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Recent Work on the reproduction of ascomycetes
RECENT WORK ON T H E REPRODUCTION OF ASCOMYCETES.
By H. C. I. Frasw, D.Sc. (Lond) F.L.S. The method of reproduction among Ascomycetes has been perhaps one of the most disputed of mycological problems, and is certainly one the unravelling of which has gone through a curious series of vicissitudes. Bulliard, in I 791, in his history of the " Champignons de France," put forward the suggestion that the ascus was a female organ fertilized by some substance emanating from the paraphyses. Systematic study of the question was initiated by De Bary and his pupils in the middle of the last century; they investigated a considerable number of species and recognised that, before spore formation took place, a special filament appeared, fused with another, branched, and a t the ends of its branches gave rise to asci. De Bary inferred that the branching filament was female, the filament with which it conjugated male, and the asci the product of an act of normal fertilization. Subsequently he pointed out that, in certain forms, one or both filaments were absent and that in these cases asci must be developed either from an unfertilized female cell or in the complete absence of sexual organs. De Bary's interpretation of his discoveries was not universally accepted. Brefeld denied the sexual nature of the filaments observed, Van Tieghem regarded them in certain cases as organs of respiration, and other botanists as boring organs. In the meantime considerable advances were being made in the detailed study of sexuality both among animals and plants. It became recognised that an essential part of fertilization was the association of male and female n ~ c l e i ,and that cell fusion was a mere preliminary to this. It was discovered also that every nucleus possesses a constant and definite number of nuclear elements or ckromosomes, and it followed that when two nuclei unite this number -must be doubled. It followed again that before a subsequent fertilization, initiating the next generation, could take place, the original, or half number must be restored, and the point in the life history at which this reduction of the chromosome number to half occurs was recognised for several forms. While the attention of biologists was thus becoming focussed on the nucleus, Dangeard (i5), in 1894, announced that he had observed a fusion 'of closely related nuclei in the ascus of
Peziza vesiculosa and some other forms. He accepted this process as an act of fertilization, defined the ascus as an egg and called upon the upholders of the sexuality of the Ascomycetes to recognise with him that fertilization occurred, and upon its opponents to deny with him that it took place at the stage indicated by De Bary. T h e importance of Dangeard's observation was still scarcely realized when Harper (19) in 1895 confirmed it in Spkaerotlteca kumuli. H e further studied the development of this species and recorded not only nuclear fusion in the ascus, but the presence and union of the filaments described by De Bary. H e saw the entrance of the nucleus of the male filament into the other and its fusion with the female nucleus. Moreover, he made out that it was from this fertilized filament that the ascus arose, and that the nuclei which fused in the ascus were the descendants of nuclei produced by the foregoing act of fertilization. Thus it was clear that in Ascomycetes occurred the unprecedented case of two nuclear fusions succeeding one another in the same life history. Dangeard (7), with little delay, himself undertook the study of Spha~rotkeca kumuZi but did not observe Harper's first fusion ; it is perhaps only necessary to add that its occurrence has since been confirmed by other workers (I) for this species and by Harper himself for the related fornls Erysipke communis (20) and Phyllactinia corylea (22). There seems thus no doubt that the two fusions, even if absent in Dangeard's material, are of common occurrence among Erysipkaceae.In rgoo,the two fusions were seen by Harper in the Discomycete Pyronzma confluens (21), but here the filaments, which we may now follow De Bary in calling sexual organs, contain each not one but several nuclei and fertilization thus consists of the union not of one but of several separate pairs. Later, in 1go5, a similar case was described in detail by Claussen (3) for Boudiera CZaussenii (=Ascodesmis nigricans?) and evidence which makes the occurence of normal fertilization had, in the meantime, been brought forward for very probable several other species. T h e next step was the investigation of the forms described by De Bary or his pupils as possessing a female organ only, and in 1906 an account of the life history of Humaria granulata was published (2). Here the female cell, like that of Pyronema confluens possesses several nuclei, but the male organ has entirely disappeared and the fusion of male with female nuclei is replaced by the union of the female nuclei in pairs. In this, as in other forms, a subsequent fusion takes place in the ascus. In 1ga7 a corresponding discovery was made for Ascobolus f u r f ~ ~ y a c e(25), u ~ and I was so fortunate as to be able to
