COMPARATIVELY little is known at present about the chemical nature of the cell wall constituents of siphonous green algae, especially that of the non-septate groups. Although the cell wall of the algae belonging to Bryopsidaceae, Caulerpaceae and Codiaceae has been stated to consist for the most part of callose (a name given originally to a polysaccharide-like substance of unknown chemical nature occurring in the callus plate of the sieve tube) and pectin and in Bryopsis and Codium in addition to some cellulose, no reliable characterization of these substances has thus far been made, their occurrence in the algal cell wall being inferred largely on the basis of staining reactions and solubility test1. Quite recently Mackie and Percival reported the presence of xylan in Caulerpa filiformis and established its structure as of β-1,3-linkage2.