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Spore liberation by water drops in some myxomycetes
[ 615 ] Trans. Brit. mycol. Soc. 46 (4), 615-619 (1963)'
SPORE LIBERATION BY WATER DROPS IN SOME MYXOMYCETES By P. A. DIXON Birkbeck College, University ofLondon (With Plate 39) Spore liberation by falling water drops has been demonstrated in four myxomycetes. In Lycogala epidendrum not only does the aethalium behave as a puffball bellows mechanism operated by falling drops, but the ostiole through which the spores escape is itself produced by drop bombardment. In Fuligo septica and Tubifera ferruginosa the aethalium is ruptured by impacting water drops and clouds of spores are released. In Leocarpus fragilis the water drop acts as a crushing agent rupturing the individual sporangia and exposing the spores.
Observations on spore liberation in the Myxogastrales are few. Brodie & Gregory (1953), using a wind tunnel, showed that the spores can be liberated from such diverse forms as the aethalium of Fuligo septica (L.) Weber and the sporangia of Craterium minutum Fr. with wind speeds of only 0'5 m./sec. Massee (1892) discussed the importance of the capillitium in spore release, and Ingold (1939) has shown how the hygroscopic movements of the capillitial elaters may bring about the release of the spores in Trichia persimilis Karst. In other aethalial and sporangial forms the way the spore-mass becomes exposed to the dispersive activity of air currents is often obscure. To ensure the dehiscence of the fructification an external disruptive force is usually required and under natural conditions this may be provided by falling raindrops. This possibility was investigated using the aethalia of Lycogala epidendrum (L.) Fr. and Fuligo septica, the pseudoaethalium of Tubiferaferruginosa (Batch) Gmel., and the discrete sporangia of Leocarpusfragilis (Dicks.) Rost. Fructifications were collected in the submature state and allowed to ripen under protected conditions to ensure that no damage occurred. They were then used as targets for bombardment by water drops of 2 or 4 mm. diam. falling from heights of o- 1 -4 m.
Lycogala epidendrum The mature fructification is a subspherical aethalium 0'2- I' 5 em, diam. and resembles a miniature, but non-ostiolate, puff-ball. The peridium, 8-1O}L thick, is entire and consists of two layers. The outer layer, which has superficial vesicles forming warts, is composed of an interlacing series of flattened tubules. These penetrate the inner homogeneous layer and are continuous with the numerous internal pseudocapillitial threads. These consist of freely branched and anastomosing, thin-walled, flattened tubes, 6-25 }L wide, marked with transverse folds and wrinkles. Intermixed with
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Transactions British Mycological Society
the pseudocapillitum are the spherical spores, 6-8 1-' diarn. At maturity the wall of the aethalium is dry and brittle. Initial experiments were made with clusters of 5-6 aethalia and water drops 4 mm. diam. falling from a height of 4 m. The first drop striking the cluster caused immediate rupture of the individual fructification struck, with the formation of an ostiole situated in a more or less apical position and the release of a small puff of spores. Subsequent drops were allowed to fall so that the whole area of the cluster was impacted and the spores were released in puffs as each aethalium was ruptured. The ostioles with successive impactions increased in size until, sometimes, the initial, approximately circular apertures became elongated gaping slits. PI. 39, fig. 3 a-e, illustrates the sequence of events occurring when 300 water drops each of 2 mm. diam. were allowed to fall from a height of 2 m. on to a single aethalium I ern, diam. The first six drops caused no damage, but the seventh ruptured the aethalium with the formation of a minute apical ostiole o· I mm. across and at the same time a very small puff of spores was observed. The