describe yet another case in Lachnea stercorea ( I 3) ; here, however, the male filament, is still present though, in my specimens it did not function and normal fertilization is again replaced by the fusion of female nuclei in pairs. At about the same time Dangeard (8) ~ublisheda most interesting memoir, in which he described the development of the sexual organs in a number of species, and showed that a passage of male nuclei into the female cell did riot take place in his material. Unfortunately he did not deal with the possibility of the union of female nuclei in pairs but assumed that, as the male filament was not functional, nuclear fusion could not take place at this stage. Towards the end of 1907Claussen carried out a further study of Pyronema conflzlens (4). H e saw the male nuclei pass, in accordance with Harper's description, into the female organ, but he asserted that they did not fuse with the female nuclei but only became associated with them in pairs; the associated nuclei travelled two and two up the ascogenous hyphae and finally fused in the ascus. In other words the ascus fusion was the final union of pairs of sexual nuclei which had become associated some time before in the female organ, and only one fusion took place in the life history of Ascomycetes. Claussen's paper was followed by an account of the life history of Humaria rutilans (14). This fungus may be very conveniently studied, as the nuclei are exceptionally large and clear. I found, with regard to the sexual organs, a stage of reduction more advanced than that in Humaria granulata, since not only the male, but also the female filament has disappeared and fertilization is replaced by the fusion in pairs of vegetative nuclei, the nuclei of the ordinary hyphae. Subsequently the descendants of these nuclei were seen to undergo a second fusion in the ascus. So far this fungus only adds another to the cases in which two fusions have been recorded, but, owing t o the size of the nuclei it was further possible to study, in some detail, their behaviour in the ascus. It had already been pointed out by various authors (Gjurasin (17)~Maire (23), Guillermond (IS), Harper (22), etc.) that the fusion nucleus in the ascus divides three times, and that the first two of these divisions correspond to the two peculiar divisions which, in other o.rganisms, have been shown to bring about that reduction of the number of chromosomes to half, which is the necessary corollary of their doubling in fertilization. In Hlmzarza rutilans it was possible to see very clearly that this was the case, and it was further found that, after the number of chromosomes has been thus reduced to half in the course of the lirst two &visions, the remaining number is again halved during the third. So that, in the course
of the life history of this fungus, the number of nuclear elements is twice doubled by the two successive fusions, and then twice halved during the divisions in the ascus. This state of affairs may be compared diagrammatically with that which occurs in an ordinary plant or animal.
- W T PLANTS AND ANIMALS ,
,ASCOMYCETES,
Humariu rzltilans does not stand alone in these peculiarities, for it has since been shown that similar processes take place in Peziza vesiculosa and Otidea azlrantia (16) ; and certain earlier investigations by Harper (22), Maire (23) and others appear open to a corresponding interpretation. To sum up, there are two main views with regard to this aspect of the sexuality of the Ascomycetes. One, foreshadowed in 1791 by Bulliard, and supported, among others by Dangeard and by Claussen, locates the sexual fusion in the ascus and denies the occurrence of any other nuclear union ; the other, initiated by De Bary and his pupils and extended by Harper and by a number of workers in this country accepts the existence of two fusions, recognises the first, that in the " female" organ, as sexual, and regards the second as peculiar t o Ascomycetes and as compensated by the equally peculiar second reduction. Probably most investigators would now agree in regarding De Bary's filaments as phylogenetically male and female organs whether they had convinced themselves that some form of fertilization still takes place in the latter or whether they held that it had shifted to the ascus. It would seem evident moreover that normal sexuality is at present disappearing among Ascomycetes and that even the same species will, under varying conditions, show various degrees in the structure and dficiency of its Sexual cells.
