy and are then able to fix atmos- pheric nitrogen (use free nitrogen and convert it into a usable molecule containing nitrogen). The soil must be aerated since the one group must have oxygen for their respiration. Azotobacter and Clostridium are two groups of nitrogen-fixing bacteria, but detailed studies of their activi- ties are difficult since the soil is so complex a medium that the factors cannot be controlled and they probably do not react the same in sterile condition in a test tube as they do in the soil. It is believed that part of the value derived from manure or other forms of humus is due to the increased activ- ity of the nitrogen-fixing bacteria. It is quite possible that these bacteria add more nitrogen to the soil than do the legume bacteria in the ordinary farm rotation but in garden- ing practice the legumes find little use. The bacterial action of decay and the preparation of that nitrogen for plant use involves several groups of bacteria. Bacteria which cause decay of other bacteria, plants, or ani- mal excrement, change the proteins to amino acids and then to ammonia. This forms ammonium compounds in the soil. As explained above and shown in the chart the ammonium compounds may be used by the plant, but if not, they are acted on by other bacteria which oxidize them in order to get 180 PLANT GROWTH energy. The first group of bacteria oxidizes the ammonia to a nitrite form, after which another kind of bacteria oxi- dizes the nitrite to nitrate. This nitrate is absorbed by the plant just as effectively as that applied as fertilizer to supple- ment the soil supply. REFERENCES Maximov, N. A., Plant Physiology, Edited by Harvey, R. B., and A. E. Murneek, McGraw-Hill Book Co., 1938. Nightingale, G. T., "The Nitrogen Nutrition of Green Plants," The Botanical Review, vol. 3, pp. 85-174, 1937. Wilson, P. W., "Symbiotic Nitrogen Fixation by the Leguminosae," The Botanical Review, vol. 3, pp. 365-399, 1937. Chapter Twenty-Four SOME SPECIAL CONSIDERATIONS OF PLANT GROWTH Growth is an irreversible change in shape or weight of a Hving organism. It usually means an increase in dry weight because of the increase in cell number and size, but in case of seedling growth the weight decreases (see table below) because the stored food is being oxidized. Growth is the result of many coordinated activities of the organism, some of which are deeply involved in growth regulators and stimu- lants. Some of our knowledge of hormones in relation to growth is briefly described in Chapter 20. The germination in soil of radish and pea seedlings in full sunlight, in 10 per cent full sunlight, and in darkness (Plate XV) shows the external influence of light on growth. The following table tells the story of the use of the food (in dry weight) and the relative amounts of water in the green weights: Light Green wt. gm. Dry tot. gm. % o/ gain or loss per plant per plant in germination % Radish Full 0.206 0.016 45 gain 10 % 0.212 0.010 19 loss Darkness 0.328 Pea 0.007 37 loss Full 2.2S2 0.221 32 loss 10 % 2.499 0.203 38 loss Darkness 3.274 0.196 40 loss The leaves are thicker, the roots are better branched, and the shoots are much shorter, with full sunlight. Shade has little effect on the size of the leaves, but makes a marked increase in stem growth, and a decrease in dry weight. The dark-grown plants show the lack of leaf growth, the extreme 181 182 PLANT GROWTH growth of the stems, and an extreme loss of dry weight. Growth hormones may be inactivated in the Hght. The radish in full sunlight carried on photosynthesis to more than regain all the loss in the early germination, but the pea has regained only a small amount of the earlier loss. Most plants should not be transplanted until the seedlings have regained a supply of stored food. Growth is often defined to include all the activities in the development of the plant, including: cell division (see Chap- ter 4) , synthesis of protoplasm, elongation of the cells, differ- entiation of the cells, and the final maturing of the cells for their special functions. The synthesis of the protoplasm and cell walls requires food as the building material and oxygen for respiration. The elongation of the cells depends on the osmotic concentration and turgor pressure against the walls and on the growth-regulating hormone. The cellulose cell walls must stretch and the growth hormone appears to regu- late the resistance to stretching. A comparison of the green weights and the dry weights under these different conditions of light, as well as the size and the shape as shown in the photograph, illustrates how these water and growth relations are manifest in seedlings when they are influenced by vary- ing amounts of light. Available water also influences growth in size and in dry weight. When water is deficient growth will be reduced because of a decreased turgor pressure and a relatively smaller increase in green weight than in dry weight will be found. Under drought conditions the stomata may close as explained in Chapter 13, causing a further decrease in the rate of growth in size and in dry