109 



are of two kinds, plurilocular (containing pre- 

 sumptive gametes) and unil ocular. 



The Thallus of Ralfsia forms leathery crusts on 

 rocks, &c., and though at first almost circular, be- 

 comes ultimately of irregular outline. It attains 

 considerable thickness in the more central portion, 

 and decreases towards the margin. The cells, each 

 with one chromatophore,are in vertical series and form 

 a parenchymatous tissue ; those of the margin effect- 

 ing by division the extension of the thallus, while 

 the superficial cells similarly add to its thickness. 

 It is attached to the substratum either directly or 

 by means of root-hairs. On the upper surface there 

 occur single hairs, or in other cases tufts, sometimes 

 springing from pits which, however, do not appear to 

 exhibit a special development like the cryptostornata 

 of other orders. By a process of overlapping of new 

 crusts on older ones, the thickness of the thallus is 

 often considerably increased. In Lithoderma there is 

 no notable difference from Rcdfsia in the develop- 

 ment and structure of the thallus. 



The Reproductive Organs. Until recently only 

 unilocular sporangia were known in the case of 

 Rcdfsia. They are obovate and ' arise as lateral 

 processes from septate hairs, which are in turn 

 direct prolongations of the superficial cells of the 

 thallus. Though of lateral origin, the sporangia 

 assume a terminal position by pushing aside the 

 true terminal shoot. The plurilocular sporangia, 

 recently discovered by Mr. Batters, are of similar 

 origin, but with them are no paraphyses. The 

 hairs or paraphyses grow mainly, if not wholly, 



