286 



ALGsE 



ORDER 3. DASYCLADACE/E. 



The Dasycladaceae are clearly distinguished from the other orders of 

 the class by their verticillate branches and their external sporanges. In 

 Acetabularia Lmx. the thallus has the form of a small hymenomycetous 

 fungus with a hemispherical or 

 funnel-shaped cap or ' pileus ' 

 at the summit of a cylindrical 

 * stipe ' or stalk. From the 

 lower end of the stalk proceed 

 a number of root-like branches 

 which fix the plant to the sub- 

 stratum. The whole plant con- 

 sists of a single ramifying cell, 



FIG. 251. Acetctbnlaria mediterranea. 

 Lmx. (natural siza). 



the walls of which are, when 

 mature, permeated by calcium 

 carbonate. The cap is divided 

 by regular radial projections 

 into a large number of cham- 

 bers of nearly equal size, which 

 are in communication with one 

 another above the insertion of 

 the stalk. The upper part of 

 the thallus perishes at the end of the season, while the lower portion 

 is perennial. After a number of sterile thalli have been produced, a 

 fertile thallus appears, similar in structure to the sterile ones. In the 

 chambers of the cap of this thallus are produced the zoosporanges, a large 

 number in each chamber, of an ellipsoidal form, and furnished at one 

 end With a lid, which subsequently becomes detached. When mature 

 the protoplasm of the sporange breaks up into a number of swarm- spores, 



FIG. 252. A. mediterranean 7, cap (magnified) ; 

 , scars of branches ; r, rudimentary whorl of branches ; 

 ?v, ring above the cap ; v, depressed apex ( x 4). 

 //, sporange with lid (x 120). ///, the same, show- 

 ing the escape of the swarm-spores (x 120). IV, 

 conjugation of zoogametes. V, VI, lower part of 

 stem, showing rhizoids, b. VII r , plant germinating 

 from a zygosperm. VIII, origin of branches at 

 summit of the stem(x 120). IX, at a later stage 

 (x 90). (After de Bary and Strasburger.) 



