Part II.—STRUCTURE AND CHEMICAL NATURE OF THE SEED COAT AND ITS RELATION TO IMPERMEABLE SEEDS OF SWEET CLOVER.[3]
[3] The writers wish to acknowledge the service rendered by Mr. H. S. Doty, Instructor in Botany, Iowa State College, Ames, Iowa, in assisting in the preparation of this article.
HISTORICAL SUMMARY.
When agriculturists first began to cultivate wild legumes they observed that many seeds would not germinate within a comparatively short time after planting. Thus the problem of impermeable seeds began to demand attention many years ago. However, impermeable seeds are not confined to the Leguminosæ, as they occur also in the Malvaceæ, Chenopodiaceæ, Convolvulaceæ, Cannaceæ, 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 Malpighian cells, the osteosclerid layer, and the inner layer of nutrient cells. Pammel's work included a study of the seed coats of Melilotus 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 Leguminosæ. Crocker ([3]) states that, exactly opposite to the common view, the cause of delayed germination 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 Haenlein ([30], p. 81) state that the absorbent power of many seeds is inhibited 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 Cæsalpiniaceæ and Mimosaceæ 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 coats were made readily permeable to water. Gola ([6]) states that the cause of the impermeability of seeds is the peculiar character of the Malpighian cells, which prevents their infiltration and consequent increase in volume, while Bergtheil and Day ([2]) found that the hardness of the seeds of Indigofera arrecta was due to their possession of a very thin outer covering of a substance resistant to water. Ewart ([5], p. 185) believes that in most impermeable seeds the cuticle prohibits the absorption of water, but gives as an exception Adansonia digitata, in which the whole integument seems to be permeable to water with difficulty. The following is quoted from White ([42], p. 205):
As a general rule in small and medium-sized seeds the cuticle is well developed and represents the impermeable part of the seed coat, while in the cases of large seeds, such as those of Adansonia gregorii, Mucuna gigantea, Wistaria maideniana, and Guilandina bonducella, the cuticle is relatively unimportant and inconspicuous. In these seeds the extreme resistance which they exhibit appears to be located in the palisade cells.
In discussing the seed coat of Melilotus alba, Rees ([33], p. 404) states that the outer layer consists of palisade cells covered, externally by a structureless membrane, which, however, did not appear to be cuticle but hemicellulose, as it stained magenta with chloriodid of zinc. The greater part of the walls of the palisade cells also appears to be composed of hemicellulose and the outer ends only were cuticularized. In order to find whether the outer membrane was in itself impermeable to water, this author treated seeds for short intervals in sulphuric acid to dissolve the outside covering without directly affecting the palisade cells. Seeds treated in this manner swelled in water and microscopic examination showed that the ends of the palisade cells were quite intact, but had separated from each other. From this it was concluded that the outer membrane is instrumental in conferring impermeability on the seed, although not directly responsible for it, as is the case with a true cuticle. It is further believed that it probably served as a cement substance by means of which the cuticularized ends of the cells were held together closely, thus forming a barrier through which water could not penetrate, but that as soon as this barrier was removed the ends of the palisade cells separated and water passed in between them.
More than 20 years ago machines were devised by Kuntze, Michalowski ([27], p. 86), Huss ([15]), and later by Hughes ([14]), to scarify impermeable seeds. Other methods have been recommended and employed to some extent for hastening the germination of seeds. Hiltner ([13], p. 44) treated seeds of red clover, white clover, and alfalfa 10, 30, and 60 minutes with concentrated sulphuric acid and found that the best germination resulted from the 60-minute treatment. Love and Leighty ([21]) also treated the seeds of various legumes with concentrated sulphuric acid and obtained a better germination in all cases. In their investigations with Melilotus alba it was found that a 2-hour treatment resulted in some injury to the seed, but that a treatment varying from 25 minutes to 1 hour gave good results. In most cases in our investigations the seed coats of sweet clover became permeable to water after a treatment of 15 minutes in concentrated sulphuric acid, and within 5 minutes all of the Malpighian cells were destroyed down to the light line. Harrington ([10]) found that the soil, season, climate, color, or size of red-clover seeds had no influence upon the percentage of impermeable seeds and that the good germination ordinarily obtained with red clover was due to the scarifying of the seed coats by the rasps of hulling machines. Harrington ([11]) also studied the agricultural value of impermeable seeds and found that alternations of temperature cause the softening and germinating of many impermeable clover seeds when a temperature of 10° C. or cooler is used in alternation with a temperature of 20° C. or warmer and that the effect of such an alternation of temperature is greatly increased by previously exposing the seeds to germinating conditions at a temperature of 10° C. or cooler and is decreased by previously exposing the seeds to germinating conditions at a temperature of 30° C. It is a well-known fact that impermeable seeds which remain in the field over winter germinate readily the following spring.