weight because of the retarded photosynthesis. Since the turgor pressure is so important it is evident that it will be greater at night because of the decreased transpira- tion and that growth (stretching) will be greater than in the day. This can readily be determined by night and morning Plate XV. Seedlings grown in sunlight, in shade (10% of sunlight), and in dark- ness. Radish (left); pea (right), (i natural size.) Plate XVI. Growth of morning-glory and moonflower in soil with optimum mois- ture (left) and low moisture (center). CONSIDERATION OF PLANT GROWTH 183 measurements of a rapidly growing plant. Growth is not related to photosynthesis, except for the food supply which is normally in excess of the immediate needs. Inasmuch as all cells of an organism come from the origi- nal fertilized egg and since all newly divided cells are similar, it is obvious that cells must differentiate to form the many kinds of tissues in an organism. This change from the origi- nal kind of cell must be controlled by hormones or organizers but it is not understood. Finally the cells take on their matured characters, as the wood cells with thick lignified walls or the thin cell walls of the pith or any one of the many forms of cells in the leaves. The rate of growth is variable depending on the condi- tions that influence growth. Normally a cell, an organ, or complete organism, such as a plant or animal, begins to grow slowly, but increases the rate for some time, following which the rate remains high for a time and then declines. It is not uniform but rhythmic. This is true of a shoot or of one side of a shoot or of a pollen tube or a fruit. This rhythm is best shown by time-lapse motion pictures. The rate of growth is influenced by temperature. The lowest or minimum temperature for growth is about freezing. As the temperature increases the growth rate increases to about 90° F., beyond which it decreases for most plants. Many plants that have been studied may grow rapidly for a few hours at a high temperature after which the rate de- creases, but if the temperature decreases for a time, growth increases again with an increase in temperature. This may be a night and day response. Many plants have a decreasing growth rate as the tem- perature increases above 90°, and may stop growing at about one hundred ten degrees. Plants can withstand higher tem- peratures and will resume growth when conditions become favorable. The transpiration of plants reduces their tem- perature; therefore, if plants are killed by naturally high 184 PLANT GROWTH temperatures in this region it is usually because of lack of moisture. Plants can be conditioned to endure low temperature. It is called "hardening" and is brought about by decreasing the water supply and subjecting them to progressively lower temperatures alternating with higher ones. The tempera- ture is often alternated at 36° and 50° F. then gradually lowered. Cold injury is usually due to a lack of water in the whole plant or in some of the cells of the plant. Transpira- tion will go on at a reduced rate in cold weather while absorp- tion is retarded considerably, especially if the ground is frozen. When cells freeze the water begins to freeze and as the water freezes more is withdrawn from the protoplasm. This may continue until the protoplasm has its water re- duced to the point where it is injured. Frequently slow thawing enables the protoplasm to regain the water without injury. Hardened plant cells have a higher colloid and sugar content which resists the loss of water and may explain the value of the hardening process. Plants that normally remain in the soil during the winter should be kept cold to keep them dormant, since they withstand cold better in a dormant con- dition. Mulching should be done when the ground is frozen. Water constitutes from two-thirds to more than 95 per cent of living, growing plants. Any deficiency of water first reduces the turgor pressure of the cells and may in extreme cases be so limited that the protoplasm will not be elabo- rated. The elaboration of protoplasm and enlarging of the cells, due to water absorption, are not synonymous but it is difficult if not impossible to measure them independently. The explanation for the rapid growth of plants with water culture solution rests on the adequate supply of water at all times. Methods of watering plants were discussed in Chap- ter 13. The writer's interest in the water relation of plants led to greenhouse experiments on the influence of soil moisture on growth. Morning glory and moonvine plants were grown CONSIDERATION OF PLANT GROWTH 185 in four-gallon crocks of soil with a known moisture content. The crocks were weighed frequently and water was added to maintain: in the first crock, the optimum amount; in the second, the minimum amount that would support continu- ous growth; in a third, the optimum was kept for the first half of the growth period and the minimum for the second half; and in the fourth crock, the first half was grown at a minimum and the second half at the optimum moisture. Only the first and