26 BULLETIN 844, U. S. DEPARTMENT OF AGRICULTURE. 
Part H.— STRUCTURE AND CHEMICAL NATURE OF THE SEED 
COAT AND ITS RELATION TO IMPERMEABLE SEEDS OF 
SWEET CLOVER. 1 
HISTORICAL SUMMARY. 
When agriculturists first began to cultivate wild legumes they 
observed that many seeds would not germinate within a compara- 
tively short time after planting. Thus the problem of impermeable 
seeds began to demand attention many }^ears ago. However, imper- 
meable seeds are not confined to the Leguminosae, as they occur also 
in the Malvaceae, Chenopodiacese, ConvolvulaceaB, Cannacese, and 
other families. 
Since the first account of the structure of legume seed coats by 
Malpighi (23 v. 1) in 1687, many investigators have contributed to 
our knowledge of the structure of the coats of seeds belonging to this 
family. 
Pammel (31) made an extensive study of legume seeds, including 
all the genera in the sixth edition of Gray's Manual, as well as 
genera not included in that publication. He found that the seed coat 
uniformly consisted of three layers, namely, the outer layer of Mal- 
pighian cells, the osteosclerid layer, and the inner layer of nutrient 
cells. Pammel's work included a study of the seed coats of Meli- 
lotus alba and M. officinalis, and the descriptions and illustrations in 
his publication agree for the most part with the results obtained in 
the investigations reported in this article. However, more variation 
was noticed in the different layers of the seed coat than he describes. 
The cause of impermeability in seeds has been investigated by 
many. It has been found to be due to the embryos in some seeds, 
such as the hawthorns, but in most cases to the structure of the seed 
coat, and especially so in the Leguminosae. Crocker (3) states that, 
exactly opposite to the common view, the cause of delayed germina- 
tion 'generally lies in the seed coats rather than in the embryos. 
Nobbe (29) thought that the hardness of leguminous seeds was due 
to the Malpighian layer, and in a later publication Nobbe and Haen- 
lein (30, p. 81) state that the absorbent power of many seeds is inhib- 
ited or entirely arrested by the cones of the Malpighian cells and the 
shields built up between them, which consist principally of cutin. 
Huss (15) agrees with Nobbe and Haenlein. Verschaffelt (39) 
found that the impermeability of the seeds of Ca^salpiniaceas and 
Mimosaceas investigated was due to, the inability of water to pass 
through the canals of the seed coat. By soaking the seeds in alcohol 
or other substances which change the capillarity of the pores, the seed 
1 The writers wish to acknowledge the service rendered by Mr. EL S. Doty, Instructor in Botany, Towa 
State College, Ames, Iowa, in assisting in the preparation of this article. 
