MECHANICAL CONSTRUCTION OF PLANT-BODY 151 



spars of racing yachts, in which internal ties of 'metal have been used 

 to meet that risk. In the haulms of Grasses, and conspicuously in 

 the large Bamboos, hard woody septa at the level of the leaf-insertions 

 serve the same purpose (Fig. 107). With- 

 out these it would not be possible for 

 hollow stems to uphold the huge head of 

 leaves one hundred feet above ground 

 against all winds, as the Giant Bamboos 

 do. 



The hollow cylindrical stem of a Bamboo, 

 or of a straw of wheat, may be imagined as 

 corresponding to a series of crossed girders 

 (Fig. 1 08), with the straps or flanges all fused 

 laterally, so as to form a firm peripheral 

 band. The effective material is placed as 

 far as possible from the centre. The straps 

 being fused laterally, the connecting plate or web of the individual girders 

 can be dispensed with, and the stem is accordingly hollow. But in many 

 cases, and especially in young stems of Dicotyledons, the relation of the 



FIG. 108. 



Diagram of crossed girders. 

 Text. 



See 



FIG. 109. 



I 'lowering stein of Astrantia in transverse 

 section. ( x 10.) The collenchyma is dotted. 



Transverse section of an internode of a 

 stem of Clematis, showing a ring of six 

 larger and six smaller vascular strands, 

 surrounding the central pith, and covered 

 externally by the thick cortex, with six 

 projecting bands of collenchyma. ( x 15.) 



structure to girder-construction is more plainly seen, since in them the straps 

 of mechanical tissue are not fused laterally. This gives reality to the con- 

 ception. Thus, in the stem of Astrantia, or more clearly in Clematis or Lamium, 

 the bands of mechanical tissue are isolated, and alternate with softer tissues 

 which keep them in their position (Figs. 109). In Astrantia, the resolution 

 of the whole arrangement into a girder-construction is less obvious because 



