PH^OPHYCE^E 57 



much the same anatomical structure though of 

 different habit. Cutleria grows erect with flat 

 bilateral dichotomous fronds, while Zanardinia is 

 resupinate and attached by root-hairs on the under 

 surface. Cutleria possesses a thallus composed of 

 three parenchymatous tissues, viz. an epidermal layer 

 of small cells with many chromatophores, a cortical 

 layer of larger isodiametrical cells with few 

 chromatophores, and an axial layer of large cells 

 the greatest diameter being in the direction of the 

 axis which are almost colourless. The dorsi-ventral 

 thallus of Zanardinia has an upper layer of cells 

 with many chromatophores, and a basal layer of 

 one row of colourless cells producing in many cases 

 root-hairs. The growth of the thallus is tricho- 

 thallic, viz. by means of ultimate hairs, each with an 

 intercalary growing-point, producing outwards a 

 row of cells forming hairs that die off at the 

 apices, and inwards another row which at a short 

 distance below the growing-point unites with 

 neighbouring rows to form a compact thallus. 

 These hairs are in several layers, and their products 

 are traceable for a short distance as definite rows of 

 cells before they become merged in the general 

 tissue-system of the thallus. Lateral extension of 

 the thallus is effected by the branching of these 

 hairs, and subsequent cell-divisions in the superficial 

 cells bring about the formation of the epidermal 

 layer. In the Aglaozonia (= reputed non-sexual 

 generation of Cutleria) the growth of the thallus is 

 not trichothallic, but by means of marginal initials 

 which by periclinal and radial division produce a 



