New PhytoL (1967) 66, 65-66. A NOTE ON THE CONIDIAL SCAR IN THE XYLARIACEAE BY G. N. GREENHALGH The Hartley Botanical Laboratories, University of Liverpool {Received 5 July 1966) The conidial scars produced by Rosellinia buxi, Hypoxylonfuscum and H. serpens are described and briefly discussed from micrographs obtained with the Cambridge Stereoscan electron microscope. The conidia produced by species of the Xylariaceae are typically acrogenous blastospores, hyahne to subhyaline, unicellular and elliptical, developed at the apices of conidiophore branches. They are subsequently pushed into a lateral position by further growth of the conidiophores from a new growing point which arises just beneath and lateral to the terminal spore. The slightly thickened base of the detached spore is truncate, tending to be slightly concave; a narrow pore may be visible running through it. Detachment of the conidia produces scars on the conidiophores which, depending on the species, may be clustered together or more or less regularly spaced out along the conidiophore (Chesters and Greenhalgh, 1964). Again, they may be conspicuous or inconspicuous. For example, those produced by Rosellinia aqtiila (Fr.) de Not. and R. bitxi Fabre are easily seen, whilst those of Hypoxylon nimmitlarium Bull, ex Fr. are less obvious. It may also be possible to see a channel, corresponding to that in the spore hilum, running through the scar. Indeed in certain species, such as Rosellinia aqtiila, there appears to be a quite conspicuous pore in the conidiophore wall (Plate 3c). Examination of the conidiophores of several species has only once shown proliferation of an apparent secondary spore through the scar left by the primary conidium. This was in Hypoxylon argillaceiim Pers. ex Berk. In the hope that high magnification micrographs of the conidiophore surface would provide more information as to the nature of these scars, conidiophores of Rosellinia buxi, Hypoxylon fuscum Pers. ex Fr. and H. serpens (Pers. ex Fr.) Kickx. were examined using the Stereoscan electron microscope recently developed by the Cambridge Instrument Company. The specimens consisted of small pieces (about 10x3 mm and about 2 mm deep) of wood, taken from herbarium specimens, bearing immature stromata with conidiophores. These were fixed to standard specimen holders with Durofix and then coated under high vacuum with a very thin film of gold/palladium alloy. Plate 3a and b show the apices of conidiophore branches produced by Rosellinia buxi. In each case the terminal conidium is still attached but has collapsed and become distorted, probably during the coating process. Both conidiophores bear relatively large and conspicuous scars. The conidia have fallen away leaving pores in the conidiophore wall which are plugged at tbe base. This plugging presumably occurs before spore detachment. The scar has the appearance of a crater and this is accentuated by the slightly raised rim. The conidial scars produced by Hypoxylonfuscum are less conspicuous but have essentially the same form. In this species, the conidia are produced in clusters at the apices of branches, though these clusters may come to occupy a lateral position following renewed extension growth of the conidiophore. A terminal cluster of spores 65
66 G.N.GREENHALGH is seen in Plate 3d. The point of attachment of the spore on the right can be clearly seen and just below this are two conidial scars. The raised rim of the scar is visible and also the central plug which here almost fills the central region of the scar. Conidiophores and spores produced by H. serpens are shown in Plate 4a c. The geniculations of the conidiophore are not very apparent, and the conidial scars are very inconspicuous contrasting with their appearance when viewed by transmitted light. They are merely slightly raised areas on the hyphal wall with, as far as can be seen, no central depression at all. The channel allowing cytoplasmic continuity between the spore and its parent hypha must presumably be completely occluded before detachment. There is thus no surface evidence that the scar represents a plugged pore, and in this it differs considerably from that of Rosellinia biixi. The origin of pores in the conidiophore walls of those imperfect fungi which produce porospores is not clear. Subramanian (1965) has pointed out that the question of whether or not the pores form before spore formation or are later developments, cannot always be answered satisfactorily. Indeed, as Ellis (1962) has suggested, pores may, in some cases at least, simply represent thin areas in the conidiophore wall which are then blown out as in blastospore formation (true blastospores are formed at points on the conidiophore where the wall simply becomes more plastic or weakened in some way and is not initially thinner). The distinction between porospores and blastospores is not perfect, and Luttrell (1963) states that the two may not be fundamentally different. It may be that in some forms the pores are formed by initial blastospore formation, the secondary spores being porospores. Certainly it seems that blastospores are capable of producing scars with a range of morphology, from pore-like craters in the wall to completely occluded, slightly raised areas. Probably, such problems of conidial development as these will be solved completely only by examination of electron micrographs of thin sections. It is evident, however, that the use of the scanning electron microscope to obtain pictures of the conidiophore surface will also yield A aluable information. For example, fungi producing annellophores could be profitably examined in this way. ACKNOWLEDGMENTS Thanks are due to the Cambridge Instrument Company Ltd, for permission to examine material with the Steroscan Electron Microscope, and to Miss P. J. Killingworth for assistance in using the instrument. REEERENCES CHESTERS, C. G. C. & GREENHALGH, G. N. (1964). Geniculosporium serpens gen. et sp. nov., the imperfect state oi Hy poxy Ion serpens. Trans. Br. mvcol. Soc, 47, 393. ELLIS, INI. B. (1962). Dematiaceous Hyphomycetes: III. Commomv. Mvcol. Inst., Mvcol. Paper, 82. LUTTRELL, E. S. {1963). Taxonomic Criteria in Helmintlwsporium. Mycologia, 55, 643. SuBR.'vM.'^NLAN, C. V. (1965). Spore types in the classification of the Hyphomycetes Mycopath Mycol appl, 26, 373. EXPLANATION OF PLATES PLATE 3 (a) and (b) Apices of conidiophore branches of Rosellinia bu.xi, witb the terminal spores (collapsed) still attached, and showing conidial scars just below the apex. ( x 20,000 approx.). (c) Conidiophore branch of Rosellinia aqiiila showing conidial scars. ( x 3000 approx.). (d) Apex of conidiophore branch of Hypoxylon fusciim (from young stroma) with terminal cluster of conidia, and showing conidial scars just behind the apex. ( x 15,000 approx.). PLATE 4 (a) and (b) Conidiophore branches of Hypoxylon serpens showing attached conidia and conidial scars. ( x 15,000 approx.). (c) As (a) and (b). Note undistorted conidium just left of centre. ( x 7000 approx.).
THE NEW PHYTOLOGIST, 66, PLATK 3 G. N, GREENHALGH CO7V/DML SCARS ifacinii page 66)
THE NEW PHYTOLOGIST, 66, i PLATE 4