178 MECHANICAL SYSTEM 



The disposition of the mechanically effective elements in the form 

 of a hollow cylinder or tube is the simplest, and hence also the most 

 primitive, arrangement of the skeletal system in inflexible organs. A 

 consideration of the mechanical arrangements of the Lower Plants 

 (Bryophyta and Thallophyta) brings this fact out very clearly. In 

 the erect stems and setae of Mosses the mechanical system takes the 

 form of a typical subepidermal hollow cylinder: on its inner side the 

 latter as a rule passes over quite gradually into the conducting 

 parenchyma, but occasionally the two regions are sharply differentiated 

 (stem of Meesia longiseta). 



(e) Fifth si/stem. Peripheral hollow cylinder or tube of stereome, 

 reinforced by isolated subepidermal girders. 



The haulms of ordinary Grasses are provided with a tubular 

 stereome, supplemented by isolated subepidermal girders, the flanges of 

 which project from the outer surface of the tube at more or less 

 regular intervals. This is evidently a particularly effective arrange- 

 ment, and as a matter of fact analogous types of construction find a 

 wide application in architectural and engineering practice. The mestome- 

 strands are mostly apposed to the inner side of the fibrous cylinder, 

 but may also run freely in the medullary tissue. Usually a small 

 peripheral mestome-bundle is embedded in the fibrous tissue just below 

 the insertion of each supplementary girder upon the cylinder. One of 

 the adjoining diagrams (Fig. 61 c) illustrates the type of bast-cylinder 

 reinforced by well-developed subepidermal flanges which is characteristic 

 of the Gramineae. 



2. Bilateral organs. 



The foliage-leaf is the most important of the bilaterally sym- 

 metrical organs of the plant-body ; in accordance with the nature of 

 the mechanical requirements of leaves, their girders are all arranged 

 parallel to one another, and at right angles to the surface. Schwen- 

 dener distinguishes between " subepidermal " [superficial], " internal " 

 and " mixed " girders, which differ in their relations to the epidermis, 

 and which serve to define the three principal systems of mechanical 

 construction employed in bilateral organs. 



In the simplest cases subepidermal I-girders extend from one face 

 of the leaf to the other. The leaves of Typha, Cordyline, Phormiutn 

 (Fig. 63 a), Pandanus and Musa, and the blades of Grasses and 

 Sedges exemplify this type of structure. The two flanges of each 

 girder are similar in shape and nearly equal in size ; the web consists 

 either of mestome in conjunction with parenchyma, or of mestome 

 alone. 



