LINUM USITATISSIMUM 393 



anastomose with the lateral arches of metaxylem in the root-like 

 portion of the lower hypocotyl while the others end blindly. 

 Interconnections between the bundles of the stem are accomplished, 

 following the maturation of the primary tissues, by the develop- 

 ment of fascicular and interfascicular cambiums which produce a 

 continuous cylinder of secondary vascular tissue. 



The Stem. — In the United States, relatively little investigation 

 of the structure of the stem and its fibers has been carried on; but in 

 Europe, where fiber production is more important, extensive studies 

 have been made. The most desirable stems for fiber purposes are 

 those which, in addition to having the genetic qualities necessary 

 for good fiber production, have been grown closely together so that 

 the stems are long, slender, and unbranched. In such cases, the 

 diameter of the mature stem of fiber flax may not exceed i or 3 mm. ; 

 but it may be considerably greater under unfavorable cultural 

 conditions, or in the more branched, stocky types of flax that are 

 grown for seed. 



The young stem is round or subterete in transection; and, as it 

 matures, it becomes hollow due to a progressive disintegration of 

 the centrally located parenchymatous cells. The relatively large, 

 thick-walled epidermal cells have a well-defined cuticle; their 

 tangential dimension is about twice the radial one; and they are 

 elongated in the axial direction, being several times as long as wide. 

 The stomata which occur in longitudinal rows are somewhat 

 depressed, and the guard cells are subtended by crescentic acces- 

 sory cells that form a part of the motor mechanism as in the leaf. 

 Tammes (xi) reports a stomatal frequency of 30-40 per sq. mm., 

 while Herzog (11) records counts of 2.5-35. 



Within the epidermis is a single-layered hypodermis in which 

 the cells resemble those of the former, but are much smaller and 

 thinner walled. The hypodermal cells contain chlorophyll, and 

 are compact rather than having the spongy organization of the 

 other four or five layers of chlorenchyma lying between the hypo- 

 dermis and the endodermis. The spongy cells are approximately 

 isodiametric and arranged in longitudinal rows that may be more 

 or less separated depending upon the age of the stem. As the stem 

 becomes older, the epidermis persists, its component cells under- 

 going radial divisions without rupturing, while the cortical cells 

 first become tangentially elongated and then much crushed and 

 radially compressed. The large, oval endodermal cells can be 



