MECHANICS 



87 



The functions of the stem arc manifold, but among the most 

 important are those of bearing leaves and flowers, and of con- 

 stituting a connecting-Unk between the root-s^-stem and the 

 fohage. The shoot of the ordinary erect plant is most hable to 

 bending, under the influence of the wind, etc., and thus contrasts 

 with the root, which is exposed chiefly to pxiUing strains. Re- 

 lated to this the mechanical elements of the stem are more or 

 less symmetrically arranged near the periphery. In the young 

 stem the upright position is maintained by the frequent sub- 

 epidermal collenchjmia (Figs. 15, 34, and 37), by the xylem of 



r'iG. 41. — Stem of the Black Bryony (Tamils communis). A, Diagram of 

 transverse section. B, Photograph of a small portion on a larger 

 scale, p., phloem ; Scl., sclerenchyma-ring ; ,r. , metaxylem vessel. 



the bundles, and by the turgidity of the living cells combined 

 Mith tissue-tensions. 1 As the stem matures, additional mechani- 

 cal tissue is often furnished by the development of pericycUc 

 fibres ((?.o'. Sunflower, Fig. 35, /".), of a thick-walled sheath to the 

 bundles (Monocotjdedons, Fig. 40, A, Buttercup), or of a zone 

 of sclerenchjTna in the cortex [e.g. Vegetable Marrow, Fig. ^j, 

 Scl.). Even in the ordinary Monocotvledonous stem the bundles 

 are far more numerous near the edge (cf. Fig. 39); moreover, Mono- 

 cotyledons frequently exhibit a copious development of peri- 

 pheral sclerenchjTna, to which, for instance, the hardness of a 



1 Cf. F. & S., p. 103. 



