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During the past twelve years, the writer has spent much time investigating the greatly neglected field of root habits of plants. Such a wide interest has been manifested in this work that it was deemed advisable to bring together into a single volume the more important results of these studies. In dealing with the various cultivated plants, the rather meager data from other investigations have been added to present, so far as possible, a general view of the root development of crops in the United States. No attempt has been made to include root studies other than those made in America, as this would have extended the work far beyond the scope of this volume. For an introduction to the more recent foreign investigations, the student is referred to the works of Rotmistrov, Schulze, Vorob'ev, and Osvald, in Europe, and the extensive work of Howard, in India.
The materials for this book, except the first three and last two chapters, have been taken largely from the following publications issued by the Carnegie Institution of Washington: "Ecological Relations of Roots," Publication 286; "Root Development in the Grassland Formation," Publication 292; "Development and Activities of Roots of Crop Plants," Publication 316; and "Root Behavior and Crop Yield under Irrigation," Publication 357. The first two are by the writer, and the third and fourth by the writer and coauthors, Dr. Frank C. Jean and Dr. John W. Crist. The author is under great obligations to the Carnegie Institution of Washington for permission to use these materials and to Doctors Jean and Crist for their willingness that the materials be used. He is also indebted to a number of his students, particularly Miss Annie Mogensen, Mrs. F. C. Jean, Miss Ruth Vernon, Mr. Joseph Kramer, Misses Maud Reed and Mary Sturmer for many of the illustrations and other valuable help. To his colleagues in botany and agronomy, the writer is indebted for valuable advice and encouragement in the preparation of the manuscript; also to Professor T. J. Fitzpatrick for reading the manuscript and proof. Criticisms looking toward the improvement of future editions will be welcomed.
JOHN E. WEAVER.
THE ENVIRONMENT OF ROOTS: THE SOIL
Origin of soil--The decomposition of rocks--Rôle of plants and animals--Soil texture and structure--Soil texture--Cause and nature of soil structure--Relations of tillage, plants, and animals to soil structure--Humus and microorganisms--Origin, nature, and effect of humus--The formation of humus--Organisms concerned--The relation to nitrogen--The soil solution--Origin and nature--Importance in crop production--Removal of nutrients by crops and by leaching--Alkali soils--Acid soils--The soil water--Relation between precipitation and water content--Kinds of soil water--Factors determining water-retaining power--Water content during the growing season--Amount of water absorbed--Absorption of water--Absorption of nutrients--Soil temperature--Relation to activities of higher plants--Relation to soil Organisms and soil reactions--Factors affecting soil temperature--Relation to disease in plants--The soil air--Composition of the soil atmosphere--Relation to root growth and other biological activities--Summary.
HOW ROOTS ARE BUILT TO PERFORM THEIR
Functions, origin, and kinds of roots--Structure Zone of division --Zone of elongation--The root cap--Zone of maturity; origin, arrangement, and functions of mature tissues--Origin and development of lateral roots--Root hairs and factors affecting their development--Loss of absorbing power and secondary thickening in roots--Rate of growth and extent of root areas--Summary.
ROOT HABITS IN RELATION TO CROP PRODUCTION
Activities of roots in soil and subsoil--Extent of root systems--Absorption of water from the subsoil--Absorption of nutrients from the subsoil--Effect upon yield--Responses of roots to environmental factors--Relation of roots to soil moisture--Influence of time and amount of water added by irrigation--Effect of drainage upon root habit--Root responses to low water content--Relation of roots to fertilizers--Effects of fertilizers upon root habit--Significance in crop production of the effects of fertilizers upon root habit--Relation of roots to cultural practices--Transplanting--Correlation between root and shoot development--Tillage practice and root physiology--Root habit and depth of intertillage--Relation to crop rotations, cover crops, and intercropping--Other root relations--Soil erosion--Weed eradication--Relation to alkali and acid soils- -Aeration and soil temperature--Relation to plant disease--Summary.
