30 BULLETIN 844, U. S. DEPARTMENT OF AGRICULTURE. 



gressively subdued in varying degrees by partial reflection. Pammel 

 (31, p. 147) studied the light line in Melilotus alba and found that it 

 consisted of a narrow but distinct refractive zone below the conical 

 layer. The refractive zone colored blue with chloriodid of zinc. 

 The whole upper part was, however, more or less refractive, while the 

 remainder of the cell wall contained pigment and colored blue with 

 chloriodid of zinc. Small canals project into the walls, in some 

 cases extending beyond the light line. 



Beck (1) found that the light-refracting power of the light line was 

 much greater than that of the undifferentiated membrane and stated 

 that there may be in addition to this a chemical difference which can 

 not be detected with the present microohemical methods. He does 

 not believe that it is outicularized or that it contains less water than 

 the rest of the cell. 



Marliere (24, p. 11) gives a physical explanation and states that the 

 true cause of the light line lies in the peculiar structure of the sec- 

 ondary membrane of the Malpighian cell. Tunmann (38, p. 559) 

 observed that it did not hydrolize in weak acids and therefore decided 

 that it was not hemicellulose. He found that it dissolved in concen- 

 trated sulphuric acid more readily than the regions surrounding it 

 and that it was composed of pectin or callose. In our investigations 

 the main portion of the light line of Melilotus alba and M. officinalis 

 was very resistant to concentrated sulphuric acid, only the narrow 

 outer portion being attacked. It showed evidence of callose. 



MATERIAL AND METHODS. 



Permeable and impermeable seeds 1 of Melilotus alba and M. of- 

 ficinalis were obtained from commercial samples and also from sam- 

 ples collected in the field. Those selected for sectioning were allowed 

 to dry after being removed from the germinator and then embedded 

 on the ends of pine blocks in glycerin gum, which was made by 

 dissolving 10 grams of powdered gum arabic in 10 c. c. of water and 

 adding 40 drops of glycerin. After the glycerin gum had dried 

 for 24 hours, the seeds were easily sectioned. This method of em- 

 bedding causes no change in the seed coat. It is more satisfactory 

 than the parafim method for holding the seeds firmly. The glycerin 

 gum dissolved readily when the sections were mounted in water. 



In the microchemical studies Sudan III, alcanin, chlorophyll solu- 

 tion, and phosphoric acid iodin were used to test for cutin or suberin; 

 sulphuric acid and iodin, chloriodid of zinc, and chloriodid of 

 calcium for cellulose; phloroglucin and hydrochloric acid for lignin; 



i The term "permeable" is used in this paper to designate seeds whose coats are permeable to water in 

 two weeks or less at temperatures favorable for germination, while the term "impermeable" is used to 

 designate seeds whose seed coats are impermeable to water for this length of time when temperatures are 

 favorable for germination. Impermeable seeds are commonly referred to as "hard seeds," and they may 

 become permeable in time. 



