Derbesiacex 



each case attached to the anterior pointed end. The fusion of the gametes 

 (vide fig. 146 3 and 5 a) results in a rounded zygote which is capable of 

 immediate germination. 



The Bryopsidacete are closely related to the Derbesiaceae and the Codiacese ; they occur 

 in all seas, but more abundantly in the warmer oceans. 



The genera are : Bryopsis Larnouroux, 1809 ; Pseudobryopsis Berthold, 1880. 



Family Derbesiaceae. 



This family includes the single genus Derbesia first described by Solier 

 in 1847. In general habit the thallus is tufted, numerous erect and more or 

 less cylindrical branches arising from narrower, creeping rhizome-like 

 branches, which are fastened to the substratum by branched holdfasts 



Fig. 147. 1 and 3, Derbesia Lamourouxii (J. Ag.) Solier. 1, general aspect, iiat. size ; 

 3, ' rhizome ' and bases of shoots, x 5. 2,4 and 5, D. marina (Lyngb.) Kjellm. 2, bases of 

 branches ; 4, zoogonidangium ; 5, zoogonidium. (From Oltmanus ; 2, after Kjellman ; 4, 

 after Kuckuck ; 5, after Solier.) 



(fig. '147 1\ The whole thallus is the development of a single coenocyte, 

 but the erect branches are often separated at the base by double cell- walls 

 resulting in the formation of a small basal segment (fig. 147 2). There is 

 no differentiation into ' stem ' and ' leaves,' if one excludes the zoogonidangia 

 which are homologous with the ' leaves ' of Bryopsis, and the erect tubular 

 branches have a great resemblance to the filaments of a thick Vaucheria. 

 The cell-wall is thin and not encrusted. In the older parts of the thallus 

 a few transverse walls may sometimes be formed, but they are seldom 

 observed and have no definite position. The parietal protoplasm contains 

 many nuclei and numerous discoidal chloroplasts. The latter are oval or 

 elliptical and generally contain one or more pyrenoids, but in some species 

 (Derbesia neglecta) pyrenoids are absent. In the large sap vacuole are 



152 



