MECHANICAL CONSTRUCTION OF PLANT-BODY ,149 



illustrated by the use of girders, or simple combinations of them, in 

 the construction of bridges, floors, and shop-fronts. More complicated 

 arrangements giving columnar construction are seen in lattice-signal- 

 posts, and large gasometer-frames. The latter offer close analogies 

 with certain types of stem-construction in plants. 



In plants girder-construction depends upon differences of mechanical 

 resistance of tissues. An illustration of a simple case is given in Fig. 106 

 from the leaf of Cyperus. On either side of the vascular strand there 

 is a band of thick-walled resistant woody sclerenchyma. Each is close 

 to the upper and lower surface respectively, indeed three cells of the 

 lower epidermis are themselves sclerotic. These bands represent the 

 upper and lower straps of the girder, while the less resistant vascular 

 strand represents the connecting plate. The girder is kept in place 

 by the softer tissues, but especially by the firm layers of epidermis. 



The requirements that are most commonly illustrated in plants fall 

 under three heads, which are typified by parts of normally growing 

 plants, though they are subject to great variety with the varying 

 form of the plant-body : 



(a) Columnar requirement, as in an upright stem. 



(b) Stiffening of flattened surfaces, as in the leaf-blade, and pro- 



tection of the margins against tearing. 



(c) Rope-requirement, as in roots of upright plants exposed to 



wind. 



(a) THE COLUMNAR REQUIREMENT. 



The columnar requirement, for support of the growing dead-weight 

 of branches, leaves, and fruits, is met in large Dicotyledons chiefly 

 by the woody column, which grows in proportion to the growing need 

 for support. It is cylindrical, so as to meet all winds equally. It is 

 composed of mixed xylem-tissues, with continuous woody walls. 

 The most important of these mechanically are the wood-fibres, and 

 the resistant quality of the wood is roughly in proportion to their 

 preponderance. But all the tissues, vessels, wood-parencyhma, and 

 medullary rays, contribute in varying degree to the mechanical effect. 

 This is enhanced as the central tissues die, and are converted into 

 heart-wood. The method of construction is that of the solid column, 

 now acknowledged by engineers to be not the most economical of 

 material. But in Dicotyledons the retention of this rather primitive 

 method is to be explained as a compromise, the success of which is 

 evidence of its fitness to meet the circumstances. For the living part 



