40 



PEACTKIAL ItOTANi' 



consists of a cellular portion (o) which belongs to the bark 

 system, iinil a fibrous and tubular portion (w) which belongs 

 to the -wood system. Briefly stated, the uses of some of 

 the several parts are as follows : (1) The epidermis serves as 



a protective covering for the 

 young stem, and to a consider- 

 able extent prevents it from 

 becommg dried up. 



(2) The layers of cork cells 

 soon formed close beneath the 

 epidermis (not separately shown 

 in the diagram) prevent loss of 

 water and consequent diying ujj. 

 (^)) The layers of green cells 

 which at first directly underlie 

 the epidermis (not shown in 

 the diagram) are useful in the 

 manufacture of plant food.i 



(4) The fibrous cells of the 

 hard bast (Ji) give toughness to 

 the stem. 



(5) The thin-walled tubes 

 (if the outer portions (o) of the 

 bundles carry manufactured 

 plant food in liquid form down- 

 ward or toward the roots. 



(6) Tlie cambium layer (r) 

 (shown proportionally thicker 

 in the diagram than it really is) 



grows, and on its outer side it forms new bark, while on its 

 inner side it forms new wood (see Sect. 47). 



(7) The woody portions (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 both 

 stiffen the stem and make it tougher. 



1 See Sect. 40. 



Fig. 31. Lengthwise section through 



the vascular part of a fibrovascular 



bundle of sunflower stem 



The tubes with spiral markings (spiral 

 vessels) are the principal channels for 

 the conduction of water. Between and 

 around them are thin-walled nucle- 

 ated cells, containing much cell sap. 

 Much magnified. After Frank 