second conditions will be explained. The photograph shows in general the nature of the growth and the bottles for adding the water. The plants with optimum water reached a length of ten feet and were trained along the roof of the greenhouse but are hanging down to the crock in the photograph (Plate XVI). The plants with minimum moisture were less than one-fourth as long. The dry weight of the tops and the surface area of the leaves had the same relative relationship. The number of the stomata did not follow so clearly the one to four relation. The plants with optimum moisture had about ninety thousand stomata per square inch and those with minimum moisture about a third more. It is generally believed that plants growing where moisture is limited have fewer stomata, as was true here on a per plant basis, but they actually have more of them in a given area. This has been found to be true on other plants. If the increased size of the leaves with the moist soil is con- sidered the number per leaf and per plant is much greater than with the dryer soil. The humidity of the air also must have a marked influence on growth but no careful experi- mental work with this condition is known to the writer. Perhaps next to temperature and water supply, the sup- ply of food, both as minerals from the soil and as carbohy- drates of photosynthesis, is most important in growth regu- lation. Second only to water the food supply is modified with greater ease and more striking results in the average garden than the other growth factors. Experiments of inter- est will suggest themselves to many gardeners. Each of us 186 PLANT GROWTH has tried, or has heard of our friends trying, the chemical vitamin Bi, as a growth stimulant. The importance of a soil with a texture loose enough to supply oxygen to the roots must be included here and a reference to other growth-regu- lating conditions described in earlier chapters. Plants grow in cycles or at certain periods, alternating with periods of rest or dormancy, as is commonly observed in the spring after the winter dormancy. This is one of the hereditary characteristics. It is true for the shoot and for the root, although the first period of root growth begins before shoots elongate and the leaves grow in the spring and a second period occurs in many plants in the fall. The growth in stem thickness may be more variable depending on the growth conditions but normally the stem thickens more rapidly during the spring growing period. Flowering cycles in some plants depend on length of day rather than time of year or the age of the plant. Any experimental work should be planned to coincide with the normal growth cycle. It is possible in many cases to modify the growth cycle, but that at once becomes an experiment of major impor- tance. The reader may refer to some of the work of F. E. Denny and his co-workers, reported in the contributions of the Boyce Thompson Institute, for the details of the treat- ments. Gardeners make use of natural growth cycles in fol- lowing the periods of planting, pruning, and harvesting the desired crop. One of the most striking and interesting illustrations of changing the normal growth cycle was announced by the Russians a few years ago concerning wheat. Normally sown in the fall, winter wheat grows a small plant before winter puts it in a dormant condition. The next spring's long days induce heading and ripening. "Vernalization" is the term applied to forcing plants to go through a part of their life cycle in an unusual controlled method. The treat- ment of wheat consists in giving a mass or pile of it about half the maximum amount of water it can absorb, and hold- CONSIDERATION OF PLANT GROWTH 187 ing it at a temperature of about 60° F. for twenty-four hours to start germination. Next the temperature is lowered to about freezing for about two weeks. This prepares the grain to begin normal growth in the spring, after which it can be dried and kept until the soil is in condition for seeding. It would appear that the same growth reactions take place with this treatment that take place in fall-sown wheat. The Rus- sians claim extensive use of this method of growing wheat. They believe vernalization can be applied to many other plants in many other ways. To illustrate in another way, plants requiring short days and warm soil may be given just enough moisture to allow some — it is not clear which — physiological activities to go on for a week or longer, but only enough to make little or no growth, while at the warm temperature of the summer and in the dark. This, the Russian investigator, Lysenko, says gives the plant its needed amount of warmth and darkness for development, and it will be able to grow to maturity with lower temperature and longer days than it requires when planted untreated. Time will be necessary to test these methods under many conditions and to simplify the pro- cedure. The rapid growth of many plants in the far north with continuous daylight indicates that at least some plants can make food and grow efficiently under what in industry we would call