206 BRITISH DESMIDIACE.E. 



cell begins actively to secrete mucilage through the special 

 pore organs of its cell-wall, by which means the thin layer of 

 lime is ruptured, and the cell is lifted bodily in the mucilage out 

 of the stony mass. In this way the tiny cylinders of lime are 

 continually increasing in length, and the cells, by secreting 

 mucilage, are raised higher and higher at the same time. 



Cell-division proceeds as in ordinary Desmids, and the two 

 daughter-cells secrete, when fully developed, their own gela- 

 tinous stalks, which in time become encrusted with lime. Thus 

 as the cells increase in number the lime cylinders also increase in 

 number, so that there is always a single cell at the top of one lime 

 tube. A layer of lime as much as '5 cm. thick may be deposited 

 in a single year. 



Genus 21. SPHJEROZOSMA Corda, 1835. 



Corda in Aim. de Carlsbad, 1835, p. 207 ; ibid. 1840, p. 205. 



Ralfs in Ann. Mag. Nat. Hist, 1845, vol. 16, p. 13. 



Hass. Brit. Freshw. Alg. 1845, p. 348. 



Ralfs, Brit, Desm. 1848. p. 65. 



De Bary, Conj. 1858. p. 76. 



Arch, in Pritch. Inf. 1861, p. 723. 



De Not, Desm. Ital. 1867, p. 20. 



Rabenh. Flor. Europ. Alg. p. 148. 



Delp. Spec. Desm. subalp. 1873, p. 77. 



Wood, Freshw. Alg. N. Amer. 1873, p. 123. 



Kirchn. Alg. Schles. 1878, p. 133. 



Gay, Monogr. loc. Conj. 1884, p. 42 (in part). 



Wolle, Desm. U. S. 1884, p. 28. 



Cooke, Brit, Desm. 1887, p. 3. 



De Toni, Syll. Alg. 1889, p. 788 (in part). 



Wille in Engler, Nat, Pflanzenfam. 1890, p. 14. 



Turn. Alg. E. India, 1893 ; p. 140 (in part). 



Comere, Desm. de France, 1901, p. 198. 



G. S. West, Brit, Freshw. Alg. 1904, p. 174. 



Cells usually very small, flattened and deeply con- 

 stricted ; sinus open or narrow and linear ; semicells 

 elliptical, oblong or subrecfcangular, cells united to form 

 long filaments by means of special apical appendages, 

 often twisted and enveloped in a mucous investment ; 

 apex of each semicell provided with one or two pairs of 

 small rounded tubercles or short capitate processes, 

 which are closely applied to the corresponding processes 

 of the next cell. Chloroplasts axile, one in each semi- 

 cell, with a central pyrenoid. 



