UNITED STATES DEPARTMENT OF AGRICULTURE 



j&r^u 



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, Ioiva State College. 



CONTENTS. 



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 



Page. 

 Part I.— Pollination Studies of Seed 

 Production — Continued. 

 Influence of the weather at blossoming 20 



time upon seed production 21 



Insect pollinators of sweet clover 22 



Effect of moisture upon the production of 



Melilotus alba seed 



Part II.— Structure and Chemical Na- 

 ture of the Seed Coat and its Rela- 

 tion 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 imper- 

 meable seed coats 34 



The action of sulphuric acid on the coats 



of impermeable seeds 35 



Literature Cited 36 



Part I.— POLLINATION STUDIES OF SEED PRODUCTION. 

 UNSATISFACTORY YIELDS OF SWEET-CLOVER SEED. 



In some sections of the country much trouble has been experienced 

 for a few years past in obtaining satisfactory yields of sweet-clover 

 seed. This difficulty has been due for the most part to the following 

 causes: (1) To cutting the plants at an improper stage of develop- 



153321°— 20— Bull. 844 1 



