250 Siphonodadiales 



Order 3. SIPHONOCLADIALES. 



In the Siphonodadiales, a group first established by Schmitz in 1 878, the 

 thallus, in most cases, is septate from the earliest stages onwards. Septation is 

 quite independent of nuclear division and the segments contain from two to 

 many nuclei. The transverse walls are more of the usual character and not 

 of the nature of ' plugs ' or ' stoppers ' such as those which arise secondarily 

 in the Siphonales. With the exception of Sphseroplea and some of the 

 Cladophoracese all the forms are marine. 



The simplest types are met with in the sub-family Valoniese of the 

 Valoniaceae, in which the thallus in its young state is strictly siphonaceous. 

 From these simple types a progressive series of forms can be traced in which 

 a gradual partitioning of the thallus takes place, leading to the formation of 

 smaller and smaller segments which contain fewer nuclei. The partitioning 

 of the thallus is accompanied by branching, and as a general rule the greater 

 the number of septa the more complex the branch -system. The maximum 

 divergence from the siphonaceous type occurs in the Cladophoraceae, in which 

 the greatest segmentation of the thallus is found. The most symmetrical 

 and elegant forms are members of the Dasycladacese. 



The cytological details of the various forms are in close agreement with 

 those of the Siphonales. 



In many genera of the Valoniacese Bb'rgesen ('05 ; '13) has observed a 

 curious mode of vegetative division which he terms segregative cell-division. 

 The entire contents of a segment become divided up into a number of small 

 ball-like parts, not necessarily equal, each of which soon becomes surrounded 

 by a membrane. The young daughter-segments grow in size, become angular 

 by pressure of contact, and ultimately fill out the whole lumen of the mother- 

 segment, which in this way becomes divided into a number of smaller parts. 

 Each new segment contains many nuclei and chloroplasts, but the nuclei 

 take no active part in the partitioning of the old segment. The daughter- 

 segments subsequently grow into adult branches, and this process may be 

 repeated several times giving rise to several orders of branches. 



Reproduction occurs by zoogonidia in all the families except the Dasy- 

 cladaceae, in which the reproductive cells are aplanospores. 



In Pithophora there are large barrel-shaped resting-spores of the nature 

 of akinetes, which Wittrock originally termed ' agamo-hypnospores.' 



Gamogenesis occurs in the Cladophoracese and Dasycladaceae by the 

 fusion of isoplanogametes. 



Recent work on this group and on the Siphonales leads one to the 

 inevitable conclusion that the Siphonodadiales have arisen from the Siphonales 



