PTERIDOPHYTA 



299 



appears as a series of separate masses of cells, separated by groups of thick- 

 walled sclerenchyma, continuous with the hypoderma. This sclerenchyma 

 constitutes the chief mechanical tissue (" Stereoine ") of the shoot, and is es- 

 pecially developed in the longitudinal ridges so conspicuous upon the surface of 

 the shoot. The green tissue lies beneath the furrows between the ridges, and 

 communicates with the stoinata, which are arranged in vertical rows in the fur- 

 rows. In the slender lateral branches the amount of green tissue is relatively 

 much greater. 



The epidermal cells are heavily incrusted with silica, which usually forms con- 

 spicuous tubercles upon its surface. Upon the inner surface the guard cells of 

 the peculiar stoinata have conspicuous transverse bars. The stomata themselves 

 are usually sunk below the level of the other epidermal cells, and completely 

 covered by two accessory cells of much the same form as the true guard cells. 



FIG. 264. Equisetum telmateia. Vascular bundle from a sterile shoot (x75); 

 i, i, lacunae; x, x, tannin-cells ; t, remains of the primary tracheae ; en, endodermis. 



The branches arise as axillary buds, one corresponding to each 

 tooth of a foliar sheath. The bud originates from a single superficial 

 cell of the stem, in which intersecting walls cut out the characteristic 

 tetrahedral apical cell. At an early period, the inner surface of the 

 leaf-sheath, above the bud, becomes grown to the surface of the stem, 

 and the young bud is thus enclosed in a cavity, and has to break 

 through the base of the foliar sheath, so that it looks as if, like the 

 roots, it arose endogenously. The buds often remain undeveloped, 

 but may be stimulated into growth under suitable conditions. Occa- 

 sionally e.g. E. sylvaticum the branches may give rise to secondary 

 branchlets. 



