Plant physiology, with special reference to plant production

Front matter

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Preface

In the preparation of this text and reference book, the writer has attempted to consider both the student and the general reader, interested alike in the fundamental requirements of plants and in plant production. Throughout biological study at the present time increased emphasis is placed on the activities and responses of organisms. It is instruction in this type of biological phenomena that is rapidly becoming a part of the cultural side of education, and the practical value of such knowledge is every day being demonstrated, — notably in agriculture and medicine. Plant physiology finds its practical application in plant production, to which it stands in much the same relation as does industrial chemistry to general manufacturing.

It is somewhat strange, therefore, to find that as a separate course plant physiology is not yet offered in some of the colleges whose purpose is primarily to train persons for practical or rural pursuits. Such students require some fundamental work, and few will become specialists. For this general class of students, and for other readers as well, there seems to be needed a text (1) that shall exhibit a considerable range of material, rather than a few topics exhaustively treated; (2) that shall include both qualitative and quantitative work; and (3) that shall keep in view, as far as possible, the relations of the science to plant production, drawing the illustrations, wherever convenient, v from plants which are familiar and directly useful. By maintaining Borne direcl contacl with practical problems, interest is aroused for further desirable fundamental preparation. "The idea that useful knowledge cannol be cultural must be dismissed. . . . Every possible application must be made of each abstract principle." (Eliot, " The Conflict between Individualism and Collectivism in a Democracy,

In the field of pure physiology, there are recent texts and guides embodying much of what is considered best in tlif modern contenl and attitude of the Bcience. An

ration of the methods of quantitative stu.lv is there indicated, and stress is laid on the materials and energy involved in plant activity. Such books will be consulted with much profit

In selecting from the great amount of available material

that which lias seemed to he most suitable tor the present purpose, consideration has been given the fact that in many colleges genera] courses are offered, not only in such distinctively plant line, a- agronomy, horticulture, and breeding, but likewise in fields overlapping physiology, or partially included in this subject, such as soils, bacteriology, pathol- . and genetics. The subject-matter included is intended to be Bufficienl tor a course <>f one-hall year involving two

re.-itations and two laboratory periods; but it may he maxl - of a shorter course by suitable .selection of

material, or of a Longer course by an extension of the collateral work.

In the preparation of this text I have used freely any available I information. 'I'he subject-matter has

been presented at one time or another in class work. I am indebted to Mr. Lewis Knudson, Instructor in PJant Physi-

Preface vii

ology in Cornell University, for many suggestions and for the form of certain sections of the laboratory notes. Some of the illustrations were furnished by others, or borrowed, as credited in the text. Certain of the drawings were prepared by Miss Anna M. Keichline ; others by Mrs. B. M. Duggar, of whose constant assistance with manuscript and proof I would express also my appreciation.

B. M. DUGGAR.

CHAPTER I Introduction

Permanent high production — The relation of physiology to production — Physiology and ecology — Physiological processes

— Environment — Crop ecology — Literature of Plant Physiology — Physiology and other sciences — References . 1-14

Chapter II

The cell a physiological unit — Early use of the term " cell'' — Meristem or embryonic cells — Cytoplasm — The nucleus — Plastids — The cell- wall — Cell-sap — Cell-forms — Parenchyma Sclerenchyma — Trachea; or vessels — Sieve tubes — Protoplasmic movement — Protoplasmic irritability and response

— Laboratory work — References 15—34

CHAPTER III The Water-content of Plants and the General Relations OF Root Systems

Hydrostatic rigidity — The water-content of plants — Variation in water-content of organs — The water-absorbing system — The root habit of crops — The production of root-hairs — Root-hairs and the water-content of the soil — The root-cap

— Structure of the root-tip — Soil particles — Soil texture x Contents and water-holding capacity — Exceptional plants — Unavailable water — Leaves poorly fitted for water absorption — Laboratory work — References 35-63

CHAPTER IV Conditions and Principles of Absorption

Imbibition phenomena — Osmosis and diffusion — The demonstration of osmotic pressure — An explanation of osmotic pressure — Plasmolysis and wilting — Variation in turgor — Substances active in producing turgor — Osmosis and the absorption of nutrient salts — Protoplasmic permeability — The r61e of diffusion and osmotic pressure — Sap or root pressure — Laboratory work — References . . . 04-83

