26 BULLETIN 844, TJ. 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 years ago. However, imper- 

 meable seeds are not confined to the Leguminosse, as they occur also 

 in the Malvaceae, Chenopodiacere, Convolvulacea?, Cannaceas, 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 Leguminosse. 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. VerschafTelt (39) 

 found that the impermeability of the seeds of Caesalpiniacese and 

 Mimosacese 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, Iowa 

 State College, Ames, Iowa, in assisting in the preparation of this article. 



