WASHINGTON GOVERNMENT PRINTING OFFICE 1920 UNITED STATES DEPARTMENT OF AGRICULTURE BULLETIN No. 844 Contribution from the Bureau of Plant Industry WM. A. TAYLOR, Chief Washington, D. C. PROFESSIONAL PAPER August 11, 1920 SWEET-CLOVER SEED Part I.--Pollination Studies of Seed Production Part II.--Structure and Chemical Nature of the Seed Coat and its Relation to Impermeable Seeds of Sweet Clover By H. S. Coe, _formerly Assistant Agronomist, Office of Forage-Crop Investigations_, and J. N. Martin, _Professor of Morphology and Cytology, Iowa State College_. CONTENTS Page Part I.--Pollination Studies of Seed Production. Unsatisfactory yields of sweet-clover seed 1 Previous investigations of the pollination of sweet clover 2 Outline of pollinating experiments 3 Structure of the flowers of Melilotus alba 4 Development of the floral organs of sweet clover 5 Fertilization in Melilotus alba 8 Development of the seed 8 Mature pollen of sweet clover 9 Germination of the pollen 9 Cross-pollination and self-pollination of sweet clover 10 Artificial manipulation of sweet-clover flowers 10 Seed production of Melilotus alba under ordinary field conditions 13 Efficiency of certain kinds of insects as pollinators of sweet clover 14 Relation of the position of the flowers on Melilotus alba plants to seed production 19 Influence of the weather at blossoming time upon seed production 20 Insect pollinators of sweet clover 21 Effect of moisture upon the production of Melilotus alba seed 22 Part II.--Structure and Chemical Nature of the Seed Coat and its Relation to Impermeable Seeds of Sweet Clover. Historical summary 26 Material and methods 30 Structure of the seed coat 31 Microchemistry of the seed coat 33 The seed coat in relation to the absorption of water 34 A comparison of permeable and impermeable seed coats 34 The action of sulphuric acid on the coats of impermeable seeds 35 Literature Cited 36