100 Trans. Acad. Sci. of St. Louis. 



microchemical tests, and its contact with the neighboring 

 porous part of the cell-wall intensifies the luster of the light 

 line. Mattirolo (401), after some careful studies of the light 

 line in Tilia, concludes that in this genus it consists of true 

 lignin ; in some other plants it approaches it. In Leguminosae 

 no lignification occurs as a rule. Overhage (415), who studied 

 the seeds of Canna, agrees with Mattirolo that this portion 

 of the Malpighian cell is lignified but thinks that this does not 

 account for all of its peculiarities, and in part these must be 

 due to a peculiar molecular structure. It is doubtful whether 

 Overhage studied the light lines carefully. Humphrey (382) 

 does not commit himself, but says: "Overhage states that 

 the walls in the region of this line are lignified, as Mattirolo 

 has shown to be true of many other seeds." 



Huss (118) states that the appearance of the light line 

 must be explained by its physical properties rather than 

 differences due to chemical composition. Marliere (164, 5) 

 gives a physical explanation. "The true cause of the light 

 line lies in the structure of the secondary membrane of the 

 Malpighian cell. When viewed tangentially the cells have an 

 irregular, strongly reduced cavity which projects into the 

 secondary membrane at the points where the narrow lumen 

 occurs. These points penetrate almost to the primary mem- 

 brane. They are cut out even to the secondary wall. The 

 canals of the cells are strongly acute, generally half cylin- 

 drical, leaning towards the primary membrane. The strong 

 thickening of the membrane which surrounds the cell-cavity 

 forms a homogeneous medium for the transmission of light ; 

 it thus becomes strongly refringent, more so than any other 

 part of the waU. The depth of the wall through which the 

 rays of light have to pass multiplies their reflections at the 

 expense of their intensity." This, to him, seems to be 

 a probable explanation. Kayser (386, 88) considers that 

 both chemical and physical modifications have taken place in 

 C onvolvulaceae . 



Light Line in Other Orders. — From the above general 

 review of the subject it will be seen that the views are by no 

 means harmonious. It would be presumptuous on my part 

 to conclude, because the reactions in Leguminosae are not 



