31 



THE SIMPLE HOLOCARPIC BIFLAUELLATE PHYCOMVCETES 



presence of a structural cellulose wall by plasnio- 

 lytic experiments and treatment with chloro-iodide 

 of zinc. According to Scherffel and Diehl, the young 

 parasite may change its shape and position and in- 

 dependently undergo slight amoeboid movement, 

 but McLarty maintained that in 0. Achylae, at 

 least, such changes are caused by the rapid stream- 

 ing of the host protoplasm in which the parasite is 

 passively carried along (figs. 12-15). In 0. andreei 

 (P. Ectocarpi) Jokl figured the young thallus as an 

 amoeba with one to several long, more or less radi- 

 ally oriented pseudopods (fig. 162) which migrates 

 towards and engulfs the host nucleus. Fisch and 

 Scherffel also have shown that the young thalli of 

 0. schenkiana {P. parasiticum) , and O. Oedogoni- 

 orum respectively are often to be found in close as- 

 sociation with the host nucleus (fig. 133) which sug- 

 gests that the food supply may be more optimum in 

 that region of the host cell. 



All present-day workers are agreed that no fusion 

 of young thalli or newly-entered zoospores to form 

 a plasmodium occurs in Olpidiopsis. The monospore 

 infection experiments of McLarty and Shanor show 

 conclusively that each zoospore gives rise to a single 

 independent thallus or sporangium. By the time the 

 thallus has attained mature size it is invested by a 

 definite wall which in most species has been shown 

 to give a marked positive cellulose reaction when 

 tested with chloro-iodide of zinc. This wall is evi- 

 dently formed by the parasite itself, but the knobs, 

 warts, and spines which may later appear on it ap- 

 parently have a different origin. As Fischer has 

 shown, globules and masses of transformed host 

 protoplasm accumulate at separate points around 

 the periphery of the thallus wall (fig. 31) and are 

 gradually transformed further into spines, warts, 

 and other excrescences (fig. 32). These spines show 

 no positive cellulose reaction when tested, which 

 further suggests that they are different from the 

 primary wall. Inasmuch as they are formed in this 

 manner, it is to be expected that they will vary 

 markedly in size, length, and shape. As is shown in 

 figures 26 to 30 they may be lacking entirely or cover 

 only a part of the sporangium (fig. 26) and vary 

 from blunt knobs to broad or fine spines. Spininess 

 of the sporangia can therefore no longer be regarded 

 as a specific character. 



The protoplasmic changes which occur during the 

 growth, development, maturation and cleavage of 

 the thalli and zoosporangia have been intensively 

 studied in living as well as fixed material of several 

 Olpidiopsis species, particularly O. Saprolegnia, O. 

 Aphanomycis, and O. Achlyae. Successive stages of 

 these changes are illustrated in figures 16 to 25 of 

 two sporangia of O. Achlyae. The young developing 

 thallus usually includes numerous fatty refractive 

 bodies, and as it grows in size, the latter increase in 

 number and size and impart a granular and slightly 

 yellowish gleam or refringent appearance to the 

 protoplasm (fig. 16). With further development 

 these globules gradually become broken up into 

 bodies of smaller size and appear as oily droplets 



suspended in the more homogeneous protoplasm (fig. 

 17). At this stage of development small vacuoles ap- 

 pear in the protoplasm (fig. 17), and as they become 

 more distinct they begin to coalesce. The protoplasm 

 at this stage appears granular and slightly brown in 

 appearance, and when stained with Sudan III it be- 

 comes brick-brown in color. Coalescence of the vacu- 

 oles continues until one or more large central ones 

 are formed (fig. 19). Within an hour following this 

 stage the vacuoles begin to undergo changes in shape 

 (fig. 20) which may continue for a few minutes to 

 half an hour. Cleavage furrows then begin to form 

 at the periphery of the vacuoles and travel centri- 

 fugallv to the plasma membrane (fig. 21) and de- 

 limit the initial zoospore segments (fig. 22). How- 

 ever, in O. Achlyae the areas previously occupied by 

 the vacuoles as such do not disappear as the cleav- 

 age furrows cut through the plasma membrane as 

 Schwartze ('22) described for 0. Saprolegniae. In- 



plate 9 



Olpidiopsis A chlyae 

 (All figures after McLarty, '41) 



Fig. 1. Slightly bean-shaped living zoospore with vacu- 

 oles and refractive granules. 



Fig. 2. Fixed and stained zoospore with large nucleus. 

 Slightly unequal flagella inserted beside a deep-staining 

 body near the anterior end. 



Fig. 3. Amoeboid zoospore. 



Fig. 4. Zoospore retracting flagella before going into a 

 temporary rest period. 



Figs. 5, 6. Zoospore at rest. 



Fig. 7. Zoospore germinated in water with a branched, 

 rhizoid-like germ tube. 



Fig. 8. Zoospore at rest and encysted on surface of host 

 hypha. 



Figs. 9-11. Successive infection stages. 



Figs. 12-15. Changes in shape of the newly-entered 

 parasite due to the streaming of the host protoplasm. 



Fig. 16. Two incipient zoosporangia surrounded by a 

 dense layer and radiating strands of the host protoplasm. 



Figs. 17-20. Successive maturating stages of two zoo- 

 sporangia. 



Figs. 21-25. Cleavage and sporogenesis of lower spo- 

 rangium shown in previous figures. 



Fig. 26. Smooth and partly-spiny zoosporangia. 



O. variant 



Figs. 27-30. Variations in the character of the sporan- 

 gium wall. Shanor, '39. 



O. fusiformis 



Figs. 31, 32. Formation of spines on zoosporangia. 

 Fischer, '82. 



0. Vexans 



Figs. 33-37. Successive stages in the development of a 

 thallus from fixed and stained material. Nuclear division 

 simultaneous and completely synchronous. Barrett, '12. 



Fig. 38. Portion of a zoosporangium following cleavage. 

 Vacuolar areas still present. Barrett, I.e. 