twenty-four-hour shifts. The great question re- mains, can we learn to control the many enzymes, hormones, and growth regulators in an artificial manner to speed up the growth of plants in an environment where they do not natu- rally grow? REFERENCES Bell, G. D. H., "Experiments on Vernalization," Journal of Agricultural Science, vol. 26, pp. 155-171, 1936. Burkholder, P. R., "The Role of Light in the Life of Plants," The Botanical Review, vol. 2, pp. 1-52, 97-128, 1936. Hill, J. B., L. O. Overholts, and H. W. Popp, Botany, McGraw-Hill Book Co., 1936. Meyer, B. S., and D. B. Anderson, Plant Physiology, D. Van Nostrand Co., 1939. Chapter Twenty-Five REST PERIOD OF PLANTS Most plants of the temperate zones have developed the characteristic of growing only during the summer. Annuals grow, bear flowers, seeds, reach old age, and die during the summer. The embryo plant of the seed is the only part or stage to have a rest period. At the end of the growing season biennials and perennials go into a dormant or rest period for varying lengths of time, part of which is usually during the winter. The physiology of these adaptations to climate are not understood. The rest period is that time during which a plant or any part of a plant remains dormant or inactive even though it is given all the external conditions necessary for growth, and should be distinguished from the inactive period of stored seeds, due to unfavorable conditions for germination. Most plants, bulbs, tubers, seeds, etc., can be kept in an inactive condition after the rest period is past, by subjecting them to a low temperature and keeping them with a low moisture content. Either a low temperature or a low moisture is often suflficient but both are better. Dormant and inactive plants carry on all the activities necessary for life, but at a very low rate. Several thousand bushels of wheat may be kept in storage for months with no thought of its oxygen supply, but the small loss in weight indicates very slow respiration. Dormant seeds usually have more stored food and less water but the water is held more securely in the colloidal mass. The cells are mature, growth has ceased, and the rate of respiration is very low. These conditions enable a plant or seed to withstand extremes of temperature which would be impossible with actively grow- ing parts. This characteristic is most striking among wild 188 REST PERIOD OF PLANTS 189 plants, where the seeds may be matured in early summer and lie unprotected in a dormant condition until the following spring or in some cases for a much longer time. The rosette stage, frequently on a fleshy root, developed by a biennial remains dormant over winter and grows a tall shoot with seeds the next summer, as in carrot (Fig. 18). Perennials remain dormant in winter but grow and produce seeds each summer. This characteristic rest period is so deeply fixed in oaks that the seedlings grown in the greenhouse for four years continued to lose their leaves and remained dormant during the winter but their leaves commenced to grow each spring only about two weeks before those of the oak trees outside the greenhouse. Dormancy is more deeply fixed in certain species than in others but in all cases it appears to be stronger in the early period and appears to weaken as the period progresses. To- ward the end of the period it is easily broken in most plants. Bulbs, tubers, seeds, and branches of flowering shrubs may be forced in most cases by treating with warmth of 30° to 35° C. for twelve to twenty-four hours, if treated near the end of their dormancy, but more drastic treatment would be required at an earlier period. The dormant period is not always in the winter; crocus, tulip, spring beauty, and many other bulbous plants begin the dormant period in late spring and begin root growth in the late fall or in mid-winter. The bearded iris has a period just after flowering that approaches a dormant condition. All bulbs and rhizomes may be transplanted most effectively during the dormant period. The seeds of many of our cultivated plants have a very short or no dormant period. Grapefruit seeds are often germinated in the fruit. Beans and peas may sprout in the pod if it falls on the ground. The farmer uses care in shock- ing his grain to prevent its getting wet enough to cause it to sprout before it is dry enough to put in storage. Some spe- 190 PLANT GROWTH cies have a short dormancy. White oak acorns which can be seen germinating almost immediately after they fall, while closely related species, black or red oak acorns, under the same conditions, remain dormant until early spring. Most of the seeds of the clover family have a dormant period. Freshly hand-hulled sweet clover or alfalfa seed has about 90 per cent of dormant seeds. They remain dormant because a band of impervious matter in the seed-coat pre- vents the entrance of water. These seeds are usually treated in some manner to crack or remove the impervious layer before planting, which causes about 90 per cent to germinate at once. This treatment is called "scarification." In other seeds the