GROUP I. THALLOPHYTA : 



251 



tubular and much, branched (usually in Vaucheria, Fig. 140) ; or it may 

 be differentiated into root and shoot, the shoot assuming various forms, 

 such as a rounded cushion ((Jodium Bursa), or a simple vesicle (Botrydium, 

 Fig. 142) ; or the shoot may be differentiated into stem and leaf (Acetabu- 

 laria, Fig. 141); it has sometimes continuous apical growth (e.g. Vaucheria) ; 

 the wall is sometimes impregnated with chalk (e.g. Acetabularia). 



Asexual reproduction is known to take place in only a few forms ; it is 

 effected by zoospores, which are uniciliate in Botrydium, or multiciliate, 

 as in Vaucheria, where they are sometimes non-motile. In Vaucheria 

 the spores are formed singly in simple sporangia formed by septation of 

 branches of the body ; in Botrydium they are formed in large numbers 

 from the protoplasm of the tubular body. 



Sexual reproduction is generally isogamous by planogametes : Vau- 



FIG. 141. -Acetabularia crenulata (after Kutz- 

 ing ; nat. size) : the terminal circular disc con- 

 sists of a whorl of coherent leaves ; in these 

 the gametangia are formed. 



FIG. 142. Botrydium granulatum 

 ( x 6) : s the green shoot ; w the 

 colourless root. 



cheria is the only known oogamous form. Isogamy is known in Botry- 

 dium, Acetabularia, and others. In Botrydium and Acetabularia the 

 gametangia are formed by the aggregation of the protoplasm (in the 

 coherent leaves of the latter) into rounded masses which become sur- 

 rounded by a wall, and are set free by the rupture of the parent organism; 

 their contents undergo frequent division to produce the planogametes 

 which are eventually set free. In Botrydium and Acetabularia the con- 

 jugating gametes are quite similar. 



In Vaucheria, the sexual organs are unilocular antheridia and oogonia, 

 and are developed as lateral branches which become shut off by a septum 

 (Fig. 140); each antheridium gives rise to a number of biciliate sperma- 

 tozoids ; each oogonium gives rise to a single oosphere which is not 



