156 SEAWEEDS 



VALONIACE.E. 



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 Algae 

 with the other Chlorophycece, or at all events pointing 

 to such a connection. 



The Thallus. The most simple type is that of 

 Valonia ventricosa (Fig. 46a), 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, 



