Howe : Phycological studies 249 



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plant, when they appear as a dense tomentum adhering to the 

 lower surface of all parts of the thallus except a very narrow zone 

 at the growing margin. They are commonly more or less inter- 

 tangled with filamentous, rock-boring Cyanophyceae. The 

 larger rhizoids are occasionally formed by a ventral evagination 

 of a part of the thallus-tube^ the lumina of the two remaining con- 

 tinuous, but the (Ordinary slender rhizoids, the measurements of 

 which are given above, appear from an early stage to be external 

 appendages to the thallus-tubes, their basal cells standing in about 

 the same relation to the walls of the main cells as do the small 



w 



fibular cells o{ Siphoiwcladus rigtdus shown in plate 14, fig. 4. 

 Like Siphoiiodadus 7'igidus^ Petrosiphoii adhaerens is accom- 

 panied by a fungus, or, at least, a chlorophylless filamentous thallo- 

 phyte, though this is not so uniformly present as in that species ; 

 we have never dissected a thallus in which it could not be found, 

 yet large parts are sometimes destitute of it and we are of the opin- 

 ion that its relation to the alga is that of a parasite rather than that 

 of a subordinated symbiont. The external hyphae lie somewhat 

 loosely among the thallus-tubes or rhizoids ; they are fuscous, 

 septate, 2,5-5// broad (cells mostly 2-15 times as long), for the 

 most part sparingly branched, straight or sometimes contorted or 



torulose ; the hyphae finally penetrate the walls of the thallus-tubes 

 and follow their cavities in the direction of their growth, mean- 

 while becoming lighter-colored and forming much-elongated, 

 ribbon-shaped or virgate-fasciculate, decompound clusters of 

 branches ; the cells of these endophytic branches are mostly 5-1 2 /i 

 long, ellipsoid, oblong, or ovoid, and are occupied chiefly by a 

 large vacuole, with 1-3 small refringent bodies usually lying close 

 to the wall. Nothing which could be identified with reasonable 

 certainty as a reproductive body of this fungus has been observed. 

 The only described species, so far as we can discover, which 

 may suggest itself as possibly congeneric with Petrosiphon adhaer- 

 ens is the minute SipJionodadus zwlnticola Hariot, which grows on 

 the shells of Vohita in Tierra del Fuego. But this, according to 

 the original description and figures, * and according to supple- 

 mentary descriptions and figures by Bornet, f is quite different and 



Jour, de Bot. i : 56. 1887 ; Mission scientifique du Cap Horn 5: 22. //. /. /. 

 2~4' 1889. 



tBull. Soc. Bot. France 36; clix, clx. //. 70. /. r, 2^ 1889. 



