THE OCCURRENCE OF ZOOSPORES IN PHTSOLINUM PRINTZ

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1 THE OCCURRENCE OF ZOOSPORES IN PHTSOLINUM PRINTZ BY E. A. FLINT Christchurch, New Zealand {Received i August 1958) (With I figure in the text) Features of considerable taxonomic importance have not been referred to in the earlier descriptions of Physoliniim monile (de Wildem.) Printz. Although Skuja (1949) suspected their existence, there is no record of biflagellate zoospores having been seen, nor is there any reference to the distinction between the erect and the prostrate systems of the thallus. These omissions may be due to the fact that P. monile grows very slowly in culture, or they may be the result of the frequent use of preserved material. In 1954 a few filaments of this alga, present in a sample of Trentepohlia sp. and leafy liverworts, were grown in a medium consisting of i % agar dissolved in either Rodhe's No. 8 solution (1948) or in Detmer's solution as modified by Bold (1942); soil extract (De, 1939) was added to both media. The cultures were illuminated constantly with a 40 watt (white) AtJas fluorescent lamp and 30 watt Ekco strip lighting, set at approximately 60 cm above the cultures. When a filament of Physolinum was transferred to nutrient agar, it first produced an irregularly branched prostrate system, the cells of which were oval and inflated (17-36 ^ long, \x wide). On the average, the ratio of the length to width was 1.4, the range of this ratio being i to 2,4, The septa were fiat and varied between 2 and 6 [x in width {see Fig, ib). The erect system of irregularly branched threads developed later and consisted of cells which were longer than those of the prostrate system (24-46 pi long, \x wide). The average of the ratio of the length to width was 2.0 and the range was 1.5 to 3,3. The septa varied between 4 and 8 Ji in width and pits were not seen in any of them {see Fig, i A, B), There was no difference between the main axes and lateral branches, but isolated threads of the prostrate and erect systems of the cultured material could be distinguished by the shape and colour of the cells. The cell wall was thin, smooth and colourless, and it was noticeable that the cells of the erect threads were not easily wetted. Pectose caps were not seen. When cultured material was examined by X-ray analysis, a very clear diagram of cellulose I was obtained (Nicolai, 1956), a diagram similar to this has been produced by at least one species of Trentepohlia. Since the threads of Fhysolintini were too short to be arranged in parallel bundles, a powder diagram was prepared with the result that no observations could be made on the orientation of the cellulose I. These results agree with the observations made by Nicolai and Preston (1952, p. 271) who found cellulose II in the lower forms of algae. Cellulose I occurred in the higher forms and appeared first in Trentepohlia. The parietal chloroplasts were either ribbon-like or broken up into small discs, the pigment being denser in the cells of the prostrate threads than in those of the erect 267

2 268 E. A. FLINT system and the globules of haematochrome more numerous in the cells of the older than in those of the younger plants. Pyrenoids and starch were absent. Although the cells usually contained one nucleus with a conspicuoiis nucleolus, a few cells, abnormal in size and shape and with dense contents, were found sometimes at the centre of the prostrate system and may have been multinucleate. Fig. I. Tientepohlia iikiiiite dc Wildcm. Al, threads of erect system; A2 and B, threads of prostrate system; C, cells showing chloroplasts (ch) and haematochrome granules (h); D-I, stages in the fonnation of buds, nucleus (n); ic-n, zoospores; O-U, sporelings; X, young sporangia Magnification: A, B, O-U, 500, C-I, X 810, K-N 870.

3 Occurrence of zoospores in Physolinum Printz 269 The filaments extended by apical growth but any cell gave rise to one or more branches. New cells arose as colourless buds, which developed at the side or the tip of the parent cell. As a bud grew, chlorophyll appeared in it, the nucleus of the parent cell divided and later a septum was formed across the narrow neck, separating the bud from the parent cell. Many budding cells were examined but the passage of the daughter nucleus into the bud and the formation of the septum were not seen {see Fig, id-i), Multiplication took place by fragmentation which was particularly common in the erect system, and by the production of zoospores or aplanospores. The sporangia, usually recognized by their subspherical shape (18-20 p long, \i. wide), short hyaline beak and by their dark green colour, were always sessile; they were terminal or lateral, solitary or two or more in a row and developed on the prostrate system of plants which had been growing for at least three months {see Fig. ix), de Wildeman (1891), Karsten (1891) and Ley (1948) did not find sporangia and Printz (1921) comments on the fact that he rarely found them in the South African specimens. When cultures containing mature sporangia were irrigated with water, between four and sixteen zoospores were hberated through the beak into a colourless vesicle, from which they quickly escaped. They were naked, biflagellate cells (9-14 u long, 6-10 ^ wide, ciha \x long), and when swimming, were elongated with a truncate posterior end and a colourless, pointed anterior end. They contained a single parietal chloroplast and numerous globules of haematochrome but contractile vacuole and eyespot were absent, the cells showing no response to light {see Fig. ik-n). Some abnormal and short-lived zoospores lacked a chloroplast, others which were at first flattened, cordate or bifid, later assumed the typical shape. After a period of motility, the zoospores came to rest, rounded off and formed a cell wall. Within 2 or 3 weeks the first bud developed and in 5 weeks the sporeling consisted of four or five cells. Although two buds sometimes arose from one zoospore, the latter was always recognizable for a long time by its spherical shape (see Fig. io-u). The contents of the sporangia sometimes behaved as aplanospores, an observation already made by Printz (1921, 1939) and by Skuja (1949), but there is no evidence that the zoospores behaved as gametes. The new facts emerging from this study are relevant to any consideration of the taxonomic position of Physolinum. The alga was first identified as Trentepohlia monile (de Wildeman, 1888) and then as T. moniliformis by Karsten (1891). Later, on account of the form of the thallus, the presence of aplanospores and the type of cell-division, Printz (1921) transferred it from Trentepohlia to a new genus Physolinum. Now, however, it is clear that in the production of zoospores, in the presence of a thallus consisting of irregularly branched erect and prostrate systems and in its habitat, Physolinum resembles rather than differs from Trentepohlia. With regard to the two interdependent characters, the shape of the cells and the way in which new cells are formed as papillae or buds, it is suggested that although these features may help in distinguishing species, they are not of sufficient taxonomic value to be used in defining genera. Consequently, the retention of the name Physolinum is no longer justified but a new diagnosis of the alga becomes necessary: Diagnosis. Trentepohlia monile de Wildem. (synonyms: T. moniliformis Karsten, T. monile f. hyalina Schmidle, Physolinum monile (de Wildem,) Printz). Thallus composed of irregularly branched erect and prostrate moniliform threads. Cells oval, swollen, those of erect system being longer (24-46 v- long, pi wide, septa 4-8 \x wide) than those of the prostrate system (17-36 u long, i wide, septa 2-6 p wide). Cell wall thin, smooth, colourless; chloroplasts are parietal, disc

