DEVELOPMENT OF PLANTS 321 



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 Equisekim 

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



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

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

 inside of which are shown three bundles. 



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 

 walls and stomata can be obtained by treating the tissue as noted 

 in the diatoms. The arrangement of the leaves and branches and 

 the distribution of the vascular bundles and other tissues presents 

 a mathematical regularity unexcelled in any plant. As in the 

 Bryophyta, the elongation of the stem is effected by a single 

 apical cell which cuts off with extreme regularity the cells from 



