DEVELOPMENT OF PLANTS 305 



produce a bushy symmetry that caused the name of Equisctiun 

 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 

 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 

 which the various tissues and organs of the plant are formed. 

 The vascular bundles, which are of the collateral type noted in 



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

 character and stereome confined to the ridges : a, air spaces ; c, endo- 

 dermis, inside of which are shown three collateral bundles. 



the Ophioglossales and Osmiuida, are arranged around a large 

 pith ( Fig. 233 ) . They rarely show secondary growth through 

 the activities of a cambium, although this was a marked feature 

 of many of the extinct species. These bundles lie directly below 



