EUPHYCOPHYTA 



91 



Dasycladales 



Formerly the members of this order were included in the 

 Siphonocladales, but since they possess certain distinct character- 

 istics they are best treated as a separate order. The thallus body is 

 first characterized by the whorled (verticillate) arrangement of the 

 lateral branches (in Acetabularia they are fused to form a disc), and 

 secondly by the fact that the plastids are independent, disc-like, 

 and not reticulate as in the Siphonocladales. In the species that 



Neomeris annulata 



^ 



•.»>v.:.-i--?' gamecangium 

 siphon 



® 



rhizoid 



siphon 



<S»oO 



gametangium 



rhizoid 



-^ © 

 a 



o 



Q. 



siphon i.©> 



gametangium 



Acetab. mediterranea 



Fig. 50 Behaviour of the nucleus in certain members of the 



Dasycladales. Break-up of the large nucleus occurs in the rhizoid 



and the fragments, which may change shape, move up the siphon 



into the gametangia. (After Dao.) 



have been investigated the vegetative thallus appears to be uni- 

 nucleate. Prior to gamete formation the single primary nucleus 

 fragments into smaller portions which may subsequently change 

 shape in either the rhizoids or the main siphon. The small daughter 

 nuclei eventually migrate into the gametangia. This peculiar 

 nuclear behaviour has been estabhshed for Acetabularia, Cymopolia 

 and Neomeris (Fig. 50). There is reason to beUeve that the adult 

 plants are all diploid. 



In a number of genera the gametes are produced from special 

 cysts (cf. Acetabularia). The order is very ancient and has appar- 



