VII] BARK 95 



extent in the Sycamore (Acer Pseudo-Platamts). The 

 successive periderms commence to form at various points 

 on the stem, deep in the cortex, and their development 

 proceeds centrifugally from each point but never extends 

 far round the stem. The result is a thin, flattened or 

 slightly concave, more or less circular patch of bark, and 

 as the periderm comes to the surface at its margins this 

 tabular patch is cut out and scales off as it dries and as 

 the thicker branch beneath tears its edges. The denuded 

 patches of the current runner are greenish or olive, older 

 ones grey or brown, and they give a very characteristic 

 appearance to the stem. 



Other forms of scaly or tabular bark are furnished by 

 the Scots Pine, Spruce, Silver Fir, Horse-chestnut, &c. 



But the commonest type of bark on older stems is that 

 known as fissured or creviced, where longitudinal cracks 

 or clefts go deep into the substance, and the inter- 

 vening parts stand up as ridges. The development of 

 these characteristic crevices usually depends on the 

 presence of fibrous tissues in the bark ; as the addition 

 of thickening layers to the wood beneath distends the 

 tegumentary tissues, these fibrous masses are pulled 

 asunder by the tangential strains, and carry with them 

 the other tissues of which the bark is composed : then 

 new masses are cut off beneath by the deeper periderms, 

 to undergo the same tangential stretching and partial 

 separation, and as the bark thus thickens the ridges are 

 added to below and their prominence increased. Further 

 tangential contraction of the ridges, resulting in cor- 

 responding increase of the gaping of the crevices between, 

 is brought about by the drying up of the dead bark- 

 tissues. 



Such fissured barks are well illustrated by the Ash, 

 Robinia, Norway Maple,Elm, Black Poplar, Oak (Fig.46),&c. 



