422 BOTANY pakt ii 



form occurs as felted patches of fine filaments on the bark of trees, with the 

 hymeuium on the parts of the thallus, which are tm-ned away from the light. 



Official, — The only representative of the Lichens is Cetraria islandica 

 (Lichen islandicus). Sticta ptdmonacea is also used in domestic medicine. 



The Manna Lichen {Lecanora esculenta) is a crustaceous Lichen that often 

 covers the ground to a depth of 15 cm. in the Steppes and Deserts of Southern 

 Russia, Asia Minor, and North Africa. The thallus falls into pieces the size of a 

 pea, and is thus readily spread by the wind ; it is used by the Tartars who prepare 

 earth-bread from it. Gyropliora esculenta is eaten in Japan. Cladonia rangiferina 

 is imiiortant as affording food for the Reindeer. 



Some species particularly rich in Lichen acids are used in the preparation of 

 the pigments orseille and litmus ; there are in the first place species of lloccclla 

 (especially R. Montagnci, R. tindoria, R. fuciformis, and R, 2}hyco2)sis) which are 

 collected on the coasts of the warmer oceans, and the crustaceous lichen, Oehrolechea 

 tartarea, in North Europe and America. 



Class XV 

 Characeae (Stoneworts) f'^*^^) 



The Cliaraceae or Charophyta form a group of highly organised green Thallo- 

 phytes sharplj' isolated from both simpler and higher forms. Their origin must 

 be looked for in the Chlorophyceae, but the complicated structure of their se.xual 

 organs does not allow of any immediate connection with the oogamous Green Algae. 

 On the other hand in some characters they show some approach to the Brown 

 Algae, from which the}' differ in the pure green colour of the chromatophores. 

 They cannot be regarded as leading towards the Bryophyta although their karyo- 

 kinetic nuclear division exhibits a great agreement with that of the Archegoniatae. 



The Characeae grow in fresh or brackish water, attached to the 

 bottom and covering extended areas with a mass of vegetation. Their 

 regular construction and habit is characteristic. In some species 

 their cylindrical main axes are over a foot in length, and are composed 

 of long internodes alternating with short nodes, from which short, 

 cylindrical branches are given off in regular whorls with a similar 

 structure, but of limited growth (Fig. 363). The lateral axes are 

 either unbranched or give rise at their nodes to verticillate outgrowths 

 of a second order. From the axil of one of the side branches of each 

 whorl a lateral axis resembling the main axis is produced. The attach- 

 ment to the substratum is effected by means of colourless branched 

 rhizoids springing from the nodes at the base of the axes. 



Both the main and lateral axes grow in length by means of an apical cell, from 

 whicli other cells are successively cut off by the formation of transverse walls. 

 Each of these cells is again divided by a transverse wall into two cells, from the 

 lower of which a long internodal cell develops without further division ; while 

 the ui)per, by continued division, gives rise to a disc of nodal cells, the lateral 

 axes, and also, in tlie lower portion of the main axis, to the rhizoids. In tlie 

 genus Nilclld the long internodes remain naked, but in the genus Cliara tliey 

 become enveloped by a cortical layer consisting of longitudinal rows of cells which 

 develop at the nodes from the basal cells of the lateral axes. A corresponding 



