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JOUKNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



stem, as they would contribute almost nothing to the resistance to flexion." 

 For if a bough be bent sufficiently it will begin to break at the circum- 

 ference on the convex side, but be contracted on the concave side ; while 

 but little strain will be felt in the middle of the stem. Consequently, 

 Nature places the woody bundles in a circle ncai' the circumference, the 

 centre being occupied by soft tissue. This arrangement supplies a com- 

 bination of girders representing the "beam" or "web," which usually 



Fig. 280. — Trans, sect, of a branch 

 (one year old) of Lime tree, with 

 six fibro-vascular bundles. (After 

 Kerner and Oliver.) 



Fig. 281. — Two combined " girders," 

 arranged crosswise ; the dotted 

 lines indicate the " web," the 

 shaded parts the " flanges." (After 

 Kerner and Oliver.) 



consists of a flat piece of iron having " flanges " or cross-pieces at its ends. 

 They form as it were two T's united by their bases (or rather the letter I). 



Each fibro-vascular cord constitutes a flange, and the combined 

 webs are formed of cellular tissue, constituting the pith. (Fig. 280.) 

 The accompanying illustrations, taken from Kerner and Oliver's 

 *' Natural History of Plants," will show some combinations of girders and 

 how hey imitate the structure of stems. 



Fig. 281 represents two combined girders arranged crosswise ; the dotted 

 lines represent their webs. This may be compared with the strengthen- 

 ing collenchymatous tissue in the four corners of the stem of the Dead- 



Fig. 282. — Three combined girders. 

 (After Kerner and Oliver.) 



Fig. 283. — Six combined girders, now 

 forming a cylindrical tube by 

 mutual pressure and union. (After 

 Kerner and Oliver.) 



nettle (fig. 274). Fig. 282 shows three combined girders, a common 

 appearance in many pedicels of the flowers of Monocotyledons. 



In fig. 283 we arrive at a complete cylinder formed of six combined 

 girders, their flanges being laterally in contact and coherent. Great 

 strength is thus gained by their mutual pressure, so that no web is needed 

 and the cylinder may be hollow. Of course this is one of the commonest 

 appearances in stems, which may or may not retain the pith. Such 

 hollow stems almost invariably develop transverse plates at the nodes, to 



