UNITED STATES DEPARTMENT OF AGRICULTURE 
j&nfrj-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, Iowa 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 
Kelation 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 ___ 
