GROUP III. PTERIDOPHYTA : EQUISETTN^. 



419 



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. 



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 ; however, the inner layer is, as a matter of 

 fact, not a pericycle, but the innermost layer of the cortex, the endodermis 



FIG. 273. Portions of transverse sections of stems of species of Equisetum, illustrating 

 schizostely (after Pfitzer: x36). B Typical schieostelic rhizome of K litorale ; C schizos- 

 telic gamodesmic rhizome of E. silvaticum ; A aerial stem of E. palustre, in which the 

 structure is the same as in C, but the markings of the internal endodermal layer are not 

 developed, so that the stem appears to be monostelic ; a central cavity; 6 vallecular 

 cavities in the cortex ; c carinal cavities in the schizosteles ; End. endodermis. 



being the last layer but one of the cortex. Hence it appears that here, as in 

 the Ferns, the lateral roots spring from the innermost layer of the cortex. 

 There is no pericycle in the root of Equisetum. 



Embryogeny of the Sporophyte. The oospore is divided by a transverse basal 

 wall, and then becomes segmented into octants, as in the Filicinte. Of the four 

 epibasal octants, one gives rise to the growing -point of the primary stem ; two 

 to the first cotyledon ; and the fourth to the second cotyledon : the two cotyle- 

 dons cohere to form a leaf -sheath round the young primary stem. Of the four 



