Acetabularieas 



271 



Fig. 174. Acetabularia mediterranea Lamx. 

 Nat. size (after Oltraanns). 



The genera, which are confined to the warmer seas, are : Neomeris Lamouroux, 1816 ; 

 Cymopolia Lamouroux, 1816 ; Bornetella Munier-Chalmers, 1877. 



Sub-family AcETABULARiE/E. In this sub-family the main axis bears 

 distinctly differentiated sterile and 

 fertile branches, the former being 

 polytomously branched. All tJie 

 genera are encrusted with calcium 

 carbonate. 



The best-known genus is Aceta- 

 bularia, one species of which (A, 

 mediterranea; fig. 174) has been 

 very thoroughly investigated. The 

 mature plant consists of a stem 

 several centimetres in height termi- 

 nated by a strongly calcified disc 

 about 1012 mm. in diameter. The 

 latter is composed of a number of 

 fertile branch-segments, radially ar- 

 ranged and laterally connate. The 

 central area of the disc is occupied 

 by a flat, circular membrane, which closes up the cavity of the large axial 

 coenocyte (or stalk), and between it and the bases of the radial branch- 

 segments is a continuous circular cushion, the superior corona (fig. 175 B sc). 

 This corona really consists of as many segments as there are radial branches 

 forming the disc, and each segment bears the scars of deciduous branches. 

 Below the disc there is another similar cushion, the inferior corona (fig. 175 B 

 ic) but on this there are no hair-scars. The cavity of each radial branch of 

 the disc is at its base in communication with the corresponding segments of 

 the superior and inferior coronas, and the latter communicate with the large 

 cavity of the axial ccenocyte by means of a small central opening in a 

 separating fold of the cell-wall. For this perforated ingrowth of the cell-wall 

 Howe ('01) suggested the name of velum partiale. 



Acetabularia mediterranea grows for several years before a fertile disc is 

 formed. In the first year the axis bears only a few irregular protuberances 

 at its apex. The plant dies down at the end of the growing season, the 

 lower part of the stalk (closed above by a wall) and the irregularly branched 

 basal attachment remaining alive during the winter. The basal part increases 

 in size with age and acts as a storehouse of reserve material for further 

 growth. In the second year the axis bears several successive whorls of poly- 

 tomously branched, deciduous, branch-segments (' leaves '), and terminally a 

 disc which is not fertile. The deciduous branches are uncalcified, and, on 