In this connection, an interesting degeneration series may be traced: in Pyronema confl?cens-to take the more fully investigated Discomycetes only-and in Boudiera, the formation of functional male and female filaments takes place and normal fertilization occurs; in Lacknea stercorea the female organ is still normal, but the male, though present is functionless and the female nuclei fuse in pairs; in Humaria granulata the male filament is absent, and in Humaria rutiCans the female organ also has disappeared and sexual fusion is reduced to a mere union of vegetative nuclei ; finally it may be suggested that in some forms fertilization will have altogether ceased. Perhaps such cases are among those studied by Dangeard and his allies, but, if this be so a corresponding absence of the easily recognisable stages of the halving of the chromosome number is to be expected, and will afford convincing proof of the non-occurrence of its doubling in fertilization. In another direction the study of reproduction among Ascomycetes seems likely to do something towards an understanding of the relationships of at any rate the great groups. In AspergilZus (or Eurotium) kerbariorum it has recently been pointed out (15) that the female organ consists of three parts, a multicellular stalk, a single fertile cell or ascogonizcm, and a single terminal cell or trickogyne through which the male nuclei must pass to reach those of the ascogonium. The fertile cell alone gives rise to ascogenous hyphae, but, before doing so, it divides into several multinucleate portions. In Pyrontmn confluens the female branch is similarly constructed, but the stalk is less conspicuous and the ascogonium, irstead of being long and narrow, is spherical and gives rise to ascogenous hyphae without becoming septate. In Lachnea stercorea the state of affairs is similar, but oddly enough the trichogyne, when mature, consists not of one but of several cells, and that although the now functionless male filament is developed close at hand. Possibly it is the difficulty connected with the breaking down of so many cell walls that has put an end to normal fertilization in this species. The female branch of Humaria granulata, though now without a trichogyne, also conforms t o this type, and that of Ascobolus furfuraceus can probably be derived from it. Similar structures have been seen in other Pezizaceae, which still await full investigation, and it may perhaps be suggested that the Pyrovtema type is characteristic of Discomycetes generally. In Boudiera, however, the structure approximates much more closely to that of AspergiZZus, and the comparatively narrow female cell becomes septate after fertilization. T h e male filament, in AspergiZlus, has a multicellular stalk like that of the female, and terminates in a small cell, the
antkeridium proper, which contains several nuclei. In Boudiera, in Pyronernn, and in such other of the investigated Discomycetes as still show a male organ, the antheridium is much larger and develops as a stout oblong body seated on a short stalk. Among Pyrenomycetes a quite different type of female organ occurs ; in Gnomonia rrytkrostoma (10, 12), in the investigated species of YoZj~stigma (II), in Poronia pzrnctata @), and in various Lichens, it forms a long, coiled structure divided into numerous cells. Beyond the coil is a straight portion terminating externally in a swollen cell and constituting the tricllogyne ; some of the c,ells at the other extremity of the coil form a stalk ; but, since fusion of the sexual nuclei has not yet been observed in these cases, it is impossible to say whether the true ascogonium is multicellular before, or only, as in Aspergillus, after fertilization. In this group the presumptive male organ is again a filament terminating in a small cell, but such tilaments are very numerous and are aggregated in flask-shaped spermagonia and the terminal cell is cut off as a spernzatiuul.t and carried by the wind to the trichogyne It thus seems that the spermatium may be readily homologised with the antheridium of AspergiZZus, and consequently with that of the Discomycetes. This con~parison is the more suggestive since it has recently been shown (26, 27) that among the Red Algae undoubted antheridia are detached from their parent plant, and are carried to the female organ in a corresponding way. There thus appear to be two main types of male and female organ : that characteristic especially of Discomycetes where both antheridium and ascogonium are more or less spherical, where the former does not become detached and the latter (probably for some reason connected with its shape) does not become septatc after fertilization ; and, secondly, that characteristic of a number of Pyrenomycetes in which the female organ is elongated and richly septate and the antheridium is minute and becomes detached and carried by external agencies to the trichogyne. An intermediate, and not improbably primative stage seems t o occur in AspergiZZzrs kerbariorzrtz, and has been indicated in other forms. These main types do not of course include all the very numerous variations of reproductive structure described among Ascomycetes. In the Erysiphaceae, for instance the female organ is without a trichogyne and the antheridium fuses directly with t!le fertile cell. A similar arrangement is found in the genus Gymlooascus (5), and it m y be suggested that the latter, with its slightly developed perithecial wall forms part of F degeneration series terminating perhaps in Eremascus (24), where the asci are borne free on the branches of a simple mycelium.
