SECT. ix. EQUISETUM. 369 



The Equisetacese differ from the Ferns in having 

 spiral instead of scalariform vessels in their structure ; 

 but they agree in having only one period of fertilization, 

 after which they produce a crop of spores year after year, 

 and these, when sown, give rise to the primary leaf with 

 its antheridia and archegonia a perpetual cycle. 



The whole substance of the stems of the Equisetums 

 is so completely penetrated with silex, that a silicious 

 skeleton remains after the herbaceous part is destroyed, 

 and in some species as much as thirteen per cent, of the 

 whole plant, and fifty per cent, of its ashes are found to 

 be pure silex. On account of the crystallization of the 

 silex, the Horsetails form some of the most beautiful of 

 microscopic objects. If a fragment of the cuticle is 

 magnified and viewed by polarized light, the colours 

 are seen to be intense and vivid, and the arrangement 

 of the silicious particles so elaborate and symmetrical 

 as to resemble necklaces of diamonds and coloured gems. 

 The rows of crystals run in lines parallel to the axis of 

 the stem, the greater number being so disposed, but the 

 rest are grouped so as to form ovals joined together by 

 a chain of particles forming a sort of curvilinear quad- 

 rangle, these rows of oval combinations being arranged 

 in pairs. The effect is brilliant when they are seen in 

 polarized light. According to Sir David Brewster every 

 particle has a regular axis of double refraction; but 

 Professor Bailey, of the United States of America, states 

 that the effect under polarized light is not produced by 

 the silicious particles, but by the organized tissues, 

 since, after these have been destroyed, the silex shows 

 no double refraction. 



The vascular tissue of the Equisetums shows them to 

 be of a higher class than the Ferns. The recent forms 

 vary much in size, but even the Equisetum giganteum of 

 Brazil, which is eighteen feet high and three-quarters 



VOL. I. B B 