CHAPTER V Transpiration and Water Movement

Observations upon transpiration — Amount of transpiration — The mechanism permitting transpiration — Distribution of stomata — The control of water-loss by stomatal movement — Modifications tending to check excessive transpiration — Conditions affecting transpiration — Effects of excessive evaporation — Guttation — Transpiration and evaporation — Transpiration and growth — Water transport — Fibrovascular bundles — Leaf venation — Rate of transport — Laboratory work — References 84-115

CHAPTER VI The Water Requirements of Crops and of Vegetation

Relative requirements of a few crops — Precipitation and crop growth — Irrigation — Potted plants and water-supply — Ecological classification based upon the water relation — Semi-xerophytism and hard-wheat production — Subsidiary work — References 116-136

Contents xi

CHAPTER VII Mineral Nutrients

The ash content of plants — Composition of the ash — Effects of conditions upon ash content — Ash content at different ages

— Translocation of mineral substances — Water cultures — Nutrient solutions and water cultures — Strength of the nutrient solution — The forms of the nutrient compounds — Plant nutrients in rock — Soil fertility — Nutrients removed by farm crops — Nutrients removed by fruit crops — Amount of nutrients in soils — Availability of the nutrients — The solvent action of roots — C0 2 excretion and the availability of phosphorus — Another view of soil fertility — The paraffined wire basket in nutrition studies — Laboratory work — References 136-168

CHAPTER VIII Special Functions and Relations of Mineral Nutrients The roles of mineral nutrients : The nature of the special roles

— The role of phosphorus — The role of potassium — The role of magnesium — The role of calcium — Iron — Sodium

— Chlorine — Sulfur — Silicon — References . . . 169-183 Balanced solutions : The injurious action of certain basic nutrients — The relation of calcium to magnesium — Other nutrient bases and antitoxic action — Laboratory work — References 181-194

CHAPTER IX The Intake of Carbon and the Making of Organic Food

The amount of carbon in the plant — Carbon dioxid the source of carbon in green plants — Chlorophyllous plants — Respecting the distribution of chlorophyll — The nature and properties of chlorophyll — The factors essential in photoxii Contents synthesis — The course of photosynthesis — The demonstration of photosynthesis — The formation of sugar and starch — The diffusion process — The amount of carbon dioxid — Light the source of energy — Efficiency of the food-making apparatus — Light, intensity and quality — Temperature — Organic matter, rate of production — Laboratory work — References 195-225

CHAPTER X The Relation to Nitrogen

Combined nitrogen — The nitrogen content of plants — Synthesis of nitrogenous bodies — Soil nitrogen — Nitrites — Nitrates — Compounds of ammonium — The sources of soil nitrates and ammonia — Ammonification — Nitrification — Nitrifying organisms — Conditions favoring nitrification — Denitrification — Nitrogen fixation — Organisms which fix free nitrogen Bacteria of leguminous tubercles — Certain saprophytic

soil bacteria— Fungi — Mycorhizal fungi — General sources of supply of nitrogen — Electric fixation of nitrogen — Laboratory work — References 226-249

CHAPTER XI Products of Metabolism ; Digestion and Translocation

Metabolism — Temporary foods, storage products, and permanent structures — Annuals, biennials, and perennials — Carbohydrates — Sugars — Starches — Cellulose — Fats and oils

— Proteins — Classes of proteins — Amides — Organic acids

— Tannins — Resins and turpentine — Digestion — Digestion in different organisms — Enzymes and enzyme action — Carbohydrate enzymes and their products — Protein enzymes

— Conduction of digested foods — Ringing — Laboratory work — References 250-279

Contents xiii

Chapter XII

Respiration, Aeration, and Fermentationpages The term "respiration '' — An obvious result of respiration —

The demonstration of respiration — Respiratory phenomena in aerobic respiration — Oxygen promotes catabolic processes

— The ratio of 2 absorption to C0 2 production — Respiratory activity — Respiration of wounded plants — Heat release — The mechanism for gas exchange — Anaerobic respiration — Fermentation — Lactic fermentation — Alcoholic fermentation — Acetic fermentation — Laboratory work

