LINUM USITATISSIMUM 379 



which is limited externally in the cutinized outer wall. As the 

 secondary layers gradually absorb water, their stratification 

 becomes very evident and they begin to swell. The middle 

 lamellae are relatively inelastic; and, as a result of the pressure 

 exerted by the swelling layers against the comparatively tough 

 cutinized outer wall, they give way and the outer walls are lifted 

 up and fissured by the prismatic masses of swollen mucilage. 



Underlying the epidermis are one or two layers of "round cells" 

 with well-developed intercellular spaces, which are so called 

 because of the circular appearance of the cell cavities in surface 

 view. The third layer consists of longitudinally oriented fiber 

 cells which are thick-walled and porous. According to Winton 

 (i8), their length may vary considerably, ranging up to 150 ix 

 with radial dimensions that greatly exceed their breadth. The 

 fourth and fifth zones are made up of several layers of very thin- 

 walled, colorless "cross cells" which are so named because their 

 long axes are oriented at right angles to the fiber cells. Tschirch 

 and Oesterle (x6) found that the cells of this zone contain large 

 deposits of starch; but, as the seed develops, this is utilized and 

 the cells become crushed and more or less obliterated. 



The uniseriate pigment layer is the innermost one of the seed 

 coat, and is made up of cells that are square or polygonal with 

 thick porous walls and contents which determine the color of 

 the seed. The straight embryo has a short hypocotyl and two 

 long fleshy cotyledons that are thicker than the surrounding endo- 

 sperm. This varies in thickness, consisting of from two to six 

 layers of parenchymatous cells which are thicker walled than 

 those of the cotyledons and contain aleurone grains and fat. 



Embryogeny. — The early stages in the development of the 

 embryo of the Linaceae have been described by Soueges (lo), who 

 used Linum catharticum L. as a type form. Following fertiliza- 

 tion, the first division of the zygote occurs in a transverse plane 

 producing an apical and a basal cell, and subsequent transverse 

 divisions of these two cells result in the formation of a linear row 

 of four cells, /, /', m and ci. (Fig. 193, x-6.) Of this linear series, 

 the apical cell, /, always divides in a vertical plane; and the basal 

 cell, d, always divides transversely. The division of cell /' is 

 usually either longitudinal or transverse (occasionally oblique); 

 and cell m may divide transversely or longitudinally. (Fig. 193, 

 7-2-1 •) 



