64 INTRODUCTION TO BOTANY 
1. The epidermis (ep.) serves as a protective covering for 
the young stem and, to a considerable extent, prevents it from 
becoming dried up. 
2. The layers of cork cells soon formed just beneath the 
epidermis (not separately shown in the diagram) prevent loss 
of water and consequent drying up. 
8. The layers of green cells which at first 
directly underlie the epidermis (not distinguish- 
able in the diagram) are useful in the manu- 
facture of plant food.t 
4, The fibrous cells of the hard bast give 
toughness to the stem. 
5. Certain thin-walled 
tubes (st) of the outer 
portions of the bun- 
dles carry manufactured 
plant food in liquid form 
downward, or toward 
the roots. 
6. The cambium layer 
Gn figure 42, 4 shown 
-proportionally thicker : : 
than it really is) grows Fie. 44. A, strengthening and other tissue 
5 from stem of balsam (Impatiens); B,a group 
and forms new bark on of hard-bast fibers 
its outer side, while on e, epidermis; c, collenchyma; i, intercellular 
its inner side it forms spaces between large parenchyma cells; a, cut- 
off ends; b, lengthwise section of fibers. Greatly 
new wood (see sect. 64) "magnified. A, after Strasburger ; B, after Tschirch 
7. The vessels, or 
ducts (v), of the bundles carry water upward, or toward the 
leaves. The fibers, which constitute a considerable portion of 
the wood part of the bundles, stiffen the stem and make it 
tougher. 
61. Strengthening cells. The cells which serve to stiffen or 
toughen the roots, stems, and leaves of plants belong to sev- 
eral different types. The two kinds shown in figure 44 are 
B 
1 See section 56. 