ostiole was enlarged by further drops and by the tenth drop was I mm. across (PI. 39, 3 b). With the increasing size of the ostiole the puffs of spores increased in size (PI. 39, fig. I). Up to the sixtieth drop the ostiole continued to enlarge (PI. 39, fig. 3c), but thereafter remained of constant size. Puffing, which remained vigorous whilst the aethalium retained its shape, decreased markedly as flattening of the peridium began to occur after 240 drops (PI. 39, fig. 3 d), and ceased almost entirely after 300 had fallen on the fructification (PI. 39, fig. 3 e). Such a fruit-body I cm. across contains 800-1000 million spores. The calculated average number of spores released in each puff, assuming that 300 effective ejections occurred, is 2-3 million. Actual determinations show that the number of spores released in a puff may vary from I to 5 million. Under identical conditions smaller aethalia 0'2-0'5 em. diam. responded in a similar fashion, except that rupture usually occurred with the first impacting drop. Larger water drops 4 mm. diam., falling from the same height (2 m.), caused more rapid dehiscence and exhaustion of the aethalia. A I em. aethalium, ruptured by the first drop, was completely collapsed by the 47th drop. Although no exact estimates were made, the rate of spore release is obviously much higher. Fruit-bodies 0'2-0'5 em. diam. were very rapidly deformed, often no more than 10-15 puffs were observed before the fruit-body was completely flattened. Using a drop 2 mm, diam. and varying the height offall, it was possible to establish the minimum fall necessary to cause rupture and puffing. With a fall ofless than I m. no rupture occurred in aethalia irrespective of their size or of the number of drops allowed to fall on them. However, once an ostiole had been formed, puffing occurred with a fall of only 0'2 m. The effect of wetting was investigated by submerging fructifications in water for 2-3 days before subjecting them to bombardment. The peridium prevented the entry of water and the contained spore-mass remained dry. These fruit-bodies behaved like ordinary dry untreated specimens.
Spore liberation in myxomycetes. P. A. Dixon
617
Fuligo septica The fruit-body is an aethalium 1-20 ern. in extent, consisting of a series of interwoven tubular sporangia with fragile walls. O verlying the mass of sporangia is a yellow calcareous spongy cortex of varying thickness. All the collections used for water-drop studies were 2-3 em. in extent and had a thick unbroken cortex. The sporangia contain slender anastomosing capillitial threads, with lime-knots at the junctions, and spherical spores 7-10 fL diam. The crust, though fragile, providing it is unruptured, is not broken down nor are spores released by wind speeds of up to 10 m./sec. However, a single drop ( 2 mm. diam.), falling 1 m., had sufficient energy to tear portions of th e crust away, exposing the spores (PI. 39, fig. 6a, b), and subsequent drops produced large clouds of spor es and eventually the entire organization of the aethalium was destroyed. The splash droplets surrounding the fruit-body radiating up to 25 ern. contained large numbers of wetted, entrapped, spores. Tubifera ferruginosa The fruit-body is composed of a large number of densely clustered, elongate sporangia, 4 x 0'5 mm. , borne on a thick sterile hypothallus and forming a pseudoaethalium. At maturity, the sporangial apices are persistent and the spores remain unexposed. Bombardment of the fruit-body by 2 mm. or 4 mm . wat er-drops from a height of only 1 m. produced an immediate rupturing of all the spora ngia in th e area of impact with the release ofa cloud of spores (PI. 39, fig. 2). Subsequ ent drops, striking the sam e region, caused considerabl e damage, penetrating deeply into the hypothallus. Pl. 39, fig. 4, shows the effect of five 4 mm. drops falling 1 m. on to a mature fruit-body. The crater penetrates as far as the hypothallus and ruptured spora ngia are shown on th e periphery of the hole.