Chapter XIII

The factors — Evidences of growth — Growth of the embryo — Polarity — Elongation of roots — The stem apex — The formation and exfoliation of leaves — The resting bud — Types of stem elongation — Fruit buds and age of shoot — Persistence of the rest period in temperate regions — Differentiation of stem tissues — Secondary thickening — Growth of the cell — Cell division — Nuclear division — Cell division and respiration — The relation of pruning to growth — Budding and grafting — Scion propagation — Relation of stock to scion — Forcing — Etherization — The effect of etherization — Forcing by immersion in warm water — Transplanting after wilting — Growth movements — Laboratory work — References 305-34G

Chapter XIV

The seed habit and vegetative production — The flower : essential structures — Pistil and stamen — Pollination and pollen-tube penetration — Fertilization — Universality of fertilization — Cross-fertilization and self-fertilization — Cross-fertilization xiv Contents apparently the rule — Darwin's conclusions — The need of further work — Experiments with self -sterility in pear — Self-sterility in other orchard trees — Parthenogenesis — Xenia in corn — Indications of xenia — False xenia — Other secondary effects of pollination — Parthenocarpic development — Parthenocarpic formation in pomaceous fruits — Seedlessness in the orange, grape, and banana— Nonsexual reproduction — Thickened roots and tubers — Cuttings — Precautions with cuttings — Vegetative reproduction and running out — Relation of vegetation to fruiting — Laboratory work — References 347-380

Chapter XV

Habitat conditions of the parent plant — Localization of seed production — Maturity — Conditions of harvesting and curing _ Duration of vitality — Environmental conditions — Buried seed — Delayed germination — Effect of weight and size of seed upon vigor — Experiments with wheat — Experiments with cotton — Experiments with tobacco — Laboratory or supplementary work — References . . 381-399

Chapter XVI

Climatic extremes and introduced plants — Temperature and production — Cardinal temperatures — Inhibition at high temperatures — Heat units— Heat units and germination — The date-palm — Control of temperature — The temperature of the plant — Adjustment of structure — Irritable response — Freezing — Buds — Laboratory work — References . 400-414

Chapter XVII

The adjustment of plant members — Light perception — Diverse requirements — Light intensity — Injurious effects — Artifi-

Contents xv cial light — Monochromatic light — Half -shade in plant propagation — Crops responding to half-shade — Morphogeny effects — Half-shade and quality — The effect of shading upon other environmental factors — Laboratory work — References 415-435

CHAPTER XVIII Relation to Deleterious Chemical Agents General relations to poisons — Comparative resistance — Toxic action and the substratum — Method of action — Inorganic and organic acids — Alkalies — Salts of the heavy metals — Formalin — Organic bodies — Root excretions — Unproductiveness — Relative toxicity of some organic compounds — Illuminating gas — Stimulation by means of weak toxic agents — Protection of crops by insecticides and fungicides

— Destruction of weeds by poisons — Deleterious substances employed — Practicability of the chemical method — Laboratory work — References 436-462

Chapter XIX

Variation: Individuals and species — Fluctuating variations — Darwin's theory of natural selection — Rate of increase — Fluctuating variation and the origin of varieties — Pure lines

— Mutations — Mutation and crop improvement . . 463-476 Heredity : Nonsexual reproduction and heredity — Sexual reproduction and heredity — The early studies — Types of inheritance — Recent studies — Mendel's experiments — Purity of

the gametes — Results of segregation — Tomato characters

— Chromosome relations — Selection — Laboratory work — References 476-493

CHAPTER XX Growth Movements Stimulus and response — Tropic curvatures — Geotropism — Thigmotropism — Chemotropism — Nutation — Nastic curvatures

— Nyctitropism — Laboratory work — References. . 494-507

PLAKT PHYSIOLOGY

Chapter I Introduction

The relation of plant physiology to crop production and vegetation requires no explanation except where physiology and plant production are alike incompletely comprehended. No one may thoroughly understand a modern agricultural problem who has not learned the full significance and scientific relations of the two eminently practical terms " production" and " conservation." The basal field of agriculture is plant production, for upon this animal production is dependent ; and throughout all agriculture conservation is necessarily the key to continuous development and success.

Conservation in the broadest sense implies neither waste of the product grown nor waste of the forces and conditions which make high production possible. These forces are the environment under which the crop is grown and the inherent hereditary possibilities within the seed or seed-material.

