STEM OF EQUISETUM. 181 



mine. The stability of the slender stem is secured by the 

 stereome, in the form of the chief sclerenchyma strings, being 

 removed as far as possible outwards into the ridges of the stem, 

 while they are aided by secondary ones which occupy the furrows, 

 and not merely stiffen the epidermis here against " buckling," but 

 protect the underlying vallecular passages. The principal scler- 

 enchyma strings cover the assimilating tissue, which reaches the 

 surface at the (mechanically) most protected places, viz., on the 

 flanks of the ridges. We have here, therefore, an admirable 

 compromise between the antagonistic needs of the assimilating 

 and mechanical system ; the former of which should be near the 

 surface, so as to expose its elements to light and air, while the 

 latter to secure against flexure should be placed as peripherally 

 as possible. The endodermis for its part makes the central 

 cylinder air-tight against the air-containing tissue of the cortex ; 

 the cuticularised bands on the radial walls particularly aiding this, 

 and preventing the formation of intercellular spaces. 



Examination of the exterior of the stem shows that on succes- 

 sive internodes the ridges alternate with one another ; hence the 

 sclerenchyma strings also change their position at the nodes, 

 being connected together by branches. Considered, therefore, 

 as a whole this external mechanical system of the stem con- 

 stitutes a lattice- work of vertical girders with very short tie-rods, 

 and is primarily intended to resist flexure. 



The structure of the roots of Pteridophytes is, in essentials, 

 like that of Phanerogams. The periphery of the central cylinder 

 of the root agrees, however, with that of the fern stele w r hich we 

 have studied, in that it is covered by a double layer of Phloeo- 

 terma, of which the inner layer constitutes the pericycle, and the 

 outer, the endodermis. 



