296 BOTANY 



sheaths, in whose axils are formed buds, which may later develop 

 into aerial shoots, or may remain undeveloped and give rise to the 

 roots. Not infrequently some of the buds develop into thickened 

 tubers, with hard outer tissues, and the inner cells filled with starch 

 (Fig. 262). A section of an internode of the rhizome shows a large 

 central cavity, and a circle of smaller ones corresponding to the fur- 

 rows upon the surface of the internode. Alternating with these are 

 the small vascular bundles. 



Some of the buds at the nodes develop into the aerial shoots. 

 These may be all alike e.g. E. robustum, E. hiemale ; or there may 

 be special sporogenous shoots, as in E. telmateia (Fig. 262), which 

 are followed by very much branched, green, sterile ones. The 

 sporogenous shoots, in such cases, die as soon as the spores are 

 scattered. 



The internodes are strongly furrowed, and the nodes concealed by 

 the whorls of rudimentary leaves, which form the characteristic 

 toothed sheaths. The number of leaves in a whorl ranges from three 

 in E. scirpoides to forty or more in some of the larger species. The 

 leaves are almost entirely destitute of chlorophyll, and are exclu- 

 sively protective in function. In size the aerial shoots range from 

 about twenty to thirty centimetres (E. scirpoides) to ten metres in E. 

 giganteum, which has a slender stem, about two centimetres in diame- 

 ter, supported by the bushes and trees among which it grows. The 

 shoots may be quite unbranched, or whorls of branches corresponding 

 to the number of leaves may be formed about the internodes, as in the 

 sterile shoots of E. telmateia. The epidermis in all species is character- 

 ized by the presence of large amounts of silica, which renders the 

 surface rough, as in the common " Scouring-rush," E. hiemale. The 

 aerial shoots are, as a rule, much thicker than the rhizome, and there 

 is a corresponding increase in the number of leaves at the nodes, and 

 in the vascular bundles and lacunae in the section. 



Apical Growth. The apex of the growing shoot is terminated by a very 

 large tetrahedral apical cell whose divisions are extremely regular (Fig. 263). 

 The first division-wall in each segment is parallel to the lateral face of the api- 

 cal cell, so that there are formed two superimposed semisegmente, each of which 

 next divides by nearly radial walls, and in cross-section each series of segments 

 shows six cells arranged like the sextants of a circle. Of the two superimposed 

 sets of sextant cells, the upper series gives rise to the nodes, the lower to the in- 

 ternodes. Early periclinal divisions in the young segments separate a central 

 cylinder of tissue, the pith, from the outer cortical region. The pith becomes 

 destroyed in the internodes by subsequent tearing apart of the tissue, leaving the 

 large central cavity found in most species. The central tissue of the nodes re- 

 mains intact, and there is thus formed a series of diaphragms between the cavi- 

 ties of the internodes. 



The leaf-sheaths arise as annular outgrowths of the nodes. The initial cells of 

 the separate teeth arise at regular intervals from the margin of the young sheath. 

 These initial cells divide rapidly by alternate dorsal and ventral walls, and the 





