Contribution to the biology of dissemination of Claviceps sclerotia.
AbstractIt is generally assumed that the sclerotia of Claviceps on reaching maturity fall to the ground close to their host and lie there till the spring, when they germinate and give rise to stromata bearing perithecia and ascospores. The ascospores are then disseminated by means of the wind or of insects. In certain cases, such as that of ergot of rye, this is undoubtedly what occurs. A study of Claviceps sclerotia from wild grasses, however, shows that the fungus is not solely dependent on the wind or insect dissemination of its spores, but is also transported in the resting mycelium or sclerotium stage. There are two methods of securing this end. Either the sclerotium makes use of its host's contrivances for dissemination, or it is itself provided with such adaptations.
The author's observations and experiments deal with both alternatives. Two cases are described in which the sclerotium utilizes the host's contrivances for dissemination. Brachypodium sylvaticum has an oval caryopsis enclosed within the paleae. The inferior palea has at its apex a hook-shaped or sinuous awn, 1.5 cm. in length. This awn, with the caryopsis, easily attaches itself to passing persons or animals. The crescent-shaped sclerotium, which is surrounded by the paleae at the base, is taken up and transported in the same manner. In Calamagrostis epigeios (a new host) the sclerotia are 2 to 4 mm. long, 1/3 to 0.5 mm. thick, rod-shaped, and twisted. They are partially enclosed in and adhere to the lancet-shaped paleae. At the base of these paleae is a circle of fine hairs which often exceed the sclerotia in length. In dry weather this ring of hairs expands like an umbrella, and the paleae and sclerotium are carried away with great ease by the wind. It is probable that the autumn gales could carry them for a distance of several miles. A similar process probably occurs in Melica ciliata and Phragmites communis[Phragmites australis].
Only one adaptation of the sclerotium itself which aids dissemination is described. Experiments with Claviceps wilsoni Cke on the aquatic grass Glyceria fluitans showed that the sclerotia of this species float in water. Further tests, particulars of which are given, showed that the sclerotia occurring on the various grasses may be divided into floaters and non-floaters, and that this is correlated with their requirements. Thus the sclerotia of Glyceria fluatans, Molinia coerulea, Phragmites communis, and Phalaris arundinacea, all of which grow in or near water or bogs, can float and are not injured by long immersion, while those occurring on rye, Lolium, Brachypodium sylvaticum, Sedeña, coerulea, Arrhenatherum elatius, Agropyrum repens, Alopecurus myosuroides, and other land grasses sink in water. The floating capacity of the former group appears to be due solely to the air enclosed in the tissue of the sclerotium, as when this is removed they sink.
In 1899 the author showed that the ergot of Phalaris arundinacea was identical with that of rye, Claviceps purpurea Tul. The only modification undergone by the fungus on Phalaris is biological, the floating habit having been acquired to suit the new surroundings. This important alteration justifies in the author's opinion the creation of the new form Claviceps purpurea f. biologica natans Phalaris arundinaceae.
The sclerotia of certain land grasses (Dactylis glomerata, Holcus mollis, H. lanatus, Poa annua, and P. nemoralis) form an intermediate group between the floaters and non-floaters. Thus, in tests carried out near Berne, 69 per cent. of the sclerotia of Dactylis glomerata, 40.4 per cent. of the sclerotia of Poa nemoralis, 60 per cent. of the Holcus sclerotia, and most of those on Poa annua, floated. Possibly the sclerotia on these grasses, like those of Calamagrostis epigeios, are primarily disseminated by the wind, but when they are freed from the paleae they can float to a greater or lesser degree. In Phragmites communis[Phragmites australis] there is little doubt that both methods are made use of.