seed-coat may prevent the entrance of oxygen to the embryo as in the common cocklebur {Xanthium). A third condition, induced seed dormancy, is due to a tough seed-coat. The swelling embryo cannot break the coat as in case of the pigweed. Dormancy caused by the seed-coat can be overcome by filing or by rubbing it with an abrasive. Other seeds remain dormant because of the condition of the embryo. The embryo may be immature and unable to germinate until it grows to a larger size. In other cases the embryo appears to be mature but it requires internal changes grouped under the heading of "after-ripening." These changes have been studied but in most cases are not under- stood. The breaking of this type of dormancy is more diffi- cult than those caused by the seed-coat. The physiology of dormancy indicates that it is due to some changes in those substances called growth regulators, which are in small amounts but have profound influences. In some plants a change in enzyme content occurs at the end of dormancy. In other plants it appears to be a growth hor- mone change. The change may be made in a branch or a single bud while the remainder of the plant is dormant. Methods of breaking or overcoming the dormant period have been found without knowing the nature of the change. REST PERIOD OF PLANTS 191 Dormancy in bulbs is desirable while they are in storage and until they are planted. Near the end of their dormant period a very short exposure to a warm temperature may break it. In the earlier period of their dormancy alternate cold and warm is effective. Most dormant perennial plants and seeds will also have this resting period shortened in either of these two ways. Treatment with any one of a number of gases is effective with most plants and seeds. Ether, ethylene dichlo- ride, ethylene chlorohydrin, carbon bisulphide, and others have been used. Dormancy has been studied more in seeds than in other parts of plants, and in addition to treatments described above, most seeds have their dormancy shortened by a natu- ral warm-cold treatment called "stratification." The seeds are mixed with peat, sand, or sawdust, kept moist, and put in the soil where the temperature will fluctuate between freezing and a few degrees above. This method is commonly followed also where incubators can be used to control the temperature at P C. to 5° C. Rose seeds may be planted in outdoor flats, where germination should be allowed to go on for at least twenty months with frequent examination to remove the seedlings. Some species of rose, as multi-flora, germinate within four months in favorable conditions, but most species and hybrids require careful stratification and the longer period. Some seeds, such as bluegrass, and let- tuce, have their germination hastened with light, and others, such as Datura, in the absence of light. The rest or dormant period of plants is used constantly in gardening. We do the main pruning and transplanting of perennial plants during the dormant period. Plants store food and grow large root and shoot systems during the active period, and the food reserve that has been stored can be used for growing the new absorbing root system, and send- ing out a new set of leaves. The least damage is done in transplanting at the dormant period because of the reserve 192 PLANT GROWTH food, the tissues are hardened, the fewest roots are killed, and the shoot withstands more drought. The gardener who has so cared for the plants during their active period as to enable them to store large quantities of food will always be rewarded by the growth they make when they come out of the dormant period. Growth should be avoided for some time before the dormant period to allow the tissue to harden. Unless buds and branches are hard- ened, they readily winter-kill. Even though plants withstand much greater extremes of temperature during the rest period those that are near their northern limit may need to be protected to avoid winter- killing. The best protection is provided during the previous summer by proper growth and food storage after which a moderately moist soil, and low transpiration should be main- tained in the winter. A small mound of earth may give addi- tional protection, but where more is needed, straw may be used if proper precaution against rodents is taken. REFERENCES Barton, L. V., "Storage of Vegetable Seeds," Contributions from the Boyce Thompson Institute, vol. 7, pp. 323-331, 1935. Crocker, W., "Mechanics of Dormancy in Seeds," American Journal of Botany, vol. 3, pp. 99-121, 1916. Denny, F. E., and L. P. Miller, "Hastening the Germination of Dormant Gladiolus Cormels with Vapors of Ethylene Chlorohydrin," Contributions from the Boyce Thompson Institute, vol. 6, pp. 31-38, 1934. Howard, W. L., "An Experimental Study of the Rest Period of Plants. Seeds," Mis- souri Agricultural Experiment Station Research Bulletin, No. 17, 1915. GLOSSARY Bacteria are microscopic one-celled plants, which may Hve as individuals or in groups called colonies, and which reproduce mainly by cell division. The cambium is a cylinder in the plant just