290 THE BIOLOGY OF FLOWERING PLANTS 



by the ground tissue of the stem, may be regarded as two 

 crossed girders joined by a lattice work. If the stress acts 

 against a face it is opposed by two girders, if against a corner 

 by one. In the older stem this arrangement is reinforced 

 by secondary woody tissue which forms an internal cylinder, 

 or tube, of fibrous elements. The complete mechanical 

 system is thus composed of two crossed girders reinforced 

 by an internal tube. 



In the stem of Molinia coenilea, the purple heath grass, 

 a broad band of sclerenchyma runs completely round the 



Fig. 43. — Arrangementof mechanical tissues : i, cross-section of stem 

 of Lamium ; 2, cross-section of stem of Molinia ; the sclerenchyma is 

 shaded, the collenchyma is cross-hatched. 



Stem, outside the irregular circle of vascular bundles. From 

 the external surface of this band there spring a number of 

 ridges of sclerenchyma, which reach to the epiderm broaden- 

 ing outwards. On the inside a somewhat similar series is 

 formed by the vascular bundles, the sheaths of many of 

 which are fused with the sclerenchyma band. Here we 

 have a hollow cylinder reinforced outside, and, to a less 

 extent, inside, by T-shaped girders, the whole forming a 

 very strong type of construction. 



Apart from the arrangement of mechanical tissue the 