1. Permanent high production. — Plants form the natural covering of the surface of the earth, and if there is at present no such covering, where the rock is sufficiently b 1 decomposed to be termed soil, it indicates that something is radically wrong with the soil or climate of that region from the standpoint of the permanent occupation of it by man or animal as well as by plants.

Of all object lessons in permanent occupation by plants,

Fig. 1. In the Rainier National Forest, Washington. [Photograph from the Forest Service.]

that of the aged forest stands out supreme. Here a vigorous growth may have endured for centuries, and except for such accidents as those of floods and fires, or of flagrant devastation by man, it might continue for centuries more. So far as may be seen, or measured by the short space of agricultural record, at least, there is with the greater growth of the forest an ever increasing fertility of the land.

From this lesson of production and conservation one turns to another in discouraging contrast, — and that other is this : much of this fertile forest land has been cleared, and year by year fields which were once highly productive are left untilled, or abandoned as of no longer interest agriculturally. If lessened production is the cause of abandonment or discouragement, the system which leads to waste of this nature should have a speedy end.

All of the results of science and practice are needed to assist in working a change in the conditions. Each science may contribute something.

2. The relation of physiology to production. — Plant physiology is an intimate part of scientific plant production. It concerns itself with plant response and plant behavior under all conditions ; that is, with all relations and processes readily evident or obscure, simple or complex, which have to do with the maintenance, growth, and reproduction of plants. It is then concerned with vegetation or crops, with the relation of the plant as a whole, and with all special responses or functions of any organ or cell. From the standpoint of physiology one should be able to get facts alike applicable in understanding or interpreting the behavior or yield of plants of all description. The principles of growth are learned by the same methods, whether the plants are those constituting the vegetation of the mill-pond or of the vast fields of cultivated grain; of the greenhouse or of the weedy growth of the neglected lot ; of the sparse vegetation of the poor prairie or of the primeval forest. Throughout all, the principles involved are ultimately those of analyzing the complex stimuli and the resulting growth, or maintenance, and reproduction.

Through physiological study it is possible to understand better that which pertains to production, since increased knowledge of plant response makes it more nearly possible to modify opportunely and to improve upon current practices of production, and to develop progressive varieties and strains. Obviously, production involves a variety of nonphysiological conditions, but it also involves physiological conditions, and little progress may be anticipated without an intimate knowledge of the relation of the growth of the plant or crop to the conditions under which it is grown.

3. Physiology and ecology. — At the outset, moreover, it is necessary to recognize two possible lines of study and observation. The one is primarily concerned with the isolated and controlled plant and the functions or responses of its diverse organs and structures. This is generally considered pure physiology. The other line of study deals with plants or the crop in the field, or stated technically, in a natural or seminatural habitat. This is field physiology or ecology. There is, of course, no sharp line between the two subdivisions indicated, and both are important in production. It is necessary to know the plant, and it is equally essential to know the environment, for that is the sum of the conditions to which the plant responds.

4. Physiological processes. — The engineer who does not understand his machine cannot expect to get effective work out of it. He should know its intimate structure, what work it can perform under all conditions, and how it may be controlled. In the same way the plant producer who knows the structure of the plant and its behavior is provided with the means of interpreting the effects of conditions upon the organism. The plant is a delicate physical, chemical, and living mechanism; and through the work which is performed, often expressed in growth, or change of some sort, it is responsive to practically all manner of stimuli.

Under whatever conditions it may be able to grow and reproduce itself there are many phenomena, or processes, which are recognized as fundamentally physiological, although the information respecting these may often be essentially physical or chemical. All modern physiologists are necessarily pursuing physico-chemical methods in interpreting all the activities of plants. Among these processes may be mentioned the absorption, movement, and incorporation of water and of gases ; the absorption and disposition of nutrient salts; the manufacture of organic food material ; the accumulation, digestion, and assimilation of foods or food materials ; respiration ; growth and variability; reproduction and heredity; and the special growth or other changes as responses to environmental factors. Much of the material presented in this book is for the purpose of demonstrating simply some of the essential principles involved. Qualitative measurements are frequently sufficient.

5. Environment. — The environment of any plant or crop is a complex of factors or conditions as a resultant of which there is the response of the plant in vigorous or weakly production, and in diverse form or habit of growth. Most of the important factors of the environment affecting the agricultural plant are perfectly obvious. In considering these one almost unconsciously thinks of those factors which operate above the soil and those which operate through the soil. This division of the factors is not wholly satisfactory, as will be seen. Agronomy is commonly subdivided nowadays into " soils " and " crops " ; but these do not exclude physiology, they make it, in its broadest relations, more nearly indispensable.

