LINUM USITATISSIMUM 389 



primary xylem disappears. The stretching of the spiral elements 

 results in their collapse; and, in plants two weeks old, practically 

 all of them in the upper hypocotyl are obliterated. (Fig. 198.) 

 However, fragments of the primary xylem may remain for longer 

 periods, or throughout the life of the plant, without being com- 



FiG. 198. Transection of portion of vascular ring in upper hypocotyl showing early dis- 

 integration and resorption of primary xylem : ca, cambium ; mx, metaxylem ; ^h, phloem ; 

 />/, pith; /).v, protoxylem; atj i, secondary xylem. (After Crooks.) 



pletely resorbed; and may be seen, especially in longisection, 

 among the parenchymatous cells. 



The Cotyledons. — The obovate cotyledons are subsessile, and, 

 in young seedlings, are somewhat broader than the hypocotyle- 

 donary axis at their point of divergence. The mesophyll is made 

 up of very compact storage cells, two or three of the adaxial layers 

 forming a palisade region while the lower four or five layers are 

 more rounded and compact. There is an early differentiation of the 

 protoxylem elements in the median bundle; and under favorable 

 conditions they are completely matured twenty-four hours after 

 germination. Cases have been observed where some of the proto- 

 xylem is differentiated in the mature seed, and scattered phloem 

 elements occur in the provascular strands of the immature cotyle- 



