<38 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



this fungus may be strikingly similar in appear- 

 ance to those of species of Sirolpidium, with the 

 exception that it does not become septate and frag- 

 ment into segments. Should the latter characters 

 prove insignificant in generic diagnoses and phy- 

 togeny, Petersenia, in the present sense, may per- 

 haps be merged with Sirolpidium. On the other hand, 

 the olpidioid thalli bear a strong resemblance to 

 those of Olpidiopsis, particularly to 0. irregularis, 

 according to Sparrow ('34). 



Zoospore germination and infection of the host 

 have not been observed in this genus, but these proc- 

 esses are probably similar to those of Sirolpidium, 

 Ectrogella, etc. Sparrow ('34) reported that in the 

 early stages the thallus is "somewhat plasmodial in 

 nature" but he did not illustrate any of the initial 

 developmental phases. At maturity, however, a well- 

 defined wall is present which stains a dark, ruby 

 color when tested with chloro-iodide of zinc. As 

 noted previously the thallus varies markedly in size 

 and shape (figs. 3-8). Within the tetraspores of the 

 host it usually assumes the shape of the confining 

 cell (fig. 4), but even under such conditions lobed 

 .specimens may occur, as Feldman ('40) has shown. 

 In the vegetative state the protoplasm is refractive 

 and vacuolate (figs. 4, 7), and as the thallus is trans- 

 formed into a sporangium, the small vacuoles pre- 

 sumably flow together and form one or more larger 

 central ones. The late stages of cleavage and spo- 

 rogenesis closely resemble those of Pi/thinm, accord- 

 ing to Sparrow. The incipient spore initials undergo 

 an individual rocking movement which becomes more 

 pronounced as they separate and acquire their ma- 

 ture form. At the same time traces of flagellary mo- 

 tion become visible at the periphery of the spore 

 'mass, and this increases in intensity until the zoo- 

 spores are mature and swarming within the sporan- 

 gium. As the tip of the exit tube deliquesces they 

 emerge (fig. 3) and swim directly away. According 

 to Sparrow, the zoospores (figs. 1, 2) are elongate, 

 pyriform, shallow-grooved, and contain a refractive 

 body at the anterior end. The flagella are reported 

 to be inserted laterally, but some of Sparrow's fig- 

 ures suggest that they arise near the anterior end. 



P. LOB ATA (Petersen) Sparrow, I.e., p. 13, pi. 2, figs. 

 I-N; 1936. Biol. Bull. 70: 2U. PI. 2, figs. 1, 2. 

 Pleotrachelus lobatus Petersen, I.e., p. 4(>0, figs. V. 1-7. 



Zoosporangia solitary or numerous, often occupy- 

 ing more than one host cell, or filling it completely 

 and conforming to the latter's size and shape; usu- 

 ally markedly and irregularly lobed, elongate and 

 tubular, or oval, ellipsoid and olpidioid with 1 to 3 

 simple or branched exit tubes of variable length. 

 Zoospores elongate, pyriform, slightly arched and 

 shallow-grooved, 3 X 4.5 /j.. Resting spores un- 

 known. 



Parasitic in the vegetative cells and tetraspores of 

 Spermothamnion tumeri, S. repens, Callithamnion 

 corymbosum and C. hookeri, in Denmark (Petersen, 

 I.e.; Sparrow, I.e.), C. roseum in Massachusetts, 



U. S. A. (Sparrow, '36) and in the disporangia of 

 Seirospora interrupta near Villefranche-sur-mer in 

 the Mediterranean (J. and G. Feldman, '40). x 



This species may occur in whitened and dead cells 

 of its hosts which suggests that it is only weakly 

 parasitic. According to Sparrow, it is not assisted by 

 other organisms in the early stages of invasion, but 

 in old infections, bacteria and protozoa are always 

 present and aid in the destruction of the content of 

 the host cell. 



Whether or not Pleotrachelus pollagaster Peter- 

 sen (figs. 9—11) belongs in Petersenia is question- 

 able. Sparrow ('34) included it provisionally in this 

 genus, because he found its zoospores to be similar 

 in shape and size to those of 7'. lobata. He did not, 

 however, determine the number and position of the 

 flagella. and until these points have been conclu- 

 sively settled the exact generic position of P. polla- 

 gaster will remain doubtful. 



Sparrow ('36) found two other fungi which he 

 assigned tentatively to Petersenia as unidentified 

 species. One occurred in the eggs of a microscopic 

 animal, possibly a rotifer, adherent to filaments of 

 Ceramium diaphanum. The parasite filled the whole 

 interior of the egg (fig. 12) and was transformed at 

 maturity into a sporangium, 20 X 50 /n, with 1 to 3 

 short broad, 8 /n in diameter, exit tubes. The forma- 

 tion and emergence of the zoospores were not ob- 

 served, although a number of the spores which had 

 failed to emerge were found within sporangia. These 

 bodies were reniform, "of the laterally biciliate 

 type," 2X*Cj and resembled the zoospores of 



1 Bull. Soc. Hist. Nat. Afrique Nord 31: 73. 



PLATE 18 

 (Figs. l-(>, 11-14 after Sparrow, '3-1, '36; figs. 7-10 after 

 Petersen, '05. Figs. 13, 1+ drawn from photographs.) 



Petersenia lobata 



Fig. 1. Group of isoeont zoospores; point of attachment 

 of flagella uncertain. 



Fig. 2. Freehand, enlarged drawing and interpretation 

 of zoospore. 



Fig. 3. Emergence of zoospore from olpidioid zjcspo- 

 rangium. 



Fig. 4. Infected tetraspores of Spermothamniv/m. 



Fig. 5. Olpidioid and nodular thalli. 



Fig. fi. Irregular thallus with four short branches. 



Fig. 7. Elongate, lobed and constricted multivacuolate 

 thallus occupying two host cells. 



Fig. 8. Empty zoosporangium. 



Pleotrachelus (Petersenia) pollagaster 



Fig. 9. Lobed thallus with three long exit tubes. 

 Fig. 10. Olpidioid sporangium. 



Fig. 11. Irregular resting spore. 



Petersenia sp. 



Fig. 12. Sporangium in rotifer (?) egg with two bi- 

 flagellate isoeont zoospores. 



Fig. 13. Young thallus of Petersenia sp. saprophytic in 

 ( V ru in in in diaphan it in. 



Fig. 14. Empty zoosporangium with three exit tubes. 



