156 SEAWEEDS 
VALONIACES. 
General Characters—There is scarcely any Order 
to which it is more difficult to assign distinctive 
general characters, owing to the much varied 
structure of the vegetative organs and our ignorance 
of the reproductive processes in most of the genera. 
The thallus ranges in variety from a single large cell 
with rhizoids up to forms of complex structure with 
stalk and frond. The cells of the thallus are fre- 
quently linked together by haptera or holdfasts. The 
reproduction by zoospores described for Microdictyon 
and Anadyomene certainly needs minute re-investi- 
gation, but that of Siphonocladus and Valonia stands 
on a firmer basis of observation. In Valonia spores 
with cell-membranes arise by free-cell-formation 
within the great lumen of the cell, but their ger- 
mination has not been observed. The following 
types indicate the character of the thallus, and ex- 
hibit a series connecting the multinucleate Algx 
with the other Chlorophycec, or at all events pointing 
to such a connection. 
The Thallus——The most simple type is that of 
Valonia ventricosa (Fig. 46), which consists of a single 
cell, generally varying in size from that of a goose- 
berry to a hen’s egg. This enormous cell, much the 
largest cell of isodiametric shape known to us, is 
attached to its substratum by rhizoids at the base, and 
presents a uniform green appearance, except on the 
cessation of its vegetative life, when the protoplasm 
with the chromatophores, which had lined the wall, 
