384 



PART IV. CLASSIFICATION. 



there is a considerable development of assimilatory tissue in the cortex, 

 strands of this tissue corresponding in position with the furrows on the 

 surface in which the stomata are developed ; whilst opposite the ridges on 

 the surface there are cortical strands of sclerenchyma. The development 

 of assimilatory tissue in the shoots and branches is correlated with the 

 absence of foliage-leaves, the functions of foliage-leaves having therefore 

 to be discharged by the shoots and branches. The epidermal cell-walls 

 are impregnated with silica. 



Within the cortex, and almost abutting upon the large central cavity, 

 is a ring of distinct vascular bundles which run down the internode from 



-End. 



FIG. 230. Portions of transverse sections of stems of species of Equisetum (after 

 Pfltzer: x 36). S rhizome of E. litorale ; C rhizome of E. sihaticum ; A aerial stem of E. 

 palustre, in which the structure is the same as in C, but the markings of the internal en- 

 dodermal layer are not developed; a central cavity ; 6 vallecular cavities in the cortex ; c 

 carinal cavities in the vascular bundles ; JGnd. endodermis. 



the leaves at the node. Each bundle is collateral, closed and common, 

 with very rudimentary xylem consisting of the few annular vessels of the 

 protoxylem and of two small groups of scalariform tracheids. 



The root grows in length by means of a tetrahedral apical cell ; in its 

 mode of growth, development of root-cap, etc., it essentially resembles 

 that of the Ferns. Its structure is rather peculiar : the vascular 

 cylinder consists (usually) of three wood-bundles and three bast-bundles, 

 and is invested by two layers of sheathing-cells, the outer of which has 

 the characteristic marks of an endodermis, whilst the inner appears to be 

 a pericycle and gives rise to the growing-points of the lateral roots ; how- 

 ever, the inner layer is, as a matter of fact, not a pericycle, but the inner- 



