STEMS 



701 



Fig. 1027. — A diagrammatic cross 

 section of a carnation stem {Dianthus 

 Caryophylltis), showing the develop- 

 ment of a mechanical cylinder (m) out- 

 side the vascular tract (v) ; c, c', cortical 

 parenchyma; p, central pith. 



gram per square millimeter, some of 



the strongest bast has within its 



limits of elasticity a tensile strength 



of twelve to twenty-five kilograms per 



square millimeter, the higher figure 



being twice that of wrought iron and 



about equal to that of German steel; 



however, the breaking point of steel 



is much beyond that of bast. Bast 



differs widely from the metals in 



having a considerable degree of 



elastic elongation, and is itself ex- 

 ceeded in this respect by the me- 

 chanical strands of lichens, those of 



Usnea being capable of elongating 



sixty to one hundred per cent (see fig. 1113). Bast is considerably 



stronger when desiccated than when it contains moisture, though it is 



more elastic in the latter condition; ligni- 

 fication usually decreases the tensile 

 strength. The great tensile strength of 

 bast is due to the amount and quality of 

 the wall thickening, to the dovetailing of 

 the prosenchymatic cells, and probably, 

 also, to a spiral arrangement of the wall 

 micellae. The fact that the limit of elas- 

 ticity is so near the breaking point is not 

 disadvantageous, since any elongation of 

 bast beyond the limit of elasticity would 

 be harmful. Collenchyma is almost as 

 strong as bast, though it has a lower 

 limit of elastic elongation. Having the 

 power of growth elongation, it is espe- 

 cially suited to growing tissues. 



Fig. 102S. — A diagrammatic 

 cross section of the erect aerial 

 stem of Juncus balticits littoralis, 

 showing an interrupted mechanical 

 cylinder, composed of alternating 

 outer {or} and inner (z) fibrovas- 

 cular bundles, in which a bast 

 cylinder (fe) surrounds the vas- 

 cular tract iy) ; note that the bast 

 is more strongly developed out- 

 ward than inward; c, cortex; a, 

 large central chamber, originally 

 occupied by pith cells; consider- 

 ably magnified. 



The role of scattered sclereids is less obvious 

 and probably less important than is that of bast 

 and collenchyma though it is of significance in 

 certain leaves (p. 639). Bast and collenchyma 

 sometimes have been regarded as advantageous 

 in checking transpiration, and collenchyma some- 