It is, however, probably still too early to speculate on the relationships which may be indicated by the structure of the sexual organs, and their disappearance has been shown to occur in groups so diverse as to make it without importance in classification. An enormous number of forms still demand investigation, and a further knowledge of their structure and cytology cannot fail to solve some, at any rate, of the problems of this group, and through them, not impossibly, of a -wider field. LIST OF PAPERS CITED.
Blackman, V. H., and Fraser, H. C. I. 1905. Fertilization in Sphaerotheca. Annals of Botany XIX., p. 567.
. 1906. On the Sexuality and Development of the Ascocarp in H u w r i a granulaia. Proc. Royal Soc. London B. LXXVII., p. 354. Claussen, P. 1905. Zur Entwickelungsgeschichte der Ascomyceten. Boudiera. Botanische Zeitung LXIII., p. I. . 1907. Zur Kenntniss der Kernverhaltnisse von Pyronema confluens. Berichte der deut. Bot. Gesell. XXV., p. 586. Dale E. I 903. Observations on the Gymnoascaceae. Ann. Bot. XVII., p. 57I. Dangeard, P. 1894-5, La Reproduction sexuelle des Ascomycktes. L e Botaniste IV., p. 2 I .
. 1897. Second MCmoire sur la Reproduction sexuelle des Ascomyc&tes. Le Botaniste V., p. 245. . 1907. Sur le DCveloppement de la PerithecCe chez les Ascomyc&tes. Le Botaniste X., p. I. Dawson, M. 1900. On the biology of Poronia punctata. Ann. Bot. XIV., p. 245. Fisch, C. 1882. Beitrage zur Entwicklungsgeschichte einiger Ascomyceten. Bot. Zeit. Jahrg 40,p. 850. Frank, B. 1883. Ueber einige neue und weniger bekannte Pflauzenkrankheiten. Berichte der deut. Bot. Gesell. Bd. I., p. 58.
.
Gnomonza erytkrostoma die Ber. der deut. Bot. GeselL Bd. IV.,
1886 Ueber
Ursache, etc. p. 200.
Fraser, H. C. I. 1 9 ~ 7 . On the Sexuality and Development of the Ascocarp in Lacknea stercorea Ann. Bot. XXI., P. 349. -- . 198. Contributions to the Cytology of Humaria rutilans. Ann. Bot. XXII., p. 35. and Chambers, H. S. 1907. The Morphology of AspergiZZus kerbariorum. Annales Mycologici V., P. 4'9. and Welsford, E. J. Further Contributions to the Cytology of the Ascomycetes. Ann. Bot. XXII., P 465. Gjurasin, S. 1893. Ueber die Kernteilung in den Schlaiichen von Peziza vesiczrLosa. Ber. der deut. Bot. Gesell. Bd. XI. Guillermond, M. A. 1904. Remarques sur la Karyokinhe des Ascomyc6tes. Ann. Mycol. III., p. 344. Harper, R. A. I 895. Die Entwickelung des Peritheciums bei Spkaerotkeca castagnei. Ber. der deut. Bot. Gesell. XIII., p. 475. . I 896. Ueber das Verhalten der Kerne bei der Fruchtentwickelung einiger Ascomyceten Jahrb. fur wiss. Bot. XXIX., p. 655. . 19". Sexual Reproduction in Pyronema confluens and the Morphology of the Ascocarp. Ann. B0t. XIV., p. 321. . IPS. Sexual Reproduction and the Organization of the Nucleus in certain Mildews. Publ. Carnegie Institution of Washington, NO. 37, p. I. Maire, R. 1905. Recherches cytologiques sur quelques Ascomyc&tes. Ann. Mycol. III., p. 123. Stoppell, R. 1907. Eremasczcs fertiZis nov. spec. Flora. Bd. 97,P-332Welsford, E. J. 1907. Fertilization in AscoboCus furfuraceus. New Phytologist VI., p. I 56. Wolfe, J. J. 1904 Cytological Studies on Nemalion. Ann. Bot XVIII., p. 607. Yamanouchi, S. 1906. The Life History of PoZysipkonia violacea, Bot. Gazette 42,p. 401.
