RESISTANCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, AND BENDING. 



731 



projections from the bast tube, e.g. in the grass Molinia ccerulea (fig. 179*), or as 

 independent collenchymatous strands in the comers of the angular stem, as in the 

 Woodruff {Asperula odorata, fig. 179^), or, again, a circle of independent bundles of 

 hard bast appears outside the bast tube, as in the stately umbellifer Euryangiiim 

 Swmbul (fig. 179 "^j. In this plant the .strengthening accessories are combined into 

 independent simple girders and a canal filled with air is situated on the inner side 

 of each of the flanges (cf. fig. 179"). 



The tliird group consists of all stems in which the flanges are developed as 



Fig. 180. — Transverse sections of erect foliage-stems with flanges developed as secondary girders. 



1 Tufted ScirpUB (Scifpus C(vspUosus). - Perfoliate .Silpliium (Silphium perfoUaUtm). ^ Blaclc-stemniod Bamboo (liambusa 

 nujra). * Hard Kush (Juncus gtaucus). ^ Common Reed (Phragmites commiuii.n'). 6 .Sugar-cane {Sacckarum oßiclnarum). 

 lu these diagrammatic figures the mechanical tissue is represented grey, and the vascular bundles black witil white spots. 



secondary girders. The web in these secondary girders always consists of vascular 

 bundles, and the flanges themselves of hard bast. Sometimes the secondary girders 

 are arranged in a single circle, but in most instances they form several concentric 

 rings. In fig. 180 some of the most striking forms of this group are given diagram- 

 matically. Fig. 180 ^ represents a transverse section of a stem of Scirpus cces- 

 pitosihs, in which the secondary girders — arranged in a single circle — alternate with 

 large air-spaces; fig. 180" shows a similar section of the stem of the composite illus- 

 trated on p. 239 {Silphiuvi perfoliatum), with its numerous series of secondary 

 girders parallel to tlie four sides; and fig. 180^ is the transvei'se section of a bamboo 

 {Bambusa nigra) in which the secondary girders are grouped in several concentric 

 rings. Here, as in the first and second groups, accessory structures are present, 



