x6 



THE STRUCTURE OF ECONOMIC PLANTS 



mary xylem, this is indicated by the terms exarch, mesarch, or end- 

 arch. In exarch development, the protoxylem is differentiated at 

 or near the periphery of the stele, usually abutting the pericycle, 

 and the metaxylem is laid down centripetally with respect to it. 

 In the mesarch type, the differentiation of metaxylem proceeds both 

 centripetally and centrifugally from the protoxylem so that it forms 

 a zone on either side of the protoxylem and may completely sur- 

 round it. In the endarch relation, the protoxylem is differentiated 



a^ b 



/ g h 



i j k I m n 



'HI 



Fig. 9. Longisection of the stem of the parsnip through a bundle: a, parenchyma of the 

 pith; i, epithelial cells of a medullary oil duct; c, mechanical tissue; ^, protoxylem; <?, meta- 

 xylem; /, secondary xylem vessels and parenchyma; g, cambium; /', sieve tubes, companion 

 cells, and parenchyma; /, pericyclic fibers; ;, endodermis; k, cortical parenchyma; /, cortical 

 oil duct and its epithelial cells; m, coUenchyma fibers; «, collenchymatous parenchyma; 

 0, epidermis. 



at or near the center of the axis and the metaxylem is laid down cen- 

 trifugally. These terms are only used in reference to primary 

 xylem and are not applied to secondary xylem, which is usually 

 laid down centripetally by the cambium. 



The Tracheids. — The tracheid is regarded as the basic and, 

 phylogenetically, the most primitive cell type in the xylem tissue. 

 It is angular or roughly four-sided in transection and much elon- 

 gated with tapered, pointed, or wedge-shaped ends. It is com- 

 monly small in transection with a reLitively large lumen that is 

 devoid of protoplasm at maturity; and the walls, which are 