Conditions acting above the soil are mainly sunshine, heat, precipitation, humidity, evaporation, and air movement and composition ; while through the soil there is afforded fixity, mineral nutrients, and water, as well as heat and air. There are, however, many other factors acting above or below the soil which may affect vegetation directly or indirectly, including the bacterial flora of the soil, and injurious substances in the soil solution; fungous diseases, insect pests, and higher animals ; the constant factor gravity; and many special stimuli.

This complexity of environmental factors renders an interpretation of the effects of any one factor in terms of plant behavior peculiar ly difficult. Progress in field study, or experimental ecology, is nevertheless being made, and much is due to the greater perfection in instrumentation as applied to the habitat.

In nature a species of plant thrives best where the requirements of the particular form are most completely met. In the case of the cultivated plant, however, while the natural factors condition production, these alone are, of course, insufficient to determine whether or not a particular crop should be grown in a particular locality, for there are a host of economic considerations which must have weight. Market, transportation, labor, machinery, and many other factors must be taken into account.

From the point of view of the natural factors there are again two lines of inquiry : (1) Is the crop suited to the general conditions of the region? (2) Are the conditions of environment and the cultural practices afforded the crop such as to result in maximum yield? It is not expected that any considerable number of facts enabling one better to answer the first question shall be presented in this book; such facts, at any rate, may be only incidentally touched upon. The fundamental facts developed should, however, enable one to observe, test, and answer for himself more completely any question which would fall in the second group of inquiry.

6. Crop ecology. — Since ecological data may be included only incidentally, a few words may be said here concerning the general relations of plants as distributed over the surface of the earth or as cultivated under special conditions. In the steppes of northern Africa and portions of Australia, in the dry prairies of the western United States and southern Russia, or in the equivalent regions of western Brazil, the vegetation is similar in physiognomy. Here tough and drought-resistant grasses thrive, and for the most part these regions are treeless tracts where rains fall infrequently or precipitation is poorly distributed, and here, too, winds often exert their highest force. The great permanent grass lands, such as these areas are, may be considered as being ecologically most closely related to true desert.

On the other hand, in tropical or temperate regions of abundant, or at least sufficient, rainfall forests of one type or another find a natural home. It is clear that upon the discovery of North America, the region now included in the United States was, practically speaking, a continuous forest extending from the Atlantic westward to the region of little precipitation.

Agriculture and commerce have already encroached to an enormous extent upon the natural domain of both the native forest and grazing lands; but in mountainous regions and towards the northern limit of vigorous growth

Fig. 2. Merriam's Life Zones of the United States ; Boreal (1), Transition (2), Upper and Middle Austral (3), Lower Austral (4), Gulf strip of Lower Austral (5), and Tropical (6). Dotted parts of the Austral zones indicate humid divisions. [After Cockerell, in Bailey's Cyclo. Agr.]

most small crops become less profitable, and the forest will, through many generations, at least, form the natural boundary line separating agriculture from the Arctic zone. In addition, of course, forests will continue to thrive in the agricultural region where permitted by man.

Furthermore, in taking a bird's-eye view of the forests of the United States there is noticed a more or less striking limitation in range, hence in general adaptability, of many well-known forest species. Thus the range of white pine as a commercial crop is practically limited to a region extending westward from the New England States to Minnesota, while the long-leaf yellow pine is restricted to the sandy coastal region of the Southern Atlantic and certain Gulf States.

A similar relation of particular crops to one or more factors of the environment is strikingly brought out by those crops especially that are commonly associated with southern climates. Cotton has a relatively long season of growth, and it is restricted in the United States to a region practically below the thirty-seventh parallel, — a region which is, for about seven or eight months of the year, free from frost and with a high mean temperature. Citrus fruits are accustomed to an almost continuous growing season, where frosts are few and severe freezing practically unknown.

