| UNITED STATES DEPARTMENT OF AGRICULTURE | ||
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BULLETIN No. 844 Contribution from the Bureau of Plant Industry |
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| Washington, D. C. | PROFESSIONAL PAPER | August 11, 1920 |
By H. S. Coe, formerly Assistant Agronomist, Office of Forage-Crop Investigations, and J. N. Martin, Professor of Morphology and Cytology, Iowa State College.
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Part I.—Pollination Studies of Seed Production. |
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| 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 |
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| 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. |
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| 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 |