FusARiA JF Potatoes 117 



substratum; and to the production of special structures such as plecten- 

 chymic stromata and sclerotia. 



In the microscopical study, the different types of conidia, chlamydo- 

 spores, and conidiophores received special attention. The observations 

 were recorded by means of camera lucida drawings^^ and necessary 

 measurements. 



When measurements were of any importance/^ ten conidia (or chlamydo- 

 spores) were measured and only the average and the extremes of these 

 measurements were recorded. In cases of special variability of the 

 material, records were made of fifteen or twenty spores of each important 

 type of septation and shape. 



In making camera lucida drawings, care was taken to picture the apical 

 and the basal ends of the conidia with the utmost accuracy; every kind 

 of conidium occurring in a culture was drawn, but the typical and the 

 exceptional cases, as they appeared, were marked off. 



As a rule, measurements and drawings usually were made of the conidia 

 taken from sporodochial or pionnotes-like masses of them, because such 

 conidia on the whole are more uniform and typical for a given organism. 

 Here it should be noted that for the measurements and drawings it is 

 highly important, at least in case of very closely related organisms, to 

 use material analogous in all respects — age, type of fructification, and 

 enviromnental condition. 



The fungi were studied, not only on different media, but. also at different 

 stages of their growth. The latter factor is almost as important as the 

 former, because in certain cases, as in F. angustum, conidia in the best 

 condition (most regular, and so forth) were observed when the cultures 

 were very young. Some characters, however, become manifest only 

 after a culture has reached a certain stage of maturity. This is often the 

 case with chlamydospores and color production. Sclerotia often appear 

 in comparatively old cultures and continue to grow for some time. 



The presentation of the species of Fusarium is based on the following 

 main principles: 



All drawings (with only a few exceptions) are intentionally made to 

 the same scale as those of Appel and Wollenweber — thus far the one 



32 All drawings were made from living material mounted in water, and, with a few exceptions, with an 

 oil immersion lens. In order to prevent movement of the spores in water, it is necessary first to use just 

 enough water to keep air from underneath the cover glass, and then to spread around the cover glass some 

 oily substance, as cedar oil, which does not dissolve in water and which does not dry out quickly. 



^ A number of fields should be examined before measuring in order to see the prevailing type, and then 

 measurements of the conidia of the prevailing type should be taken. This leaves rrmch to personal inter- 

 pretation, but otherwise it would be necessary to take many more measurements. 



