SECT. ix. EQUISETACE&. 367 



SECTION IX. 



EQUISETACE^E, OR HORSETAILS. 



THE Equiseta are leafless, herbaceous plants, annually re- 

 newed from a creeping rhizome, and growing in marshy 

 land, in pools and ditches, on the banks of rivulets, and 

 in rivers, from Lapland and Siberia to within the tropics. 

 There is but one genus, and few species. The largest 

 of the ten or eleven species, which are indigenous in 

 Great Britain, is not more than five or six feet high, but 

 they are of greater size in warm climates. The Horse- 

 tails begin their lives precisely like the Ferns ; for, when 

 a spore begins to germinate, it forms a marchantioid leaf 

 or prothallus lying flat on the ground, upon which are 

 produced antheridia, full of cells, in each of which there 

 is a spermatozoid with numerous cilia. Archegonia 

 are also formed on the prothallus. These, after fertili- 

 zation, give rise to the perfect plant, which throws out 

 a rhizome, whence new shoots are produced. ' The 

 structure of the rhizome is very different from that of 

 ferns. In an early stage it consists of a central column 

 of cellular tissue, sending off about eight radiating 

 plates, which connect it with an external cylinder of 

 the same tissue, and opposite to each of which there is, 

 in the central column, a vascular bundle consisting of 

 annular vessels passing into spiral. At a later period, 

 tissue grows from the walls into the cavities, in such 

 wise that they are more or less perfectly obliterated. 5 2 



2 Berkeley's ' Cryptogamic Botany.' 



