184 MECHANICAL SYSTEM 



it must lie inflexible in structure; for this reason its mechanical system 

 is not like that of an ordinary bilateral leaf-blade, but on the con- 

 trary resembles that of a hollow stem. The fibrous strands display a 

 centrifugal tendency ; in other words they are well developed beneath 

 the outer epidermis, whereas the inner half of the sheath is altogether 

 devoid of fibrous flanges, or at most provided with very feeble strands 

 of bast (Fig. 64). This instance once more clearly demonstrates the 

 fact that the disposition of fibrous strands is solely determined by 

 mechanical considerations. 



The same office which is performed by the leaf-sheaths of the 

 Gkamixeae and Cypekaceae pertains to the peculiar tubular spathe 

 that surrounds the apical portion of the scape of Armeria. Here also, 

 as Westermaier has convincingly demonstrated, everything points 

 to the conclusion that the spathe provides the necessary mechanical 

 support for an intercalary growing-zone. 



B. THE STRUCTURE OF INEXTENSIBLE ORGANS. 



All the organs that serve to attach a plant to its substratum, 

 whether the latter be the soil, the face of a rock, a tree trunk, a branch, 

 or other object, are frequently subjected to longitudinal tension. This 

 statement applies more particularly to roots, which may in fact be 

 regarded in general as " models of inextensibility." It has already 

 been explained that the most advantageous disposition of the mechani- 

 cally effective tissues, in organs which have to be inextensible, consists 

 in the concentration of the resistant elements into a single compact, 

 cable-like central strand. In all ordinary roots, accordingly, the 

 mechanical tissue combines with the conducting elements to form 

 an axile bundle. A number of different types of inextensible construc- 

 tion may be distinguished, according to the varying disposition of the 

 actual mechanical tissue in the composite central strand. In its 

 simplest form the axile strand consists of a central stereome-bundle sur- 

 rounded by several hadrome- and leptome-groups (lateral roots of 

 StarJiys sylvatica, Mentha aquatica and Hedera Helix). The axile strand 

 may on the other hand be produced externally into ridges of mechanical 

 tissue, which separate the leptome- and hadrome-bundles from one 

 another (lateral roots of most Grasses and Sedges). Sometimes the 

 stereome is restricted to strips corresponding to the ridges of the 

 preceding type, while the central area is occupied by a parenchymatous 

 pith (Smilax), which may contain additional isolated fibrous strands 

 (many Palms). The stereome may finally take the form of semi-lunar , 

 sheaths embracing the outer faces of leptome-bundles (Pimm, Phaseolus 

 and other Leguminosae). 



Those rhizomes which serve to fix the plant in the ground agree 