Leocarpus fragilis The obovoid, 3 mm. long, yellow-brown sporangia ar e discrete, but usually gregarious. They ar e borne on short lax stalks which are extensions of th e common membraneous hypothallus. The peridium is duplex with a shiny brittle cartilaginous outer wall and a thin membraneous inner wall. Within the sporangium is a system of capillitial threads with angular limeknots and a mass of sph erical spinulose spores 9-13 fL diam. Dehiscence of th e sporangial wall is described as 'irregular', the wall sometimes formin g revolute floriform lobes (Lister, 1925). Bombardment with 2 mm . wat er drops from heights of 1-4 m. caused no dam age to the sporangia. T wo effects, however, were observed when 4 mm . drops fell from 4 m. After a few drops had struck the target sporangia began to rupture, exposing the spores. The sporangial wall broke into irr egular lobes which on dr ying became slightly revolute (PI. 39, fig. 5). Splash droplets reflected around th e targ et contained many entra pped spores. The lax stalks allowed the sporangia to swing and change their position with every impact of a drop and several sporangia were
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Transactions British Mycological Society
eventually torn from their stalks and flung away in reflected droplets to distances of 6-10 em. The stalk remained firmly attached to the hypothallus, but the sporangium was torn away leaving a small ring of peridium at the apex of the stalk. DISCUSSION
De Bary (1887) noted that the 'rind in Lycogala and Reticularia tears irregularly and perhaps spontaneously at the apex' and Martin (1949) stated that dehiscence is apical but did not describe the mechanism of dehiscence. The behaviour of the Lycogala fruit-body under the action of water drops indicates that the aethalium once ruptured behaves like the puff-ball (Lycoperdon). Gregory (1949), using L. perlatum Pers., described the bellows action in which momentary invagination of the peridium by an impacting raindrop leads to the ejection of a puff of spores through the ostiole; the peridium returns to its original position owing to the resilience of the internal capillitial threads. The observation on Lycogala suggests that the larger specimens of Reticularia may behave in a similar manner and act like puff-balls after rupture of the peridium has occurred. The results of experiments with aethalia of Fuligo and Tubifera indicate the importance of mechanical rupture for spore release, and the possible role that water drops may play. Other large aethalial forms, e.g. those of Enteridium and Mucilago, may well behave in a similar manner. The rupture of the sporangia of Leocarpus is again a mechanical effect. An interesting feature is the lifting away from the fruiting region of almost entire sporangia. These being dry and open at one end could act as mobile sifters and be moved about by wind after their initial splash liberation. A typical raindrop is 2 mm. diam. (Mason, 1962) and has a steady terminal velocity of650 csu.lscc. In the experiments reported here the drop will have reached 90 % of the terminal velocity after falling 4 m. and almost reached 80 % when the fall is only 2 m. Thus the drops used are closely comparable, as spore liberators, with raindrops or with drips from trees. Actual observations of spore liberation in Lycogola and Tubifera caused by raindrops under natural conditions are consistent with the experimental evidence. These studies indicate the possible importance of water drops in spore liberation in myxomycetes and suggest that in many genera, though wind can play an important part, as shown by Brodie & Gregory (1953), water drops may well be initiators of the processes of spore dispersal. REFERENCES
BRODIE, H.]. & GREGORY, P. H. (1953). The action of wind in the dispersal of spores from cup-shaped plant structures. Canad.]. Bot. 31,402-410. DE BARY, A. (1887). Comparative morphology and biology oj thefungi, mycetozoa and bacteria. Oxford: Clarendon Press. GREGORY, P. H. (1949). The operation of the puff-ball mechanism of Lycoperdon perlatum. Trans. Brit. mycol. Soc. 32, 11-15. INGOLD, C. T. (1939). Spore discharge in landplants. Oxford. LISTER, A. (1925). A Monograph oj the Mycetozoa, 3rd edn. Revised by G. Lister. London: Brit. Mus. (Nat. Hist.). MARTIN, G. W. (1949). North American Flora. I: Myxomycetes. New York Bot. Gdn. MASON, B.]. (1962). Clouds, rain and rainmaking. Cambridge University Press. MASSEE, G. (1892). A monograph of the Myxogastres. London: Methuen.
T rans. Brit. mycol. Soc.
Vol. 46, Plate 39
d
601
6b (Facl1lg p. 619 )
Spore liberation in myxomycetes. P. A. Dixon EXPLANATION OF PLATE
619
39
Fig. 1. Lycogala epidendrum. Puff of spores released by 4 mm. water-drop falling 1 m. x 3. Fig. 2. Tubifera ferruginosa, Cloud of spores released by 4 mm. water-drop falling 1 m. x 2. Fig. 3. Lycogala epidendrum. Effect of 2 mm. water-drops falling 2 m. a, Unruptured fruit-body; b, ostiole formed after impact of 10 drops; c,ostiole size after 120 drops; d, beginnings of compression after bombardment with 240 drops; e, increased compression after 300 drops. x 3. Fig. 4. Tubiferaferruginosa. Effect of five 4 mm. drops falling 1 m, x 4. Fig. 5. Leocarpus fragilis. Ruptured sporangia after bombardment with 10 drops (4 mm, diam.) falling from 4 m. x 4. Fig. 6. a, b. Fuligo septica. Effect of two 4 mm. drops falling 1 m. x 2.
