196 DERMAL AND OTHER NON-VASCULAR TISSUES [ch. 



the lateral walls. Since the slope of the pits is in the same direction in all 

 the cells those of adjoining cells appear to cross, giving the characteristic 

 marking of such tissues (Fig. 183). 



The distribution of the sclerotic tissues in Ferns appears to be ruled 

 according to two distinct functions, mechanical resistance and water-reserve. 

 It is difficult to dissociate these in individual cases, and they probably overlap. 

 A mechanical use clearly lies with the sclerotic band, often pale-coloured, 

 which is found close to the periphery of leaf-stalks, giving the "stramineous" 

 appearance of systematists (Figs. 156, 161). It is well seen in Pteridium, 



Fig. 183. Pteridimn aqitilinum. A, half 

 of a brown-walled sclerenchyma fibre 

 from the stem : B, part of one of these 

 more highly magnified (x 550):/ = one 

 of the slit-like pits seen in section : C, 

 transverse section : a, limiting lamel- 

 la : b, c, inner layers of the wall. 

 (From De Bary. After Sachs.) 



Fig. 184. Outer sclerotic sheath which invests 

 the dictyostele of Dicksonia, with the edges 

 of the foliar gaps turned outwards, thus giving 

 added rigidity to the structure. Reduced. 

 (Compare Figs. 150, 151.) 



and in Dryopteris Filix-mas. The outer stony covering of the rhizome of 

 Pteridimn, or of Plagiogyria, is probably effective in meeting soil-pressures 

 (Figs. 3, 138). The thick and hard outer cylinder of the stems of Tree Ferns 

 certainly gives great strength. Their internally-lying masses of sclerenchyma 

 are also mechanically effective. No one can examine the form of the sclerotic 

 sheath surrounding the dictyostele of a Tree Fern, and note the outward- 

 curved lips round each foliar gap, without realising that it is constructed on 

 the principle of a corrugated metal sheet, thus gaining great mechanical 

 effect with little cost of material (Fig. 1 84). Lastly, in the leaves the veins are 