inside the true bark made of a single layer of cells which can divide to add cells to the bark on its outside and to the wood on its inside. It supplies the cells for the growth in thickness. Chlorophyll is the complex chemical green pigment of plants. A chloroplast is a small amount of specialized protoplasm which contains chlorophyll. Chrom,atin is a protein compound found in the nucleus of a cell, which determines the characteristics of the organism. ChromosoTTie is a short thread-like structural unit of chromatin visible during the division of the cell. A clone is a group of plants vegetatively propagated from the same parent. The coleoptile is a tube or sheath covering the plumule of the grass-type seedling until it emerges from the soil, after which the coleoptile dies. A corm is a shortened fleshy stem with leaves growing frOm the upper side and roots from the lower side, as in gladiolus. The cortex is a group of thin-walled cells inside the epidermis with a variable function, but most frequently it stores food and in many cases has chlorophyll, hence it makes food. Cross-pollination is the transfer of pollen from the anther of a flower to the stigma of a flower on another plant. Cytology is the study of the internal structure of the cell, i.e., with chro- mosomes. The cytoplasm is the living matter of a cell outside the nucleus. The nucleus and cytoplasm constitute the protoplasm of a cell. Diffusion is the spontaneous intermingling of the molecules of two (or more) liquids or gases in combination, which results in an equal distribution of the molecules of each liquid or gas among those of the other. Diff"usion is caused by the characteristic of all ions or molecules to go from regions of their respective greater concen- trations. A dicotyledonous plant is one which produces seeds with two seed leaves (cotyledons) such as the two fleshy bodies in a pea or bean seed. 193 194 PLANT GROWTH The embryo is a many-celled early stage of an organism receiving food and protection from the parent. In the case of the plant it is the reproductive stage in the seed. Endosperm is the stored food which develops independently of the embryo in an albuminous seed. Enzymes hasten the chemical reactions of organisms without entering into their compounds (such as in the digestion of foods). An etiolated plant is one with characteristics resulting from deficient light, i.e., less chlorophyll, larger cells, longer internodes and, in complete darkness, very small leaves. Exalbum.inous seeds are those in which the endosperm is not developed, but the stored food is in enlarged cotyledons. Fertilization is (may refer to) (a) the application of plant food to the soil, or (b) to the union of the male sex cell with the egg cell or with the endosperm nuclei of the ovule. Fungi form the group of simple non-green plants known as bacteria, molds, and toadstools. A gene is the smallest unit of a chromosome which can determine a characteristic of an organism. Heredity is the tendency of the offspring to be like the parents. It is based on the chromosome as the basis for the transmission of such tendency. The hilum is the scar caused by the breaking of the seed from its attach- ment in the ovary. Hormones are secretions of certain parts of an organism which are carried to other parts of the organism where they promote some special activity. Humus is any decaying organism found in the soil. It usually gives the soil a darker color. A hybrid is the offspring of two parents whose characteristics differ in one or more ways. Hybridizing is the act of developing new plants by bringing in contact the sex cells of two unlike plants, i.e., cross-pollination. The hypocotyl is that part of the seedling between the cotyledons, or seed leaves, and the root. An internode is that portion of a stem between two nodes, or joints from which leaves and buds appear. A m,em,brane is a thin often invisible layer of cells or molecules sepa- rating two kinds or conditions of matter. (A surface-tension mem- brane separates the water molecules in a drop of water from the air.) GLOSSARY 195 Meristematic cells are those which divide to form new cells, i.e., the cells in the cambium and the tips of stems and roots. The micropyle is a small pore in the seed coat below the hilum through which the pollen tube enters the seed coat. A molecule is the smallest particle of any compound, i.e., H2O, or CeHizOe. Monocotyledonous seeds have a single seed leaf, i.e., in the embryo of corn, Figure 1. Mutations are sudden changes or variations in organisms caused by changes in the chromatin and therefore are transmitted by all later cell divisions. Mycorrhiza is a symbiotic fungus which grows on the outside In the form of a mantle and Into the root either between the cells or into the cells, of many plants, chiefly the woody types. A node Is the part of a stem from which one or more leaves and buds grow. The nucleus is the part of a cell containing the chromatin of the cell. It appears to have certain special functions necessary to the life of the cell. Osmosis is the diffusion of