MECHANICAL CONSTRUCTION OF PLANT-BODY 151 



spars of racing yachts, in wliich nitcrnal ties of metal fiave been used 

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

 tlie 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 c>-lindrical stem of a Bamboo, 

 or of a straw of wheat, n^ay be imagined as 

 corresponding to a series of crossed girders 

 (Fig. 108), with the straps or Ifanges 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 



Uiagraiii of crossed girders. See 

 Text. 



Fig. iolj. 

 Flowering stem of Astiantia ia trans\'erse Transverse section of an internode of a . 



section. ( ■ 10.) The coIlench\'ma is dotted. stem of Clci}t^tis, stiowing a ring of six 



larger and six smaller vasctilar strands, 

 surrounding the central pith, and covered 

 externally by the thick cortex, with six 

 projecting bands of coUenchyma- { ■ 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 nrore clearly in Clematis or Lamium, 

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

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

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