By H. C. I. Frasw, D.Sc. (Lond) F.L.S. The method of reproduction among Ascomycetes has been perhaps one of the most disputed of mycological problems, and is certainly one the unravelling of which has gone through a curious series of vicissitudes. Bulliard, in I 791, in his history of the " Champignons de France," put forward the suggestion that the ascus was a female organ fertilized by some substance emanating from the paraphyses. Systematic study of the question was initiated by De Bary and his pupils in the middle of the last century; they investigated a considerable number of species and recognised that, before spore formation took place, a special filament appeared, fused with another, branched, and a t the ends of its branches gave rise to asci. De Bary inferred that the branching filament was female, the filament with which it conjugated male, and the asci the product of an act of normal fertilization. Subsequently he pointed out that, in certain forms, one or both filaments were absent and that in these cases asci must be developed either from an unfertilized female cell or in the complete absence of sexual organs. De Bary's interpretation of his discoveries was not universally accepted. Brefeld denied the sexual nature of the filaments observed, Van Tieghem regarded them in certain cases as organs of respiration, and other botanists as boring organs. In the meantime considerable advances were being made in the detailed study of sexuality both among animals and plants. It became recognised that an essential part of fertilization was the association of male and female n ~ c l e i ,and that cell fusion was a mere preliminary to this. It was discovered also that every nucleus possesses a constant and definite number of nuclear elements or ckromosomes, and it followed that when two nuclei unite this number -must be doubled. It followed again that before a subsequent fertilization, initiating the next generation, could take place, the original, or half number must be restored, and the point in the life history at which this reduction of the chromosome number to half occurs was recognised for several forms. While the attention of biologists was thus becoming focussed on the nucleus, Dangeard (i5), in 1894, announced that he had observed a fusion 'of closely related nuclei in the ascus of
Peziza vesiculosa and some other forms. He accepted this process as an act of fertilization, defined the ascus as an egg and called upon the upholders of the sexuality of the Ascomycetes to recognise with him that fertilization occurred, and upon its opponents to deny with him that it took place at the stage indicated by De Bary. T h e importance of Dangeard's observation was still scarcely realized when Harper (19) in 1895 confirmed it in Spkaerotlteca kumuli. H e further studied the development of this species and recorded not only nuclear fusion in the ascus, but the presence and union of the filaments described by De Bary. H e saw the entrance of the nucleus of the male filament into the other and its fusion with the female nucleus. Moreover, he made out that it was from this fertilized filament that the ascus arose, and that the nuclei which fused in the ascus were the descendants of nuclei produced by the foregoing act of fertilization. Thus it was clear that in Ascomycetes occurred the unprecedented case of two nuclear fusions succeeding one another in the same life history. Dangeard (7), with little delay, himself undertook the study of Spha~rotkeca kumuZi but did not observe Harper's first fusion ; it is perhaps only necessary to add that its occurrence has since been confirmed by other workers (I) for this species and by Harper himself for the related fornls Erysipke communis (20) and Phyllactinia corylea (22). There seems thus no doubt that the two fusions, even if absent in Dangeard's material, are of common occurrence among Erysipkaceae.In rgoo,the two fusions were seen by Harper in the Discomycete Pyronzma confluens (21), but here the filaments, which we may now follow De Bary in calling sexual organs, contain each not one but several nuclei and fertilization thus consists of the union not of one but of several separate pairs. Later, in 1go5, a similar case was described in detail by Claussen (3) for Boudiera CZaussenii (=Ascodesmis nigricans?) and evidence which makes the occurence of normal fertilization had, in the meantime, been brought forward for very probable several other species. T h e next step was the investigation of the forms described by De Bary or his pupils as possessing a female organ only, and in 1906 an account of the life history of Humaria granulata was published (2). Here the female cell, like that of Pyronema confluens possesses several nuclei, but the male organ has entirely disappeared and the fusion of male with female nuclei is replaced by the union of the female nuclei in pairs. In this, as in other forms, a subsequent fusion takes place in the ascus. In 1ga7 a corresponding discovery was made for Ascobolus f u r f ~ ~ y a c e(25), u ~ and I was so fortunate as to be able to
