336 STRUCTURE OF EQUISETUM 



and take little or no part in photosynthesis. These scale leaves 

 are arranged with great regularity- at the nodes of the stem, and 

 owing to their close association they grow together, forming a 

 papery sheath with teeth-like points about the stem (Fig. 232, /). 

 It is noteworthy that in allied fossil forms large chlorophyll- 

 bearing leaves occurred. 



The stems are of two kinds, subterranean rhizomes that branch 



FIG. 233. Cross-section of a portion of the stem showing its grooved 

 character and stereome conned to the ridges: a, air spaces; e, endodermis, 

 inside of which are shown three bundles. 



extensively through the soil, and aerial stems that arise as 

 branches from the rhizomes. The aerial stem is simple or 

 branched and is characterized by nodes made conspicuous by the 

 sheathing teeth-like leaves and strongly furrowed internodes. In 

 such species as branch, these organs originate with great regular- 

 ity in the axils of the leaves and perforating the sheathing leaves 

 produce a bushy symmetry that caused the name of Equisetmn 

 or horsetail to be applied to these plants. The epidermal cells 

 are hard and rough, owing to the abundant deposit of silica in 

 the cell walls. For this reason certain species were used in early 

 times for scouring purposes and so they became popularly known 

 as scouring rushes. Complete silicious casts of the epidermal 



