166 SEAWEEDS 
zoospores each with four cilia. Vegetative propaga- 
tion also takes place. 
The Thallus.—In Chetomorpha the thallus remains 
unbranched, in Rhizoclonium there are very short 
branches, while in Urospora there are mere lateral 
outgrowths from the cells, not cut off by cross-walls ; 
in Cladophora the thallus is densely branched, the 
branches being given off at the upper part of the cell, 
just below the cross-wall separating it from the one 
above. The base, which is attached to the substratum 
by a holdfast, is in some cases, ¢g Cladophora 
rupestris, much strengthened by the growing to- 
gether of a number of rhizoid filaments. The 
filaments of Gomontia, which inhabits the shells of 
molluses, penetrating their substance very densely, 
are less regularly branched, and the cells become 
irregularly swollen, sometimes more or less beaded. 
The whole thallus may be easily isolated by placing 
a fragment of the green-stained shell in Perenyi’s 
fluid (see p. 30). 
The number of nuclei in each cell varies consider- 
ably, and the chromatophores also vary in shape, 
but are mostly perforated and flat; or net-like, with 
abundant pyrenoids. 
The Reproductive Organs—Gametes are produced 
by both Cladophora and Urospora. They arise within 
the vegetative cells in large numbers, and escape by 
a round hole; they are bi-ciliated and have a red 
spot. They are all exactly alike in appearance, and 
conjugate in pairs. The zygote germinates at once 
in Cladophora, and after a period of rest in Urospora. 
Zoospores are known in all the genera except Rhizo- 
