THE DEVELOPMENT AND STRUCTURE OF THE CONIDIA OF ERYSIPHE POLYGONI DC. AND THEIR GERMINATION AT LOW HUMIDITY

The germination of the conidia of Erysiphe Polygoni DC. takes place through a range of relative humidity from approximately zero to 100% and, therefore, independently of the moisture content of the surrounding atmosphere. In germinating thus, they differ from the spores of some other erysiphaceous fungi and of non-erysiphaceous fungi in general.In E. Polygoni, the conidium is cut off from the conidiophore by a ring of wall material which is added to inwardly until a perforate disk is formed. Later, the pore is closed and the mature conidium remains attached to its conidiophore only by a minute papilla. The conidia have never been observed to germinate in situ, and they are passively discharged.The conidium wall is relatively impervious to water, stain passing into the spore only at the papillate end. Assuming the wall to be relatively impervious to gases also, an explanation is offered for the mechanism of germination of the conidia when they are dislodged from their conidiophores and allowed to fall on dry slides. The papilla provides a permeable spot in the spore wall. It is not exposed until after the spore has been detached. Upon exposure to air, the papilla allows carbon dioxide to pass out from the protoplast and oxygen to pass in, causing respiration and other germination processes to begin.Evidence in support of this suggestion is presented. When freshly detached conidia were held in an atmosphere containing 10% carbon dioxide, germination was checked. These conidia germinated perfectly when removed from the carbon dioxide. Germination was similarly checked by holding the spores in an atmosphere of nitrogen.No shrinkage of the conidia during germination was observed, but shrivelling and collapse take place when death is imminent.