702 



ECOLOGY 



times is considered to be a sort of " water storage " tissue. The latter theory 

 may be dismissed summarily, and the former theory is at least doubtful, in view 

 of the permeability of the walls. 



Flexile strength. The flexion of a stem induces tension on one side 

 and compression on the other, each strain decreasing to zero at the center. 

 Consequently a stem with a strong cylinder of peripheral mechanical tis- 

 sue may have a central region of pith or may even be hollow (as in 

 Equisetum and in the grasses) and yet have considerable rigidity. It 

 has been computed that if the mechanical periphery forms one seventh 

 of the diameter, the strength will be sufficient to meet all usual strains. 



Further development of mechanical tissue 

 would be not only useless but actually 

 disadvantageous, because it would re- 

 quire a considerable amount of struc- 

 tural energy and material, and also be- 

 cause space would be used that might 

 be taken by other tissues. The aver- 

 age erect herbaceous stem illustrates 

 admirably these mechanical principles. 

 Sometimes (as in Dianthus, fig. 1027) 

 there is a mechanical cylinder sur- 

 rounding the leptome, while in other 

 cases strands of bast form an interrupted 

 cylinder. External to the bast is the 

 collenchyma cylinder, and internal to 

 the bast is the cylinder of secondary 

 wood which is of great mechanical im- 

 portance. Although the conductive and mechanical bundles in mono- 

 cotyl stems are scattered, the decrease of bast elements toward the 

 center results essentially in a broken peripheral cylinder of mechan- 

 ical tissue (fig. 1028). In angled stems (notably in the mints, fig. 

 1029) strains are accentuated at the angles, where there is consider- 

 able collenchyma just inside the epidermis, and often a bast crescent 

 just outside the leptome. 



The advantage of flexile strength is well illustrated on mountain slopes that 

 are subject to snowslides and avalanches, flexible trees and shrubs (such as alders 

 and willows) being uninjured, whereas some rigid trees (as conifers) snap off like 

 pipestems. In lowlands the accumulation of ice during a cold rain frequently 

 causes the rupture of branches that withstand all ordinary strains. 



FIG 1029 A diagrammatic 

 cross section of an erect aerial stem 

 of a mountain mint (Pycnanthemum 

 virginianum), showing columns of 

 mechanical tissue (m) at the stem 

 angles; c, cortical parenchyma; v. 

 vascular cylinder; p, central pith 

 region. 



