SIPHONED. 



63 



Order 9. Dasycladaceae. The thallus consists of an axile 

 longitudinal cell, destitute of transverse walls, attached at the 

 base by root-like organs of attachment, and producing acropetally 

 whorls of united, single or branched, leaf-like structures with 

 limited growth. Asexual reproduction is wanting. Sexual re- 

 production by conjugation of gametes which arise in separate, 

 fertile leaves/either directly or from aplanospores, which develope 

 into gametangia. The principal genera are : Acetabularia, Dasy- 

 cladus. Neomeris, Cymopnlia. All marine. 



The curiously shaped 

 Acetalndaria mediterra- 

 nea grows gregariously 

 on limestone rocks, and 

 shells of mussels in the 

 Mediterranean ; it re- 

 sembles a minute um- 

 brella with a small stem, 

 sometimes as much as 

 nine centimetres in 

 height, and a shade 

 which may be more than 

 one centimetre in di- 

 ameter. The cell-mem- 

 brane is thick, and 

 incrusted with carbonate 

 and oxalate of lime. 

 Only the lower, root- 

 like part of the thallus, 

 which penetrates the 

 calcareous substratum- 

 survives the winter, and 

 may grow up into a new 



plant. The sterile leaves, which drop off early, are dichoto- 

 mously branched and formed of cylindrical cells separated 

 from each other by cross-walls, but they are not grown to- 

 gether. The shade is formed by a circle of 70-100 club-shaped 

 rays (fertile leaves) grown together, in each ray 40-80 aplano- 

 spores are formed, which become liberated at the breaking of the 

 shade, and later on are changed to gametangia (compare Botry- 

 dium} which open by a lid and allow a large number of egg- 

 shaped gametes with two cilia to escape. Gametes from various 



FIG. 60. Halimeda opuntia. Plant (natural size). 

 B Part of a longitudinal section. 



