136 THE STRUCTURE OF ECONOMIC PLANTS 



and, according to Heuser (15), their frequency is only ii per square 

 centimeter as compared with approximately 3000 per square centi- 

 meter in flax. Briosi and Tognini (7) reported a frequency of 5 per 

 square millimeter in a young stem 2. mm. in diameter, pointing out 

 that the frequency per unit area diminishes greatly as the stem 

 matures. The radial and tangential diameters of the epidermal 



Fig. 113. Transection of portion of young stem: ca, cambium; co, cortex; col, collen- 

 chyma ; ep, epidermis ; hi; epidermal hair ; la d, latex or resin duct ; m r, medullary ray ; 

 pel, pericycle; ph, phloem; pi, pith; xy i, primary xylem; xy 2., secondary xylem. 



cells are approximately equal, and they are three to four times as 

 long as broad. 



Adjacent to the epidermis is a zone of chlorenchyma, two or three 

 cells in width, which extends completely around the periphery of 

 the stem including the ridges where it separates the collenchyma 

 from the epidermis. The chlorenchymatous cells form a compact 

 zone at first; but, later in ontogeny, they become considerably 

 stretched tangentially and large intercellular spaces develop. The 

 cells of the hypodermal layer are larger than the others, and also 

 become thicker-walled as maturation proceeds. Within the zone 

 of chlorenchyma, strands of thick-walled, compact collenchyma- 

 tous cells form a part of the ridges of the stem. The cortical zone 

 is not wide; and, aside from the chlorenchyma and collenchyma, 

 there are only a few layers of parenchymatous cells. (Fig. 114.) 



