7 6 



PLANT LIFE. 



and are ordinarily cork-like, i.e., thin-walled and impervious 

 to water. Those cells which lie outside a layer of cork are 

 therefore cut off from a supply of food and soon perish. 



The inner growing layer, or stelar cambium, is developed 



within the stele and follows a 

 tortuous course, lying outside the 

 xylem and inside the phloem 

 bundles (fig. 88). As a result 

 of tangential divisions in this 

 region, tissues similar to those 

 already existing in the stele are 

 produced. On the outer side 

 the cells differentiate mainly 

 into the tissues of the phloem, 

 and on the inner side mainly 



FIG. go.-Transverse section of the older in tO those of the Xylem, often 



part of the main root of bean (Phase- f rkrrn : ricr n npirlv nnhrnVfn 

 olus multi-floras) after secondary lOrmmg a lieaily UnD 

 thickening has progressed consider- r i (f. a c 8r> r>r>\ 



ably. Compare with fig. 89, which is ( V n s - 9> 9)- 



about five times as highly magnified. r ~1 ntl Vo amnnnf nf trip 

 6, 6, 6, 6, four primary phloem bundles; ] 



6', secondary phloem produced by f - whirh rmkf nn 



stelar cambium, as are the four wedges L *^ucs 



" PsSwn"' TheySeS^ bundles goes for to determine 



the character of the mature root. 

 87. (a) Woody roots. If mechanical tissues predominate, 

 particularly in the xylem, the root will become strong and 

 rigid, as in the case of trees and shrubs. When the root is 

 long-lived, the activity of this stelar cambium is usually 

 resumed with each season, a layer of tissue being thereby 

 added to the outside of the xylem region, and a thinner layer 

 to the inside of the phloem. The woody part, especially, 

 shows in cross-section concentric rings indicating the yearly 

 additions. Since the material produced by the stelar cam- 

 bium usually greatly increases the diameter of the root, the 

 outside parts become fissured lengthwise. Thus, in an old 

 and much-thickened root of the woody type, the periderm 