molecules through a membrane. In the plant the membrane is one of cytoplasm inside the cell wall. The ovary is the basal portion of the female organ of a flower in which the seeds will grow. The pericycle is a layer of cells in stems and roots between the phloem and the cortex. The petiole is the stem-like structure of a leaf by which it is attached to the stem. The phloem is a group of elongated cells, usually outside the cambium, which conducts food material. Photosynthesis is the making of sugar by the use of the energy from light, using water and carbon dioxide as the raw materials. This can take place only in the presence of active chlorophyll. Plumule is that small portion of the embryo of the seed which develops into the shoot of the plant. Polyploidy refers to multiples of the normal number of chromosomes for a given species. To propagate means to multiply or increase the number of organisms. The protoplasm is the living matter of a cell, consisting of the nucleus and the cytoplasm. The radicle is that part of the embryo of a seed which becomes or develops as the root of the seedling. 196 PLANT GROWTH The stele is the conducting portion of a root including the xylem, phloem, cambium, and pericycle. It is surrounded by the endodermis and cortex. Stipules are the two appendages at the base of most leaves. They are of various forms, but frequently are small bract-like structures. The stoma is an opening through the epidermis of a leaf between two guard cells through which the gases pass, i.e., carbon dioxide, oxy- gen, and water. Symbiosis is the living together of two kinds of organisms in such a way that each is benefited. Transpiration is the evaporation in the leaf and loss of moisture from a plant to the atmosphere. Vacuole is a space or cavity in the protoplasm of a cell containing other matter, i.e., the cell sap. Variation is a change in the characteristics of the offspring from those of the parent. It is usually the result of the environment during growth or of a new combination of the chromatin at the time of the fertilization of the egg cell. The xylem is the water conducting portion of a plant made of elongated cells, which generally become woody as they get older. INDEX Absorption, 41-45, 87, 88, 166, 184 diffusion (see) mineral salts, 19, 44, 45, 87, 166 potassium, 44, 45 selective, 45 water, 15, 88, 184 Acidity in soils, 159, 171 After-ripening, 190 Alleles, 132-134 Aluminum sulphate, 171 Ammonia, 178, 179 Ammonium nitrate, 176 Ammonium sulphate. 111, 171, 176 Aneuploidy, 146 Annual, 104-107 Anther, 113-115, 116 Aphids, 100, 101 Apple, 115, 120 Auxin-a, 151-157, 153 Auxin-b, 154 Azalea, 159, 171 Azotobacter, 161, 177-180 B Backcross, 136, 137 Bacteria, 51, 161, 162, 178-180 Bark, 55 Bean seed, 8, 32 Biennial, 104, 105, 107, 110, 189 Bios, 151, 152 Black Leaf Forty, 101 Bluegrass, 109, 110, 191 Bonemeal, 47, 172, 177 Bordeaux mixture, 102 Boron, 164, 165, 168, 169 Bracts, 118 Buckwheat, 168, PI. XI Bud, 53-58, 61 active, 56 adventitious, 57 dormant, 56 flower 56, 61 kinds, 56 Budding, 70 Burbank, 69, 137, 138 Cabbage, 144 Calcium, 43, 159, 164-166, 169, 171, 172 Calcium carbonate, 171 Calcium oxide, 171 Calyx, 115 Cambium, 53, 54, 55 Capillary water, 48 Carbohydrate, 79, 93-96, 185 Carbolic acid. 111 Carbon, 164, 165, 175 Carbon-bisulphide, 191 Carbon-nitrogen ratio, 93-96, 112 Carpels, 116 Carrot, 30, 105, 113 Cell, 21-29 differentiation, 183 division, 22, 28, 131 parts, 21-24 sap concentration, 45, 48, 88 constituents of, 42 size, 21,5^ structure, 23-2S wall, 23, 24, 42 Cellulose, 23, 80 Chlorophyll, 76, 79, 82 Chloroplast, 25, 73, 76 Chromatin, 25 Chromosomes, 25-29, 22, 27, 131-135, 141- 143, 149, 150 homologous, 132-134, 143 Classification of plants, 10 Clone, 67 Clostridium, 179 Clover, 190 Cocklebur, 190 Colchicine, 147, 148, PI. VIII & IX Coleoptile, 14, 153, 154 Complete fertilizer, 169 Complete flower, 115 Composite flower, 118 Compost, 161, 162 Contour, 163 Copper, 164, 165, 169 Cork, 55 197 198 PLANT GROWTH Corn, 7 Corolla, 116 Corymb, 113 Cotyledon, 7, 8, 12, 13, PI. I Cowpeas, 168, PI. XII Crabgrass, 106, 107, 109, 110 Crocus, 189 Crossing over, 135, 136, 143 Cross-pollination, 116, 119, 120, 121 Cultivation, 158, 160 Cutin, 98 Cuttings, 156 Cytology, 150 Cytoplasm, 23-25 D Dandelion, 110, 118 Date, 121 Datura, 191 Day, length, 84, 85 DDT, 101 DeVries, 130 Dicotyledon, 10, 53, PI. I Diffusion, 41, 48, 50, 79, 87 Disc flower, 113, 118 Diseases, 97-103 control of, 101-103 infected leaf, 99, 100 tobacco mosaic virus, 122 Dock, 105 Dominance, 125 Dominant characters, 125 Dormancy, 186, 188-192 - Drainage, 160 Earthworm, 159 Ecology, 1 Elements, 164-170, 173 boron, 164, 165, 168, 169 calcium, 43, 159, 164-166, 169, 171, 172 carbon, 164, 165, 175 copper, 164, 165, 169 deficiencies, 169 essential, 164, 165, 169 hydrogen, 164 iron, 164, 169, 173 magnesium, 164, 166, 169, 171 manganese, 164, 165, 169 nitrogen, 164-170, 173, 175-180 oxygen, 164, 179, 186 phosphorus, 164, 166, 168-172 potassium, 159, 164-170 selenium, 43, 98, 164 sulphur, 164, 169 zinc, 164, 165, 168, 169 Embryo, parts of, 7, 9, 