ROOT HABITS OF NATIVE PLANTS AND HOW
THEY INDICATE CROP BEHAVIOR
Grassland communities--The tall-grass prairie--The bluestem grasses (Andropogon)--Tall panic grass (Panicum virgatum)--Tall marsh grass (Spartina michauxiana)--Wild rye (Elymus canadensis)--Porcupine grass (Stipa spartea) and June grass (Koeleria cristata)--Root habits of non-grassy species--Conditions indicated for crop growth--The short-grass plains--Blue grama grass (Bouteloua gracilis)--Hairy grama (Bouteloua hirsuta)--Buffalo grass (Bulbilis dactyloides)--Muhlenberg's ring grass (Muhlenbergia gracillima) --Wire grass (Aristida purpurea)--Root habits of non-grassy species--Conditions indicated for crop growth--The mixed prairie--Western wheat grass (Agropyron smithii)--Needle grass (Stipa comata)--Little bluestem (Andropogon scoparius)--Wire grass (Aristida purpurea)--Short grasses and sedges--Root habits of non-grassy species--Root habits of sandhill plants --Blow-out grass (Redfieldia flexuosa)--Sand reed grass (Calamotilfa longifolia)--Sandhill bluestem (Andropogon hallii)--Little bluestem (Andropogon scoparius)--Other sandhill species--Conditions indicated for crop growth--Root habit and crops in the grassland of the Pacific Northwest--Root studies in other regions--Summary.
ROOT HABITS OF WHEAT
Spring wheat--Early development--Half-grown plants--Mature root system--Root variations under different soils and climates--Variations in root habit under irrigation--Early development--Half-grown plants--Mature root systems--Root development under increased rainfall--Winter wheat--Early development--Late autumn development--Relations of the development of roots and tops--Absorbing and transpiring areas--Mature root system--Root variations under different soils and climates--Relation of roots to tops under different climates--Other investigations on the root habits of wheat--Relation of root habits to cultural practice--Summary.
ROOT HABITS OF RYE
Mature root system--Root variations under different soils and climates--Summary.
ROOT HABITS OF OATS
Early root development--Roots of half-grown plants--Mature root system--Root variations under different soils and climates--Variations in root habits of different varieties--Summary.
ROOT HABITS OF BARLEY
Early development--Half-grown plants--Mature root system--Root variations under different soils and climates--Summary.
ROOT HABITS OF CORN OR MAIZE
Early development--Midsummer growth--Mature root system--Relation of root habits to tillage practice--Variations in root habit under different degrees of irrigation--Early development--Midsummer root habits--Mature root systems--Root development under increased precipitation--Other investigations on corn--Summary.
ROOT HABITS OF SORGHUM
Mature root habit of Black Amber sorghum--Mature root habit of Folger sorghum--Mature root habit of kafir--Root development of Blackhull kafir and Dwarf milo--Relation of root habit to drought resistance and growth in poor soil--Relation to tillage and crop rotations--Summary.
ROOT HABITS OF VARIOUS MEADOW AND
Brome grass--Root growth during the first year--Root habits of older plants--Orchard grass--Root growth during the first year--Root habits of mature plants--Meadow Fescue Root growth during the first year--Bluegrass--General root habits--Root growth during the first season--Variation of mature roots under different soils and climates--Summary and discussion--Root development of other cultivated grasses--Grass roots in relation to soil structure and productivity.
ROOT HABITS OF SUGAR BEET
The young root system--The half-grown root system--Mature root system--Root development under increased rainfall--Relation of root habit to tillage practice--Root growth of "mother beets" used for seed production--Summary.
ROOT HABITS OF ALFALFA
Early root habit of common alfalfa--Later development--Root habit of two-year-old plants--Effect of environment on root habit--Root behavior under irrigation and in dry land--Early development--Midsummer growth--Root habit at the end of the first year--Root extent during the second year--Other investigations of alfalfa--Varietal differences in root habit--Relation of root habit to crop production--Summary.
ROOT HABITS OF VARIOUS CLOVERS
Common red or purple clover--Early development--Mature root system--White clover--White sweet clover--Early development--Four-months-old plants--Twelve-months-old plants--Other root relations--Suminary.
ROOT HABITS OF THE POTATO
Early development--Mature root system--Root habit in relation to tillage--Variations in root habit under different degrees of moisture--Other investigations on the potato--Summary.
ROOT HABITS OF SUNFLOWER
Early growth--Later development--Variations in root habit under competition--Summary.
METHODS OF STUDYING ROOT DEVELOPMENT
Methods employed by earlier investigators--The direct method of root examination--Selecting the plants-Excavating the trench--Excavating and describing the roots--Photographs and drawings.