MECHANICS 87 



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

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

 stituting a connecting-link between the root-system and the 

 foliage. The shoot of the ordinary erect plant is most liable to 

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

 with the root, which is exposed chiefly to pulling 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 collenchyma (Figs. 15, 34, and 37), by the xylem of 



Ej.s. 



FIG. 41. Stem of the Black Bryony (Tamus communis). A, Diagram of 

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

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



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

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

 cal tissue is often furnished by the development of pericyclic 

 fibres (e.g. Sunflower, Fig. 35, /.), of a thick-walled sheath to the 

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

 of sclerenchyma in the cortex (e.g. Vegetable Marrow, Fig. 37. 

 Scl.). Even in the ordinary Monocotyledonous stent the bundles 

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

 cotyledons frequently exhibit a copious development of peri- 

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



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