4 270 E. A. FLINT or ribbon shaped; haematochrome is present but pyrenoids and starch are absent. Growth is apical, new cells arising as lateral or terminal buds. Multiplication occurs by fragmentation, aplanospores or zoospores. Sporangia are sessile, sub-spherical, beaked, terminal or lateral on the prostrate system. Zoospores (four to sixteen in a sporangium) are naked biflagellate cells (9-14 ^ long, 6-10 M wide, cilia pi long) and contain one parietal chloroplast and globules of haematochrome. Eyespot and contractile vacuole are absent. Gametes and akinetes are not known. The alga is widespread in the Tropics where it is found on stone walls or as an epiphyte on tree trunks. It may not be inappropriate to comment on the report of 'Physolinutn' having been found in Britain (Kahn, 1951). The alga which was collected from a tree trunk and grown on nutrient agar, differs from cultured Nigerian material in the shape of the cells and in the presence of T-shaped cells (the latter have not been recorded in Trentepohlia monile); in the presence of arched septa and in the lamellose condition of the cell wall; in the development of a long beak and in the liberation of separate zoospores from ripe sporangia. In view of these differences it is suggested that until more is known of this alga, it should not be regarded as T. monile de Wildeni. and that the geographical range of the latter species, so characteristic of the tropical regions (Nielsen, 1954) should not, at present, be extended to include a temperate zone of the world. The writer would thank Dr. J. W. G. Lund and Dr. E. Nicolai for their help in this study, and Miss M. Eox who collected the original specimens from the bark of a cocoa tree growing near Ibadan, Nigeria. REFERENCES BOLD, H. C. (1942). The cultivation of algae. Bot. Rev., 8, 69. DE, P. K. (1939). The role of blue-green algae in nitrogen fixation in rice fields. Proc. roy. Soc, B., 127, 121. KARSTEN, G. (1891). Untersuchungen uber die Familie der Chroolepideen. Ann. Jard. bot. Buitenz., 10, I. KHAN, A. S. (1951). On the occurrence oi Physul/niiin monile (de Wildem.) Printz in England. Hydrobiohgia, 3, 79. LEY, S. H. (1948). The subaerial algae from the Paracel Islands in the South China Sea. Bot. Bull. Acad. sinica., 2 (4), 235. NICOLAI, E. (1956). Private communication. NICOLAI, E. & PRESTON, R. D. (1952). Cell wall studies in the Chlorophyceae. I. A general survey of submicroscopic structure in filamentous species. Proc. rov. Soc, B., 140, 244. NIELSEN, C. S. (1954)- The distribution oi Physolimim. Bull.'Torrey bot. CL, 81, 176. PRINTZ, H. (1921). Subaerial algae from South" Africa. K. norske vidensk. Setsk. Skr., i, 3, PRINTZ, H. (1939). Vorarbeiten zu einer Monographie der Trentepohliaceen. jvv^. Mag. Naturv., 80, 137. RoDHE, W. (1948). Environmental requirements of fresh water plankton algae. Svmb. bot npsaliens 10 (i), I. SKUJA, H. (1949). Zur Siisswasseralgen Flora Burmas. Nova Acta Soc Sci. upsal., ser. 4, 14, 5. DE WiLDEMAN, E. (i88s). Sur quelques formes du genre Trentepohlia. Bull. Soc. Bot. Belg. 27, 178. DE WiLDEMAN, E. (1891). Les Trentepohlia des Indes Neerlandaises. Ann. Jaid. bot. Buitenz., 9 "127

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