THE PTERIDOI'IIYTA 



339 



Equisetineae or Horsetails. — This vory distinct class consists of 

 but one order, tlie Equisetales, and this of but the single genus 

 Equisetum, the horsetails or scouring rushes (Fig. 214). Some of 

 its species grow in dry sterile soil and others in marshy situa- 

 tions. From a perennial rootstock arise the very characteristic 

 stems, which are jointed, ridged, and hollow. Leaves are repre- 



Fio. 213. — Isoetes. A, general appearance of the plant. B, a megaspore. 

 C, a microspore. 



sented merely by a circle of scales which surround the stem at 

 each joint or node, the green stems carrying on most of the pro- 

 cess of photosynthesis. Aside from the large central air space, 

 a smaller one occurs just inside each furrow in the stem (Fig. 215). 

 Opposite each ridge is a small and very poorly developed fibro- 

 vascular bundle, the stoutness and strength of the stem being due 

 chiefly to the layers of heavy-walled sclcrenchyma which it 



