ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. 



675 



-<xtM to the strength and avoid breakage by transverse strains, as may be 

 seen in a straw and in the stem of the Dead-nettle. 



Secondary girders are not uncommon in stems, and are imitated by 

 such a structure as is illustrated in fig. 284, in which it will be seen that 

 the flanges are themselves formed of secondary girders. This may be 

 <jompared with fig. 286, in which, in addition to such secondary girders 



Fig. 284.— Four combined girders ; 

 their flanges are formed of second- 

 ary girders. (After Kerner and 

 Oliver.) 



JFiG. 285. — Trans, sect, of stem of 

 Crow Garlic {AlFiuDi vineale). The 

 pericycle has formed a cylinder of 

 supportive tissue. (Afier Kerner 

 and Oliver.) 



;arranged around a hollow centre, there is a band of sti»engthening tissue 

 on the circumference. 



In fig. 274, representing a section of the stem of the Dead-nettle, we 

 ■see two pairs of collenchymatous girders at the corners and another pair at 

 the sides, and, in addition, four pairs made by the fibro-vascular cords. 



In monocotyledonous stems a solid cylinder is made by means of the 

 ;active layer of tissue known as the "pericycle," situated just below the 

 cortical region. Besides forming a rigid circular band in the ground- 

 tissue, several fibro-vascular cords may be developed within it, outside the 

 primary ones. Fig. 285 represents such a condition in the stem of All mm 

 L'ineale. 



Fig. 286 IS a section of the centre of the Common Reed {Plivacjmitcs 



Fig. 286.— Trans, sect, of stem of 

 Common Eeed {Plwagmites com- 

 munis), Cp. Fig. 284. The funda- 

 mental tissue has formed a cylinder 

 of supportive tissue. (After Kerner 

 and Oliver.) 



F'iG. 287.— Trans, sect, of stem of 

 Purple Molinia {Molinia cicridta). 

 The pericycle has formed a cylinder 

 of supportive tissue, connected 

 with the circumference by radial 

 supportive bars. (After Kerner 

 and Oliver.) 



'comimmis), in which, besides the circumferential cylinder formed in the 

 i^round tissue, each fibro-vascular cord is a girder by itself, the vessels 

 constituting the web, and the more solid tissue at the two ends constituting 

 the flanges. 



In fig. 287, exhibiting a section of the culm of the grass Molinia 

 ■cczrulea, besides the pericycular strengthening cylinder (represented 



JE 2 