(Acceptedfor publication 23 March 1963)
SPORE LIBERATION BY WATER DROPS IN SOME MYXOMYCETES By P. A. DIXON Birkbeck College, University ofLondon (With Plate 39) Spore liberation by falling water drops has been demonstrated in four myxomycetes. In Lycogala epidendrum not only does the aethalium behave as a puffball bellows mechanism operated by falling drops, but the ostiole through which the spores escape is itself produced by drop bombardment. In Fuligo septica and Tubifera ferruginosa the aethalium is ruptured by impacting water drops and clouds of spores are released. In Leocarpus fragilis the water drop acts as a crushing agent rupturing the individual sporangia and exposing the spores.
Observations on spore liberation in the Myxogastrales are few. Brodie & Gregory (1953), using a wind tunnel, showed that the spores can be liberated from such diverse forms as the aethalium of Fuligo septica (L.) Weber and the sporangia of Craterium minutum Fr. with wind speeds of only 0'5 m./sec. Massee (1892) discussed the importance of the capillitium in spore release, and Ingold (1939) has shown how the hygroscopic movements of the capillitial elaters may bring about the release of the spores in Trichia persimilis Karst. In other aethalial and sporangial forms the way the spore-mass becomes exposed to the dispersive activity of air currents is often obscure. To ensure the dehiscence of the fructification an external disruptive force is usually required and under natural conditions this may be provided by falling raindrops. This possibility was investigated using the aethalia of Lycogala epidendrum (L.) Fr. and Fuligo septica, the pseudoaethalium of Tubiferaferruginosa (Batch) Gmel., and the discrete sporangia of Leocarpusfragilis (Dicks.) Rost. Fructifications were collected in the submature state and allowed to ripen under protected conditions to ensure that no damage occurred. They were then used as targets for bombardment by water drops of 2 or 4 mm. diam. falling from heights of o- 1 -4 m.
Lycogala epidendrum The mature fructification is a subspherical aethalium 0'2- I' 5 em, diam. and resembles a miniature, but non-ostiolate, puff-ball. The peridium, 8-1O}L thick, is entire and consists of two layers. The outer layer, which has superficial vesicles forming warts, is composed of an interlacing series of flattened tubules. These penetrate the inner homogeneous layer and are continuous with the numerous internal pseudocapillitial threads. These consist of freely branched and anastomosing, thin-walled, flattened tubes, 6-25 }L wide, marked with transverse folds and wrinkles. Intermixed with
616
Transactions British Mycological Society
the pseudocapillitum are the spherical spores, 6-8 1-' diarn. At maturity the wall of the aethalium is dry and brittle. Initial experiments were made with clusters of 5-6 aethalia and water drops 4 mm. diam. falling from a height of 4 m. The first drop striking the cluster caused immediate rupture of the individual fructification struck, with the formation of an ostiole situated in a more or less apical position and the release of a small puff of spores. Subsequent drops were allowed to fall so that the whole area of the cluster was impacted and the spores were released in puffs as each aethalium was ruptured. The ostioles with successive impactions increased in size until, sometimes, the initial, approximately circular apertures became elongated gaping slits. PI. 39, fig. 3 a-e, illustrates the sequence of events occurring when 300 water drops each of 2 mm. diam. were allowed to fall from a height of 2 m. on to a single aethalium I ern, diam. The first six drops caused no damage, but the seventh ruptured the aethalium with the formation of a minute apical ostiole o· I mm. across and at the same time a very small puff of spores was observed. The ostiole was enlarged by further drops and by the tenth drop was I mm. across (PI. 39, 3 b). With the increasing size of the ostiole the puffs of spores increased in size (PI. 39, fig. I). Up to the sixtieth drop the ostiole continued to enlarge (PI. 39, fig. 3c), but thereafter remained of constant size. Puffing, which remained vigorous whilst the aethalium retained its shape, decreased markedly as flattening of the peridium began to occur after 240 drops (PI. 39, fig. 3 d), and ceased almost entirely after 300 had fallen on the fructification (PI. 39, fig. 3 e). Such a fruit-body I cm. across contains 800-1000 million spores. The calculated average number of spores released in each puff, assuming that 300 effective ejections occurred, is 2-3 million. Actual determinations show that the number of spores released in a puff may vary from I to 5 million. Under identical conditions smaller aethalia 0'2-0'5 em. diam. responded in a similar fashion, except that rupture usually occurred with the first impacting drop. Larger water drops 4 mm. diam., falling from the same height (2 m.), caused more rapid dehiscence and exhaustion of the aethalia. A I em. aethalium, ruptured by the first drop, was completely collapsed by the 47th drop. Although no exact estimates were made, the rate of spore release is obviously much higher. Fruit-bodies 0'2-0'5 em. diam. were very rapidly deformed, often no more than 10-15 puffs were observed before the fruit-body was completely flattened. Using a drop 2 mm, diam. and varying the height offall, it was possible to establish the minimum fall necessary to cause rupture and puffing. With a fall ofless than I m. no rupture occurred in aethalia irrespective of their size or of the number of drops allowed to fall on them. However, once an ostiole had been formed, puffing occurred with a fall of only 0'2 m. The effect of wetting was investigated by submerging fructifications in water for 2-3 days before subjecting them to bombardment. The peridium prevented the entry of water and the contained spore-mass remained dry. These fruit-bodies behaved like ordinary dry untreated specimens.
