118 



A TEXTBOOK OF BOTANY 



[Cn. IV, 2 



young stems, though other tissues share in that function. All 

 of these features are shown with particular clearness in Fig. 71. 



In sections taken well back of 

 the tip, two other tissues appear. 

 One is a mere line extending 

 right' through the fibro-vascular 

 bundles, and from one to another 

 (Figs. 70, 71), uniting them into 

 one ring, or (since they run 

 lengthwise) one sheath. This 

 is the important CAMBJ^,M J or 

 growth tissue, whicnTater builds 

 new tissues on both its outer and 

 inner surfaces. The other is a 

 band of whitish-glistening tissue 

 just beneath the epidermis, 

 called coLLENqHYMA. It has a 

 firm elastic texture, and aids 

 the young stem to support the 

 strains imposed by the presence 

 of the leaves. Its position close 

 to the outside is typical of the 

 strengthening tissues of stems, 

 which are developed upon the 

 principle of the hollow column 

 or tube. This principle is 

 known to engineers as that 

 which provides the greatest re- 

 sistance to lateral strains with 

 of ma- 

 terial, on which account it is 

 used by them in many construc- 

 tions, most familiar perhaps in architectural columns and 

 bicycle frames. 



The fiVfr^-v*][S i ci]lfl.r bundles (or veins) of the stem extend 

 downward all the way to the tips of the roots, and upward 



FIG. 73. The fibro-vascular 

 system, showing its nodal branch- 

 ing, in the young stem of Clematis the least expenditure 

 mticella. (After Nageli, from 

 Strasburger.) 



