Siphonodadese 



255 



Ernodesrnis there are not only annular constrictions at the l>;<sr <>| t h,- main 

 basal segment, but also a single annular con- 

 striction at the base of each branch. In Apjoh n fa 

 there are several annular constrictions at the base 

 of each branch and the branching is very regular. 

 Moreover, in this genus the branches are not 

 separated from their parent coenocyte (at any 

 rate in the younger parts of the thallus) by cross- 

 walls. In fact, Apjohnia may be a transition-form 

 between the Siphonales and the Siphonocladiales. 



In Struvea, in which there is a flat frond on 

 the end of a conspicuous stalk, there may be 

 branches of the fourth order, and in the younger 

 specimens the branching is very regular and 

 the stalk consists of a single elongated coenocyte 

 with annular corrugations at the base. Later 

 on in the life of the plant it becomes divided 

 by transverse walls, more especially near its 

 upper end. The branching is largely in one 

 plane, and the upper part of each segment of 

 a branch generally develops rhizoid-like organs 

 of attachment (' tenacula ') which fasten them- 

 selves to the wall of the branch nearest above. 

 In this way there is formed an open net-work of 

 branches bound together by 'tenacula' at the 

 points of contact. As in other genera of the 

 Siphonocladese, there are numerous decumbent 

 creeping filaments attached to the substratum by 

 rhizoids, and from them the erect ' leafy shoots ' 

 arise. 



In Chttmtedoris the stipe is 4 or 5 cms. in 

 length, with annular constrictions from base to 

 apex, where it terminates in a cup shaped head, 

 about 3 cms. in diameter, composed of large num- 

 bers of whorled branches of several orders, all 

 arising by segregative cell-divisions. The branches are felted together and 

 their coherence is increased by small rhizoids (' tenacula ') growing out here 

 and there and firmly attaching themselves to neighbouring filaments. 



In all the Siphonocladese there are numerous small plate-like polygonal 

 chloroplasts, many of which contain a central pyrenoid. Very often, but not 

 always, the chloroplasts are connected by fine prolongations from their 

 corners, so that in each coenocyte there is apparently a net-like chloroplast 



Fig. 164. Siphonocladus tropicus 

 (Crouan) J. Ag. x 15 (after 

 Borgesen) . a , laterally branched 

 shoot, the basal part marked *. 

 Only one branch is septate. 

 b, appearance presented by 

 septate branch after soaking in 

 glycerin. 



