PHAOPHYCEA 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 
