MICROSCOPIC STRUCTURE OF WEED SEEDS 537 



fifth. Subsequent treatment with hydrochloric acid dissolves out 

 the calcium oxalate, and staining with chlorzinc iodine or safranin 

 renders the outer layers more distinct. 1. The Epidermal Cells 

 (ep) are polygonal and of enormous size, corresponding to the net- 

 work on the seed. As appears in cross section, the cells are col- 

 lapsed except in the neighborhood of the radial walls. In surface 

 view the radial walls are sinuous and thin, what are often con- 

 sidered the thick dark walls of this layer being not the walls at all, 

 but the ribs formed by the thickening of the second and third 

 layers. This conclusion is consistent with Meyer's and Hanausek's 

 figures of cross sections, also with Meyer's drawings and Mach's 

 photomicrographs of surface preparations. The statement of 

 Tschirch and Oesterle that the epidermis consists of elongated cells 

 situated over the ribs, with large polygonal cells between, has since 

 been corrected by the authors themselves. Doubtless they mistook 

 some of the cells of the second layer for epidermis. Hanausek's 

 surface view, on the other hand, might convey the impression that 

 the ribs were the cell walls, but his description and cross section 

 clearly show their true nature. 2. Crystal Layer (k). On the 



Fig. 414. Poppy Seed (Papaver somniferum). Transverse section, s, testa 

 consisting of epidermis ep, crystal layer k, fiber layer f, cross-cells a and 

 netted-cells n ; e, endosperm containing aleurone grains al. X 160. 

 (After Winton, Conn. Agr. Exp. Sta.) 



ribs, the cells of this layer are more or less tangentially elongated, 

 but between the ribs are isodiametric and polygonal, the elongated 

 cells having longer radial walls than the others, thus contributing 

 to the formation of the ribs. They contain fine, granular crystals 

 of calcium oxalate. Meyer has demonstrated that the blue color 

 of the poppy seed is due, not to a blue pigment, but to the inter- 

 ference of light by the crystals over the brown cells in the back- 

 ground, and is the same phenomenon that causes the apparent 

 blue color of the sky and the iris of the eye. As soon as these 

 crystals are dissolved in hydrochloric acid, the seed appears brown. 



