236 



Siphon-ales 



javensis, U. glaucescens, Tydemania, Rhipocephalus), or they are more or less 

 completely conglutinated to form a pluriseriate flabelliform or cyathiform 

 frond (as in U. conglutinata, U. cyathiformis, etc.) ; (2) the simple or 

 branched lateral appendages of the filaments, which in this type cohere so 

 as to form a cortical covering to the frond and its stalk, are thickened by a 

 calcareous deposit along the laterally cohering walls. The extent of the 

 calcification varies with the species, penetrating in some cases deeply into 

 the thallus. This method of incrustation is found in the cortex of the stalks 

 of Udotea, Penicillus, and Rhipocephalus, and in the frond of the corticated 

 species of Udotea. It will be thus seen that the calcareous cement really 

 fills the grooves between the filaments and binds them into a firm thallus. 



Fig. 153. On the left, Penicillus Lamourouxii Decaisne, f nat. size. On the right, 

 P. dumetosus (Lamx.) Decaisne, nat. size. 



In the first type of calcification 'pores' in the calcareous sheath are 

 extremely numerous. Each pore is a bubble-like chamber in the thickness 

 of the incrustation, being covered externally by a delicate calcareous pellicle 

 in which is a minute ostiole. They mark the spots where bubbles of 

 oxygen are evolved during photosynthesis (A. & E. S. Gepp, '11). 



In the second type of calcification there are numerous ' windows ' con- 

 sisting of the apices of the lateral branches of the filaments which are free 



