64 



INTRODUCTION TO BOTANY 



1. The epidermis (ep.) serves as a protective covering for 

 the yonng.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. 



3. 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. 1 



4. The fibrous cells of the hard bast give 

 toughness to the stem. 



5. Certain thin-walled 

 tubes (sz) of the outer 

 portions of the bun- 

 dles carry manufactured 

 plant food in liquid form 

 downward, or toward 

 the roots. 



6. The cambium layer 

 (in figure 42, A shown 

 proportionally thicker 

 than it really is) grows 

 and forms new bark on 

 its outer side, while on 

 its inner side it forms 

 new wood (see sect. 64). 



7. The vessels, or 



ducts (w), 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 



1 See section 56. 



A B 



FIG. 44. A, strengthening and other tissue 



from stem of balsam (Impatiens) ; B, a group 



of hard-bast fibers 



e, epidermis ; c, collenchyma ; i, intercellular 

 spaces between large parenchyma cells ; a, cut- 

 off ends; b, lengthwise section of fibers. Greatly 

 magnified. ^4,afterStrasburger; B, after Tschirch 



