MECHANICAL CONSTRUCTION OF PLANT-BODY l8l 



lower epidermis are themselves sclerotn . These bands represent the 

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

 strand represents the con- 

 necting 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 sub- 

 ject to great variety with 

 the varying form of the 

 plant-body : 



(a) Columnar require- 

 ment, as in an up- 

 right 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. 



Fig. 113. 

 Transverse section through a leaf of Cyperus, showing a 

 vascular strand with a strand of resistant sclerenchyma 

 above and below, constituting a girder. ( x 300.) F. O. B. 



(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-parenchyma, and 

 medullary rays,— contribute in varying degree to the mechanical ettect. 

 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 