Spore liberation in myxomycetes. P. A. Dixon
617
Fuligo septica The fruit-body is an aethalium 1-20 ern. in extent, consisting of a series of interwoven tubular sporangia with fragile walls. O verlying the mass of sporangia is a yellow calcareous spongy cortex of varying thickness. All the collections used for water-drop studies were 2-3 em. in extent and had a thick unbroken cortex. The sporangia contain slender anastomosing capillitial threads, with lime-knots at the junctions, and spherical spores 7-10 fL diam. The crust, though fragile, providing it is unruptured, is not broken down nor are spores released by wind speeds of up to 10 m./sec. However, a single drop ( 2 mm. diam.), falling 1 m., had sufficient energy to tear portions of th e crust away, exposing the spores (PI. 39, fig. 6a, b), and subsequent drops produced large clouds of spor es and eventually the entire organization of the aethalium was destroyed. The splash droplets surrounding the fruit-body radiating up to 25 ern. contained large numbers of wetted, entrapped, spores. Tubifera ferruginosa The fruit-body is composed of a large number of densely clustered, elongate sporangia, 4 x 0'5 mm. , borne on a thick sterile hypothallus and forming a pseudoaethalium. At maturity, the sporangial apices are persistent and the spores remain unexposed. Bombardment of the fruit-body by 2 mm. or 4 mm . wat er-drops from a height of only 1 m. produced an immediate rupturing of all the spora ngia in th e area of impact with the release ofa cloud of spores (PI. 39, fig. 2). Subsequ ent drops, striking the sam e region, caused considerabl e damage, penetrating deeply into the hypothallus. Pl. 39, fig. 4, shows the effect of five 4 mm. drops falling 1 m. on to a mature fruit-body. The crater penetrates as far as the hypothallus and ruptured spora ngia are shown on th e periphery of the hole.
