THE STEM 195 
tinguishable from the cortex, except tor the knowledge of their 
relative position, since each has become so similar to the cortex 
through passage of food, etc.) 
1. EpIweRMis, a single layer of somewhat flattened cells whose 
outer walls are cutinized. Some of the cells show outgrowths as 
hairs. Stomata also occur. 
mr mr 
Fic. 125.—Diagram of a representative portion of a transverse section cut 
through an old part of the Sunflower stem. ep, epidermis; col, collenchyma; 
co, cortex; p.f., pericyclic fibers in pericycle; ph, protophloem; ph’, secondary 
phloem; cam, intrafascicular cambium; cam’, interfascicular cambium; x, proto- 
xylem; x’, secondary xylem; mr, primary medullary ray; mr’, secondary medullary 
ray; m, pith. The three longest bundles represent primary bundles which have 
been broadened by the cambium adding secondary phloem outwardly and second- 
ary xylem inwardly. The smaller bundles represent secondary bundles, each 
consisting of secondary phloem, cambium and secondary xylem. 
2. Primary CorTEX, composed of two zones: an outer region 
of several layers of narrow, thick-walled cells (hypodermis) and a 
broad zone of larger, thin-walled cortical parenchyma cells, many 
of which contain chloroplasts. 
3. The innermost layer of cells of the cortex is called the 
Enpopermis. (Not generally distinguishable.) 
4. Pertcycie (Pericambium). Not generally distinguish- 
able in most dicotyl stems but showing early formation of groups 
of fibers in sunflower stems. 
