214 Pla?it Biology 



walled parejichyma cells; (2) vascular bundles which are arranged in 

 a circle toward the periphery of the stem and composed of (a) xylem (to- 

 ward the pith), composed of thick-walled cells (single-celled tracheids 

 and vessels), (b) phloem (toward the periphery of the stem), composed 

 of nonnucleated, sieve tubes (with perforated sieve-plates) and elongated, 

 nucleated companion cells, and (c) a thin layer of meristematic tissue 

 called cambium which separates the xylem and phloem; (3) the pericycle 

 which is a cylinder of mechanical tissue, external to the vascular bundles, 

 composed of thick-walled cells, with some thin-walled cells; individual 

 bundles are separated by radial strands or rays, composed of parenchyma 

 cells to conduct materials across the stem; the entire central core of the 

 stem described so far constitutes what is called the stele; (4) a layer 

 of cortex just external to the stele composed of large, thin-w^alled cells; 

 (5) a layer of mechanical tissue beyond the cortex composed of thick- 

 walled cells; (6) the epidermis external to the mechanical tissue com- 

 posed of elongated, flat cells whose outer walls are impregnated with a 

 waxy cutin to make them impermeable to water. Certain epidermal cells 

 may produce extensions known as hairs. A connection between a vascu- 

 lar bundle and a leaf is called a leaf trace. 



Dicotyledonous stems (as well as gymnospermous stems) usually pro- 

 duce so-called secondary tissues in contrast to the monocotyledons which 

 do not produce secondary tissues. The secondary tissues arise from the 

 cambium and consist of secondary xylem and secondary phloem. In the 

 stems of annual plants the secondary xylem and secondary phloem are 

 formed for only one season, but in perennial plants (especially shrubs 

 and trees) the cambium forms secondary xylem and phloem year after 

 year. In stems of woody trees and shrubs, where secondary xylem and 

 phloem are formed year after year, the xylem formed in the spring is 

 composed of large, relatively thin-walled elements (spring wood), while 

 the xylem formed during the summer is composed of small, thick-walled 

 elements (summer wood). The annual rings are concentric lines of 

 demarcation between the large-celled, thin-walled spring wood of the 

 current year and the small-celled, thick-walled summer wood of the pre- 

 vious year. The number of rings is not always an accurate criterion ofl 

 age, because under unusual conditions two rings may be formed in onei 

 year. The thickness of an annual ring is often greatly influenced by 

 environmental conditions, such as water supply, weather conditions, etc. 

 The secondary phloem of most woody dicotyledons may contain thick- 

 walled, elongated cells with pointed ends and are called bast fibers. The 