describe yet another case in Lachnea stercorea ( I 3) ; here, however, the male filament, is still present though, in my specimens it did not function and normal fertilization is again replaced by the fusion of female nuclei in pairs. At about the same time Dangeard (8) ~ublisheda most interesting memoir, in which he described the development of the sexual organs in a number of species, and showed that a passage of male nuclei into the female cell did riot take place in his material. Unfortunately he did not deal with the possibility of the union of female nuclei in pairs but assumed that, as the male filament was not functional, nuclear fusion could not take place at this stage. Towards the end of 1907Claussen carried out a further study of Pyronema conflzlens (4). H e saw the male nuclei pass, in accordance with Harper's description, into the female organ, but he asserted that they did not fuse with the female nuclei but only became associated with them in pairs; the associated nuclei travelled two and two up the ascogenous hyphae and finally fused in the ascus. In other words the ascus fusion was the final union of pairs of sexual nuclei which had become associated some time before in the female organ, and only one fusion took place in the life history of Ascomycetes. Claussen's paper was followed by an account of the life history of Humaria rutilans (14). This fungus may be very conveniently studied, as the nuclei are exceptionally large and clear. I found, with regard to the sexual organs, a stage of reduction more advanced than that in Humaria granulata, since not only the male, but also the female filament has disappeared and fertilization is replaced by the fusion in pairs of vegetative nuclei, the nuclei of the ordinary hyphae. Subsequently the descendants of these nuclei were seen to undergo a second fusion in the ascus. So far this fungus only adds another to the cases in which two fusions have been recorded, but, owing t o the size of the nuclei it was further possible to study, in some detail, their behaviour in the ascus. It had already been pointed out by various authors (Gjurasin (17)~Maire (23), Guillermond (IS), Harper (22), etc.) that the fusion nucleus in the ascus divides three times, and that the first two of these divisions correspond to the two peculiar divisions which, in other o.rganisms, have been shown to bring about that reduction of the number of chromosomes to half, which is the necessary corollary of their doubling in fertilization. In Hlmzarza rutilans it was possible to see very clearly that this was the case, and it was further found that, after the number of chromosomes has been thus reduced to half in the course of the lirst two &visions, the remaining number is again halved during the third. So that, in the course
of the life history of this fungus, the number of nuclear elements is twice doubled by the two successive fusions, and then twice halved during the divisions in the ascus. This state of affairs may be compared diagrammatically with that which occurs in an ordinary plant or animal.
- W T PLANTS AND ANIMALS ,
,ASCOMYCETES,
Humariu rzltilans does not stand alone in these peculiarities, for it has since been shown that similar processes take place in Peziza vesiculosa and Otidea azlrantia (16) ; and certain earlier investigations by Harper (22), Maire (23) and others appear open to a corresponding interpretation. To sum up, there are two main views with regard to this aspect of the sexuality of the Ascomycetes. One, foreshadowed in 1791 by Bulliard, and supported, among others by Dangeard and by Claussen, locates the sexual fusion in the ascus and denies the occurrence of any other nuclear union ; the other, initiated by De Bary and his pupils and extended by Harper and by a number of workers in this country accepts the existence of two fusions, recognises the first, that in the " female" organ, as sexual, and regards the second as peculiar t o Ascomycetes and as compensated by the equally peculiar second reduction. Probably most investigators would now agree in regarding De Bary's filaments as phylogenetically male and female organs whether they had convinced themselves that some form of fertilization still takes place in the latter or whether they held that it had shifted to the ascus. It would seem evident moreover that normal sexuality is at present disappearing among Ascomycetes and that even the same species will, under varying conditions, show various degrees in the structure and dficiency of its Sexual cells.
