RESISTANCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, AND BENDING. 729 



First we will give a general idea of the distribution of mechanical tissue, in as 

 far as it enables erect stems to resist bending. We can distinguish three groups of 

 forms in this respect. The first group includes forms with simple girders whose 

 flanges of hard bast are placed as near the periphery as possible, but are not fused 

 together into a cylindrical tube. The line connecting every pair of flanges passes 

 through the axis of the stem. To this group belong almost all young stems of 

 woody plants, e.g. those of willows, oaks, elms, maples, and limes (c/. fig. 178 1). 

 Special emphasis must be laid on the words "young stems", since in the older stems 

 of these trees — when the wood has become thickened — the hard bast on the outer 

 side of the cambium-ring, and therefore outside the vascular bundle, has finished its 

 task, and its functions are transferred to the wood, more especially to the woody 

 fibres (libriform cells) (c/. p. 726). 



In the erect stems of undershrubs belonging to this group the simple girders are 



10 



®\ 



\^gg^ 



(gy j/ 



Fig. 178. — Transverse sections of erect foliage-stems with simple girders not fused together into a tube. 



1 One-year-old branch of the Broad-leaved Lime {Tilia grandifoUa). 2 White Dead-nettle {Lamium album), s Date Palm 

 (Phoenix daciylyfera). In these diagrammatic figures the mechanical tissue is grey and the vascular bundles black with 

 white spots. 



very often assisted by collenchymatous strands which lie close to the periphery of 

 the stem, and are arranged so that each strand appears to strengthen the bundle 

 of hard bast forming a flange. Fig. 178 ^ shows a transverse section of a stem of 

 an undershrub belonging to this group, the White Dead-nettle {Lamium album), 

 in which the further peculiarity is noticeable, that the strengthening, collenchy- 

 matous strands in the comers of the four-sided stem are thick and pillar-like, 

 while those at the sides of the stem are broad and flattened. This condition is not 

 an uncommon one. In palms, of which the diagrammatic cross section of the Date 

 Palm (Phcenix dactylifera, flg. 178 2) may serve as a type, the strands accessory to 

 the simple girders are in the form of numerous bundles of hard bast developed on 

 the periphery of the stem, but not exactly in front of the flanges of the girders. 

 These bundles of hard bast are always in pairs opposite one another, and may 

 be regarded as the flanges of special girders. In these cases the number of girders 

 is always very large, and the flanges appear in two, three, or even more circles in a 

 cross section of the stem. Sometimes also two or three adjoining flanges are fused 



