240 PART III. THE CLASSIFICATION OF PLANTS. 



the tubular body; in Derbesia and Halimeda there are definite gonidangia, 

 each producing a number of zoogonidia. 



Sexual reproduction is generally isogamous by planogametes : Vaucheria is 

 the only known oogamous fonn. Isogamy is known, certainly or probably, in 

 Botrydium, Bryopsis (?), Codium (?), Dasycladus, Acetabularia. In Botrydium 

 and Acetabularia the gametangia are formed by the aggregation of the proto- 

 plasm (in the coherent leaves of the latter) into rounded masses which become 

 surrounded 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 Dasycladus and Codium the unilocular gametangia are 

 rounded or oval, and are borne, in the former, terminally on the leaves, and in 

 the latter, as lateral outgrowths on the thalloid shoot ; in Bryopsis the small 

 tubular leaves act as gametangia, being shut off by a septum from the branch 

 bearing them. 



In Botrydium, Acetabularia, and Dasycladus, the conjugating gametes are 

 quite similar ; but in Bryopsis and in Codium, the one (male) is small and 

 brown or yellow, the other (female) is larger and green. 



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

 are developed as lateral branches which become shut off by a septum (Fig. 169) ; 

 each antheridium gives rise to a number of biciliate spermatozoids ; each oo- 

 gonium gives rise to a single oosphere which is not extruded, and has a hyaline 

 receptive spot directed towards the opening of the oogonium. 



So far as the life-history of the Siphonaceas is known, there is no indication 

 of an alternation of generations ; the plant is, in all cases, the gametophyte.aud 

 the product of the sexual process is a zygospore or an oospore which, on ger- 

 mination, developes directly into a new gametophyte. 

 The following are the families of the Siphonacese : 



Fam. 1. Vaucheriece ; oogamous ; body, simply tubular and branched ; in- 

 cludes the single genns Vaucheria ; marine and fresh-water. 

 Fam. 2. Derbesiete : probably isogamous ; body simply tubular and 



branched ; single genus Derbesia ; marine. 



Fam. 3. Codiete : isogamous, so far as known ; body, tubular and much 

 branched) the branches interwoven into a tissue, assuming various 

 forms ; principal genera, Codium, Halimeda, Udotea ; marine. 

 Fam. 4. Dasycla dece : isogamous; body tubular, short, differentiated into 

 stem and leaf, the stem being an erect main axis ; leaves whorled, 

 or bilaterally arranged (Bryopsis) ; principal genera, Dasycladus, 

 Acetabularia, Bryopsis ; marine. 



Fam. 5. Caulerpea : reproduction not known ; body, differentiated into root 

 stem and leaf, dorsiventral ; stem creeping, bearing roots be- 

 low and leaves above ; genera, Caulerpa, Cblorodictyon ; marine. 

 Fam. 6. Botrydiece : isogamous; body differentiated into root and shoot; 

 shoot, vesicular, thalloid and unbranched ; genera, Botrydium 

 on mud, Codiolum marine. 



Fam. 7. Phyllosiphonece : sexual reproduction not known ; asexual repro- 

 duction by non-motile gonidia ; body, tubular branched, or vesi- 

 cular and unbrancbed, thalloid; endophytic; genera, Phyllosiphon 

 in leaves of Arisarum vulgare, Phytophysa in shoots of Pilea. 



