Udotede 



237 



from calcification. These ' windows,' which are flush with the surface of the 

 thallus, are for the entrance of light, and presumably escape calcification 

 owing to the clinging of bubbles of oxygen evolved as the result of the 

 photosynthetic work of the chromatophores lodged within (vide A & E S 

 Gepp, 11). 



In most species the stalk consists of many parallel filaments, with 

 numerous lateral branches forming a calcified cortex, but in Udotea javensis, 

 a species in which the flabellate frond consists of a single layer of filaments, 

 the stalk is rnonosiphonous. 



A marked feature of the frond of Udotea is its beautiful zoning due to 

 periodic growth. The thallus also shows marginal proliferation. 



Fig. 154. Halimeda incrassata (Ellis & Sol.) Lamx. var. simulans (Howe) Borgesen. 

 Nat. size (after Borgesen). 



The highest type of the Udotese is Halimeda (fig. 154). This genus is 

 certainly the most singular in the form of its fronds, which consist of 

 branched chains of incrusted segments joined together in the manner of a 

 cactus. The segments are compressed, more or less irregularly rounded, and 

 in some species subreniform in shape. The structure of the segment is 

 much like that of the frond of the more advanced types of Udotea, only 

 between the axial strand of filaments and the peripheral cortical branches 

 there is a subcortical layer, consisting of lateral branches of the axial 