10 Endosperm, 7, 10, 119 Energy, 79 Environment, 47 Enzyme action, 17, 100 Enzymes, 119, 151, 152, 190 Epidermis, 72-74 Epithelium, 10 Erosion, 161, 162 Evaporation, 87, 88 Fats, 80 Ferns, 171 Fertilization, 8, 117, 119 Fertilizers, 42, 109, 110, 164-174, 177, PI VI insoluble, 47 lime, 159 soluble, 47 Field capacity, 50, 91 Flowers, 112-121 anther, 113-115, 116 calyx, 115 carpel, 116 clover, 117 complete, 115 composite, 118 corolla, 116 corymb, 113 dandelion, 118 defined, 112 disc, 113 egg cell, 117, 119 embryo, 119 embryo sac, 119 endosperm, 119 fertilization, 117 filament, 114, 117 fruit, 120, 121 glume, 118 grass, 117, 118 INDEX 199 head, 113 inflorescence, 112, 113 irregular, 116 length of day, 112 lodicule, 118 micropyle, 117 nectar glands, 116 ovary, 113-115, 116, 117, 120 ovule, 114, 115, 117, 119, 120 pappus, 118 panicle, 114 pedicel, 113 peduncle, 112 petal, 113-115, 116 petunia, 114 pistil, 4, 113-115, 116, 117 polar bodies, 119 pollen, 116-119 pollen tube, 117-119 pollination, 116, 118 cross, 119 self, 119 raceme, 113 ray, 113 receptacle, 115, 116 rose, 115 sepal, 114, 115 sperm, 117 spikelet, 118 stamen, 114, 116 standard, 117 stigma, 114, 115, 116-119 style, 114, 115, 116-119 sunflower, 117, 118 sweet pea, 116, 117 types, 117, 118 umbel, 113 whorls, 114, 115, 115 zinnia, 113, 114, 116, 118 Food making, 79-86 Food storage in roots, 37 Fruit, 7 corn, 7 iris, 9 Fungi, 51, 68, 97-102 Genes, 26-28, 132-135, 137, 142, 143 Genetics, 138 Genotype, 135 Germ tubes, 99 Germination conditions, 15, 19 delayed, 8 enzymes, 17 oxygen, 17 process, 17 temperature, 16 time, 20 Gladiolus, 113 Glumes, 118 Grafting, 68-70, 70, 137, PI. IV Grass, 7, 159 Grass flowers, 118 Gravitational water, 48, 49 Growth, 18, 19, 84, 85, 181-187 cycle, 186, 187 rate of, 183, PI. Ill, XV regulators, 151-157, 190 Guard cells, 72 H Hardening, 176, 184, 192 Haustoria, 100 Heredity, 141-157 Hibiscus, 113 Hilum, 8 Homologous chromosomes, 132-134, 143 Homologues, 132-134 Hormones, 151-157, 161, 182, 183, 190, PI. X Humus, 158, 160, 161, 179 Hybridizing, 122-140 Hybrids, 139 Hydrogen ion, 109 Hydroponics, 172, PI. XIV Hygroscopic water, 49 Hypocotyl, 14 Inflorescence, 112 corymb, 113 head, 113 panicle, 114 raceme, 113 umbel, 113 Inheritance, 122-150 Insecticides, 101, 102 200 PLANT GROWTH Insects, 97-103 control of, 97-103, 162 general groups, 101 Internode, S3 Ions, 50 Iris bearded, 189 flower, 116 fruit and seed, 9 Iron, 164, 169, 173 Iron sulphate, 111 Irregular flower, 116 Irrigation, 48 Japanese beetle, 162 K Keel, 117 Larkspur, 112, 114 Lawn grass, 95, 96, 108, 109 Layering, 71 Leaf, 72-78 chlorophyll, 76 chloroplast, 73, 76 classes, 72 compound, 72, PI. I, II mesophyll, 76 modifications, 11, 78, 112 position, 57 simple, 72 starch, 76 structure, 72-78, 73 sugar, 1(> venation, 72 Legumes, 159, 178 Length of day, 84, 85, 112, 186, PI. V Ligulate flowers, 118 Lime, 159-166, 171, 172, 176 Linkage, 27 , 135-137 Lodicules, 118 Long-day plants, 84 Meiosis, 131-134 Mendel, 123-139, 141 Mendel's garden, PI. VII Mendelian characteristics, 137 Mendelian inheritance, 123-139, 141, 143 Mesophyll, Id Metaxenia, 120 Micropyle, 8,\\1 Minerals, 41-47, 50, 55, 166, 172, 185 Mitosis, 131 Modified leaves, 11, 112, 116 Modified stems, 106 Molecules, 41, 87 Moisture, soil, 90 Monocotyledon, 10, 53 Moonvine plants, 184, PI. XVI Morning-glory, 184, PI. XVI Mulch, 159, 160, 184 Mulching, 184 Mutation, 143-145, 148 Mycorrhiza, 39 N Nectar glands, 116 Nitrogen, 52, 80-82, 93-96 high, 64, 65 in soUs, 161, 164-170, 173, 175-180, PI. XIII Nitrogen cycle, 176-178, 178 Nitrogen-fixing bacteria, 161, 177-180 Node, 53, 59, PI. I Nodules, 177, 178 Nucleus, 23-25 Nutrient solutions, 152 0 Orchids, 171 Organic matter in soils, 47, 51, 52 Osmosis, 42 Osmotic pressure, 36 Ovary, 113-115, 117, 119 Ovules, 117, 119 Oxygen, 50, 51, 68, 75, Id, 79, 82, 83, 164, 179, 186 M Magnesium, 164, 166, 169, 171 Manganese, 164, 165, 169 Manure, 51, 161, 170, 177, 179 Palisade cells, 72, 73 Panicles, 114 Parental combination, 128, 136 INDEX 201 Pea, 181, 182 Peduncle, 112 Perennial, 55, 104, 105, 107, 110, 189, 191 Petal, 113-115, 116 Petiole, 72 Petunia, 112, 114, 115, 117 PH, 109, 159, 166, 171-173, 177 Phenotype, 135 Phloem, 34, 54, 55 Phosphorus, 159, 164-170, PI. XIII Photoperiodism, 84, 85 Photosynthesis, 75-85, 96 rate of, 80, 82 Pistil, 113-115, 116, 117 Pitcher plant, 77 Pith, 54, SS Plant ash, 164 Plant breeding, 138 principles of, 123 Plant disease, 97-102 leaf infection, 99 radish leaf, 100 Plantain, 110 Plastids, 25 Plowing, 158 Plumule, 8, 9, 11, 53, PI. I Pollen, 116-119 Pollen-sac, 114 Pollination, 116, 118, 119 Polyploid plants, 26, PI. IX Polyploidy, 146-148 Poison, 43, 44, 98, 99, 162, 164 Poison ivy. 111 Potassium, 44, 45, 159, 164-170 Propagation, 66-71 asexual, 66-71 budding, 70 clone, 67 cutting, 67, 68 grafting, 68-70, 70, PI. IV layering, 71 sexual, 66 Protein, 41, 42, 80 Protoplasm, 2, 21, 23-25, 42, 43, SO, 184 Pruning, 57-65 defined, 59 reasons for, 59, 94 suckering, 61, 62 thinning, 60, 62 time, 62, 64 training, 59 vigor, 64 wounds, 63 Quick lime, 171 Q R Radicle,