The well-developed light line in Melilotus alba and M. officinalis is found just below the bases of the cones. In some seed coats only a few and in others none of the canals which are common in the cones and cuticularized region cross the light line. A very distinct line of small canals filled with air and thus forming a dark band is present just above the fight line, thus making the light line more conspicuous. ([Pl. V, fig. 3.]) When the lumina of the cells extend across the light line, they are exceedingly small. The light line is the most compact region of the Malpighian layer and is conspicuous because it refracts the light much more than the regions above and below it.
Just below the Malpighian is a layer of cells variously modified and known as the osteosclerid. The cells of this layer are often referred to as the hourglass cells on account of their shape. In some regions of the seed coat they are expanded at both ends and their walls are much thickened, the thickenings forming ridges on the radial walls, while in other regions only the upper tangential wall and a portion of the radial walls are thickened and the cells are expanded only at the inner end, thus having the shape of the frustum of a cone. Beneath the osteosclerid layer is the nutrient layer.
The nutrient layer contains chloroplasts. It varies not only in the number of layers of cells composing it, but also in the modifications of these cells. This layer ranges from four to seven cells in thickness in the different parts of the seed coat.
Structure of the Seed Coat of Sweet Clover.
Fig. 1.—Section of the seed coat of Melilotus officinalis. × 450. Fig. 2.—Another section of the seed coat of Melilotus officinalis, showing the variation in size and modifications that occur in the three layers. × 450. Fig. 3.—Section of the Malpighian layer of a Melilotus alba seed, showing a line of canals just above the light zone. × 450. Fig. 4.—Section of the Malpighian layer of a permeable Melilotus alba seed. × 450. Fig. 5.—Tangential section of the Malpighian cells cut between the cuticle and tops of the cones, showing pores. × 530. Fig. 6.—Section through the Malpighian layer of an impermeable Melilotus alba seed. × 450. Fig. 7.—Section through the Malpighian layer of an impermeable Melilotus alba seed, showing the region through which water and stains readily passed. × 450. Fig. 8.—Cross section of a Malpighian cell of a permeable Melilotus alba seed through the region of the light zone, showing the lumen not entirely closed. × 530. Fig. 9.—Section through the Malpighian layer of a Melilotus alba seed shaded to show the portions which react to the cellulose and pectose tests. × 450. Fig. 10.—Section through the Malpighian layer of a Melilotus alba seed which shows the condition of the seed coat after 60 minutes' treatment of concentrated sulphuric acid. That portion above the light zone was destroyed, and the lumina as small pores through which much of the stain now passed were seen extending across the light line. The lines between the cells were much more distinct, appearing as small intercellular spaces through which some stain passed. × 450. a, Cuticle; b, cuticularized layer; c, conelike portion of the thickening of the Malpighian cells; d, light line; e, region of a hard seed coat through which water and stains readily passed; l, lumen; M, Malpighian cells; N, nutrient cells; O, osteosclerid cells; p, canals just above light zone.
MICROCHEMISTRY OF THE SEED COAT.
Tests for cutin showed that there was very little present in the seed coat. Slight reactions for cutin were observed in the cuticle, in the outer margin of the cuticularized layer, and in the basal portion of the cones. These reactions were so slight as to be almost negligible. It is evident that the cuticle and cuticularized layer are not well named in Melilotus alba and M. officinalis. Tests for cellulose showed that it was present in the cuticle, cuticularized layer, cones, the walls of the Malpighian cells below the light line, and the walls of the cells of the osteosclerid and nutrient layers. ([Pl. V, fig. 9.]) The reaction for cellulose in the Malpighian cells was quite distinct in the walls below the light line, less distinct in the cones and cuticle, and least distinct in the cuticularized layer.