PTERIDOPHYTA 



251 



1 m. tall. The sporophyte of Eqiiisetum has a horizontal branching rhi- 

 zome with whorled leaves at the nodes. It gives rise to erect green shoots 

 that may be either simple or monopodially branched, the branches, like 

 the leaves, arising in whorls (Fig. 208). In some species the shoots 



Fig. 208. Erect shoots of Equisetum hyemale. 

 bear whorls of scale-like leaves at the nodes. 



The stems are green, unbranched, and 



branch repeatedly. The stems are longitudinally ridged and grooved, the 

 ridges of one internode alternating with those of the internode immedi- 

 ately above and below. The stems are more or less impregnated with 

 silica, giving them a rough, harsh feel. Roots occur on the rhizome and at 

 the base of the erect stems. They arise in whorls at the nodes. 



The nodes are soUd but the internodes have a large central cavity. 

 The aerial stems carry on practically all the work of photosynthesis. 

 The leaves are scale-like, their tips being free but their bases united to 