A large proportion of the varieties of rice, also restricted to warm regions, may not be grown beyond those sections in which irrigation is possible. Hard wheats gradually lose the quality of " hardness " (high per cent of gluten) when grown in moist regions. The potato is grown from Canada to Texas and from Scotland to Italy. It is interesting to note, however, that in the United States, usualfy, the yield diminishes toward the South ; and, except under special conditions, the crop matures relatively early throughout the United States. Under such conditions average production is but about 85 bushels per acre. With intensive culture 400 bushels is a maximum for some of the most productive lands in the eastern United States, although 1000 bushels have been reported under peculiar conditions in the far West. In Scotland, with its more or less continued cool climate, affording a long growth and slow maturity of the potato, we find an average of nearly 250 bushels per acre ; while a maximum of 1000 to 1200 bushels is commonly attained. At the well-known seed farm of Lord Rosebery a yield at the rate of over 1700 bushels per acre was reported for a particular plot during the past season. Such facts as these cannot fail to be suggestive from the ecological standpoint.

If the more fundamental lines of general physiology seem less a part of plant production or of practical agriculture than the broader relations above referred to, it must be that it is so partly because of the name which has been applied to this subject, and partly to the fact that the methods of instruction necessarily take the student or reader to a far greater degree away from the cultivated field. To a considerable extent this is necessary, for physiology must remain one of the fundamental sciences, and the fundamental attitude should be kept prominent. It has been considered, too often, a subject with merely laboratory applicability. This erroneous view is vanishing as plant producers become more and more interested in the causes which produce results and not merely in the results themselves. Both horticultural and agronomic work have in recent years extended more and more into the realm of pure plant physiology, which should mean that they have extended into that of accurate experimental study, with the plant response as the central feature.

7. The literature of plant physiology. — The literature of this subject is extensive and scattered, as is that of any other science. The student will do well to bear in mind that while both the brief and the extensive standard works are important, the subject is one which, through its diverse relationships, encourages breadth of preparation and of application ; so that frequently physiological texts alone are insufficient. Any standard text is in large part a logical arrangement and correlation of the facts of many separate papers or monographs; and the detailed data respecting any phenomenon should be sought in the special paper.

The rapid strides which have been made in scientific agriculture and horticulture, especially in plant chemistry, soils, and intensified production, have developed a great array of interesting phenomena. This has given a decided impetus to physiological study. Often, unfortunately, the agriculturist has been compelled to go forward without due knowledge of physiology in the interpretation of his results, but this is no excuse for the neglect of the large amount of valuable and sound work which has been done.

Again, it should be further emphasized that many physiological phenomena are only properly understood when they are viewed in the light of physical and chemical theory, and it is frequently necessary that the student who is encouraged to go further shall turn to the sources of information in these fundamental sciences.

8. Physiology and other sciences. — The aim of plant physiology is a definite one, like that of other sciences; it is ultimately to obtain precise information concerning all those factors and forces which are operative within or through the living plant. Facts are derived and laws established in exactly the same manner as in other sciences. It does not stand apart from physics and chemistry, but utilizes and advances these or any other sciences which may assist in deducing the facts of plant life. Just as chemistry may utilize the plant as an indicator of chemical reaction or chemical fact, so physiology may use or develop chemical facts in analyzing the phenomena of plant life.

In general, a study of physiology must assume or include facts regarding the form and structure of plants; that is, morphology and histology. The more elaborate the morphology of an organism, as a rule, the more specialized and intricate are its reactions. These reactions are those of its constituent units, and the cell is a convenient and necessary unit of structure. The cell is likewise an important physiological unit, and as such requires special consideration.

Reference Books and Texts l

Bailey, L. H. Cyclopedia of American Agriculture. 1 : 618 pp.,

756 figs., 25 ph., 1907 ; 2 : 699 pp., 907 figs., 19 ph., 1907. Barnes, C. R. Physiology (in Coulter, Barnes, and Cowles,

College Botany, Part II), pp. 297-484, figs. 619-699, 1910. Clements, F. E. Research Methods in Ecology. 334 pp., 85

Plant Physiology and Ecology. 315 pp., 125 figs., 1907.

Curtis, C. C. Nature and Development of Plants. 471 pp.,

342 figs., 1907.

Darwin, F., and Acton, E. H. Practical Physiology of Plants.

Detmer, W. Practical Plant Physiology. (Transl. by S. A.

1 In this list of reference works it is intended to include some of the more useful texts and general works which contain physiological information along many lines. Other books of greater specialization are included under the selected references for particular topics.