72 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



ing length, diameter, contour, and shape; content of 

 sporangium usually emerging to form a globular 

 mass at the mouth of the exit tube. Zoospores bean- 

 shaped, reniform and somewhat pyriform, laterally 

 biflagellate, with several small refractive globules, 

 mono- or diplanetic ; primary swarmers isocont, 

 secondary swarmers heterocont ; formed ( 1 ) extra- 

 matrically by progressive cleavage of the extruded 

 globular mass of protoplasm which may be naked or 

 surrounded by a vesicular membrane as in Pythium; 

 (2) delimited in the sporangium, emerging singly 

 and completing development at the mouth of the 

 exit tube; or (3) completely developed in the spo- 

 rangium, emerging in succession and swimming di- 

 rectly away. Antlieridia borne on the same or differ- 

 ent thalli; oogonia terminal or intercalary, variously 

 shaped ; differentiation of egg cell or oosphere prior 

 to fusion absent or very doubtful; epiplasm lacking; 

 content usually contracting toward the conjugation 

 canal during plasmogamy. Antlieridia when present 

 usually more slender, elongate and cylindrical, fre- 

 quently forming a conspicuous perforation and con- 

 jugation tube which extends into the oogonium; 

 fused protoplasts contracting and becoming in- 

 vested with a definite wall. Oospores sexual or 

 parthenogenetic, lying free in the oogonium, usually 

 spherical, oval or ellipsoidal, smooth or warty, hya- 

 line or colored, thick-walled with one or more large 

 refractive globules; germinating by giving rise di- 

 rectly to biflagellate zoospores. 



This is the largest genus of the family and in- 

 cludes approximately fifteen species, some of which 

 are doubtful, incompletely known, and possibly 

 synonymous. The majority are parasites of algae; 

 ,two occur in tissues of higher plants, and two live in 

 the body of insects. A number of species occur on 

 the same host and are structurally similar. It is thus 

 probable that when extensive cross inoculations have 

 been made and the range of variation of the indi- 

 vidual species is known some of them will prove to 

 be identical. As to geographic distribution, they 

 have been reported from Asia, Europe, and North 

 America. 



As is shown in figure 1, the zoospores come to 

 rest on the host cell and penetrate it by a germ tube 

 of varying length and diameter, tip of which enlarges 

 as the content of the spore passes into it. This tip 

 soon elongates into a comparatively thick hypha- 

 likc tubular strand as in L. rabenhorsiii (figs. 2-4) 

 or enlarges into a globular, oval, vesicular, sac-like 

 and somewhat irregular structure as in L. brachy- 

 stomum, L. enecans, L. Oedogonii, etc. (figs. 30, 39, 

 45, 49). This enlargement of the tip eventually de- 

 velops into the mature thallus and in several species 

 is attached to the host wall by the persistent zoo- 

 spore case and infection tube. With further growtli 

 and increase in diameter, the thallus of the more ex- 

 tensive and elongate species develops a few side 

 branches and numerous protuberances which often 

 make it very irregular and crooked. As its ends ap- 

 proach the limits and cross walls of the host, they 

 may either penetrate into adjacent cells or double 



back in the same cell (fig. 34). Very shortly trans- 

 verse septa are formed at more or less regular inter- 

 vals in such thalli, and they thus are divided into a 

 linear series of elongate, cylindrical or irregular 

 segments. In some species like L. Closterii, L. 

 marchalianum, L. giganteum, etc. (figs. 34, 37, 54) 

 the thallus is quite narrow, mycelioid and Pythium- 

 like, while in L. pygmaeum, L. Cyclotellae, L. 

 Oedogonii, and L. oophilum it may be reduced to a 

 single globular cell as in Olpidium (figs. 25, 39, 49, 

 60). 



The protoplasm of the thallus includes numerous 

 fairly large refractive bodies which give it the whit- 

 ish, refringent gleam characteristic of the family 

 Lagenidiaceae. In addition several small vacuoles 

 are usually present. The segments of the thallus or 

 whole thalli are transformed directly into sporangia, 

 oogonia and antlieridia. In the case of sporangia the 

 small vacuoles may run together to form a large cen- 

 tral one by the time the exit tubes have developed. 

 As sporogenesis approaches, the large refractive 

 bodies apparently break up into smaller fragments 

 and become highly dispersed, so that the protoplasm 

 loses much of its refractive appearance and becomes 

 more greyish granular. The exit tubes vary consider- 

 ably in length, diameter, shape, contour, and the ex- 

 tent to which they project beyond the host wall. 

 They may be inflated at the base or just before they 

 pass through the host wall, constricted or uncon- 

 stricted. straight, curved, irregular or tortuous, and 

 end almost flush with the surface of the host, or ex- 

 tend considerably beyond it. 



In all species except L. pygmaeum, L. Cyclotellae, 

 and L. oophilum the protoplasm of the sporangium 

 is reported to emerge at maturity and form a spheri- 



plate 19 

 Lagenidium rabenhorstii 



(Figs. 1-14, 16 after Zopf, '84; figs. 15, 17 after Cook, '35; 

 figs. 18-22 after Wildeman, '96.) 



Fig. 1. Early infection stages of Spirogyra cells. 

 Figs. 2-4. Successive developmental stages of thallus. 

 Fig. 5. Sporangium with emerged contents surrounded 

 by a membrane. 



Figs. 6-8. Successive stages of cleavage and maturation 

 of the zoospores. 



Fig. 9. Small, reduced thallus with zoospores swarming 

 in a vesicle. 



Fig. 10. Mature zoospore. 



Figs. 11-14. Stages in fusion of the contents of antheri- 

 dium and oogonium. 



Fig. 15. Prefusion stage showing differentiation of an 

 egg cell in the oogonium. 



Fig. 16. Mature oospore in oogonium of an elongate 

 thallus. 



Fig. 17. Single, large zoospore produced by germinated 

 oospore, 



Lagenidium ( Lagenidiopsis) redmetwm 



Figs. 18, 19. Young and elongate unicellular thalli. 

 Fig. 20. Antheridium, oogonium, and warty oospore. 