Leocarpus fragilis The obovoid, 3 mm. long, yellow-brown sporangia ar e discrete, but usually gregarious. They ar e borne on short lax stalks which are extensions of th e common membraneous hypothallus. The peridium is duplex with a shiny brittle cartilaginous outer wall and a thin membraneous inner wall. Within the sporangium is a system of capillitial threads with angular limeknots and a mass of sph erical spinulose spores 9-13 fL diam. Dehiscence of th e sporangial wall is described as 'irregular', the wall sometimes formin g revolute floriform lobes (Lister, 1925). Bombardment with 2 mm . wat er drops from heights of 1-4 m. caused no dam age to the sporangia. T wo effects, however, were observed when 4 mm . drops fell from 4 m. After a few drops had struck the target sporangia began to rupture, exposing the spores. The sporangial wall broke into irr egular lobes which on dr ying became slightly revolute (PI. 39, fig. 5). Splash droplets reflected around th e targ et contained many entra pped spores. The lax stalks allowed the sporangia to swing and change their position with every impact of a drop and several sporangia were
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Transactions British Mycological Society
eventually torn from their stalks and flung away in reflected droplets to distances of 6-10 em. The stalk remained firmly attached to the hypothallus, but the sporangium was torn away leaving a small ring of peridium at the apex of the stalk. DISCUSSION
De Bary (1887) noted that the 'rind in Lycogala and Reticularia tears irregularly and perhaps spontaneously at the apex' and Martin (1949) stated that dehiscence is apical but did not describe the mechanism of dehiscence. The behaviour of the Lycogala fruit-body under the action of water drops indicates that the aethalium once ruptured behaves like the puff-ball (Lycoperdon). Gregory (1949), using L. perlatum Pers., described the bellows action in which momentary invagination of the peridium by an impacting raindrop leads to the ejection of a puff of spores through the ostiole; the peridium returns to its original position owing to the resilience of the internal capillitial threads. The observation on Lycogala suggests that the larger specimens of Reticularia may behave in a similar manner and act like puff-balls after rupture of the peridium has occurred. The results of experiments with aethalia of Fuligo and Tubifera indicate the importance of mechanical rupture for spore release, and the possible role that water drops may play. Other large aethalial forms, e.g. those of Enteridium and Mucilago, may well behave in a similar manner. The rupture of the sporangia of Leocarpus is again a mechanical effect. An interesting feature is the lifting away from the fruiting region of almost entire sporangia. These being dry and open at one end could act as mobile sifters and be moved about by wind after their initial splash liberation. A typical raindrop is 2 mm. diam. (Mason, 1962) and has a steady terminal velocity of650 csu.lscc. In the experiments reported here the drop will have reached 90 % of the terminal velocity after falling 4 m. and almost reached 80 % when the fall is only 2 m. Thus the drops used are closely comparable, as spore liberators, with raindrops or with drips from trees. Actual observations of spore liberation in Lycogola and Tubifera caused by raindrops under natural conditions are consistent with the experimental evidence. These studies indicate the possible importance of water drops in spore liberation in myxomycetes and suggest that in many genera, though wind can play an important part, as shown by Brodie & Gregory (1953), water drops may well be initiators of the processes of spore dispersal. REFERENCES
BRODIE, H.]. & GREGORY, P. H. (1953). The action of wind in the dispersal of spores from cup-shaped plant structures. Canad.]. Bot. 31,402-410. DE BARY, A. (1887). Comparative morphology and biology oj thefungi, mycetozoa and bacteria. Oxford: Clarendon Press. GREGORY, P. H. (1949). The operation of the puff-ball mechanism of Lycoperdon perlatum. Trans. Brit. mycol. Soc. 32, 11-15. INGOLD, C. T. (1939). Spore discharge in landplants. Oxford. LISTER, A. (1925). A Monograph oj the Mycetozoa, 3rd edn. Revised by G. Lister. London: Brit. Mus. (Nat. Hist.). MARTIN, G. W. (1949). North American Flora. I: Myxomycetes. New York Bot. Gdn. MASON, B.]. (1962). Clouds, rain and rainmaking. Cambridge University Press. MASSEE, G. (1892). A monograph of the Myxogastres. London: Methuen.
T rans. Brit. mycol. Soc.
Vol. 46, Plate 39
d
601
6b (Facl1lg p. 619 )
Spore liberation in myxomycetes. P. A. Dixon EXPLANATION OF PLATE
619
39
Fig. 1. Lycogala epidendrum. Puff of spores released by 4 mm. water-drop falling 1 m. x 3. Fig. 2. Tubifera ferruginosa, Cloud of spores released by 4 mm. water-drop falling 1 m. x 2. Fig. 3. Lycogala epidendrum. Effect of 2 mm. water-drops falling 2 m. a, Unruptured fruit-body; b, ostiole formed after impact of 10 drops; c,ostiole size after 120 drops; d, beginnings of compression after bombardment with 240 drops; e, increased compression after 300 drops. x 3. Fig. 4. Tubiferaferruginosa. Effect of five 4 mm. drops falling 1 m, x 4. Fig. 5. Leocarpus fragilis. Ruptured sporangia after bombardment with 10 drops (4 mm, diam.) falling from 4 m. x 4. Fig. 6. a, b. Fuligo septica. Effect of two 4 mm. drops falling 1 m. x 2.
(Acceptedfor publication 23 March 1963)