In this connection, an interesting degeneration series may be traced: in Pyronema confl?cens-to take the more fully investigated Discomycetes only-and in Boudiera, the formation of functional male and female filaments takes place and normal fertilization occurs; in Lacknea stercorea the female organ is still normal, but the male, though present is functionless and the female nuclei fuse in pairs; in Humaria granulata the male filament is absent, and in Humaria rutiCans the female organ also has disappeared and sexual fusion is reduced to a mere union of vegetative nuclei ; finally it may be suggested that in some forms fertilization will have altogether ceased. Perhaps such cases are among those studied by Dangeard and his allies, but, if this be so a corresponding absence of the easily recognisable stages of the halving of the chromosome number is to be expected, and will afford convincing proof of the non-occurrence of its doubling in fertilization. In another direction the study of reproduction among Ascomycetes seems likely to do something towards an understanding of the relationships of at any rate the great groups. In AspergilZus (or Eurotium) kerbariorum it has recently been pointed out (15) that the female organ consists of three parts, a multicellular stalk, a single fertile cell or ascogonizcm, and a single terminal cell or trickogyne through which the male nuclei must pass to reach those of the ascogonium. The fertile cell alone gives rise to ascogenous hyphae, but, before doing so, it divides into several multinucleate portions. In Pyrontmn confluens the female branch is similarly constructed, but the stalk is less conspicuous and the ascogonium, irstead of being long and narrow, is spherical and gives rise to ascogenous hyphae without becoming septate. In Lachnea stercorea the state of affairs is similar, but oddly enough the trichogyne, when mature, consists not of one but of several cells, and that although the now functionless male filament is developed close at hand. Possibly it is the difficulty connected with the breaking down of so many cell walls that has put an end to normal fertilization in this species. The female branch of Humaria granulata, though now without a trichogyne, also conforms t o this type, and that of Ascobolus furfuraceus can probably be derived from it. Similar structures have been seen in other Pezizaceae, which still await full investigation, and it may perhaps be suggested that the Pyrovtema type is characteristic of Discomycetes generally. In Boudiera, however, the structure approximates much more closely to that of AspergiZZus, and the comparatively narrow female cell becomes septate after fertilization. T h e male filament, in AspergiZlus, has a multicellular stalk like that of the female, and terminates in a small cell, the
antkeridium proper, which contains several nuclei. In Boudiera, in Pyronernn, and in such other of the investigated Discomycetes as still show a male organ, the antheridium is much larger and develops as a stout oblong body seated on a short stalk. Among Pyrenomycetes a quite different type of female organ occurs ; in Gnomonia rrytkrostoma (10, 12), in the investigated species of YoZj~stigma (II), in Poronia pzrnctata @), and in various Lichens, it forms a long, coiled structure divided into numerous cells. Beyond the coil is a straight portion terminating externally in a swollen cell and constituting the tricllogyne ; some of the c,ells at the other extremity of the coil form a stalk ; but, since fusion of the sexual nuclei has not yet been observed in these cases, it is impossible to say whether the true ascogonium is multicellular before, or only, as in Aspergillus, after fertilization. In this group the presumptive male organ is again a filament terminating in a small cell, but such tilaments are very numerous and are aggregated in flask-shaped spermagonia and the terminal cell is cut off as a spernzatiuul.t and carried by the wind to the trichogyne It thus seems that the spermatium may be readily homologised with the antheridium of AspergiZZus, and consequently with that of the Discomycetes. This con~parison is the more suggestive since it has recently been shown (26, 27) that among the Red Algae undoubted antheridia are detached from their parent plant, and are carried to the female organ in a corresponding way. There thus appear to be two main types of male and female organ : that characteristic especially of Discomycetes where both antheridium and ascogonium are more or less spherical, where the former does not become detached and the latter (probably for some reason connected with its shape) does not become septatc after fertilization ; and, secondly, that characteristic of a number of Pyrenomycetes in which the female organ is elongated and richly septate and the antheridium is minute and becomes detached and carried by external agencies to the trichogyne. An intermediate, and not improbably primative stage seems t o occur in AspergiZZzrs kerbariorzrtz, and has been indicated in other forms. These main types do not of course include all the very numerous variations of reproductive structure described among Ascomycetes. In the Erysiphaceae, for instance the female organ is without a trichogyne and the antheridium fuses directly with t!le fertile cell. A similar arrangement is found in the genus Gymlooascus (5), and it m y be suggested that the latter, with its slightly developed perithecial wall forms part of F degeneration series terminating perhaps in Eremascus (24), where the asci are borne free on the branches of a simple mycelium.
It is, however, probably still too early to speculate on the relationships which may be indicated by the structure of the sexual organs, and their disappearance has been shown to occur in groups so diverse as to make it without importance in classification. An enormous number of forms still demand investigation, and a further knowledge of their structure and cytology cannot fail to solve some, at any rate, of the problems of this group, and through them, not impossibly, of a -wider field. LIST OF PAPERS CITED.
Blackman, V. H., and Fraser, H. C. I. 1905. Fertilization in Sphaerotheca. Annals of Botany XIX., p. 567.
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