. Minnesota plant diseases. Plant diseases. I50 Minnesota Plant Diseases. count of the infrequency of the occurrence of proper condi- tions, it might forget how to form sac-spores and would thus become independent of the sac-spore form. Both the produc- tion of different kinds of spores by one fungus plant and the production of spores on different hosts in one life-cycle would tend to furnish fungi where such a separation might occur. In addition to those forms where this actual separation and inde- pendence occurs there is a considerable assembly of spores, where the connection of apparently
. Minnesota plant diseases. Plant diseases. I50 Minnesota Plant Diseases. count of the infrequency of the occurrence of proper condi- tions, it might forget how to form sac-spores and would thus become independent of the sac-spore form. Both the produc- tion of different kinds of spores by one fungus plant and the production of spores on different hosts in one life-cycle would tend to furnish fungi where such a separation might occur. In addition to those forms where this actual separation and inde- pendence occurs there is a considerable assembly of spores, where the connection of apparently inde- pendent forms with sac-spore stages is known, and in such cases the term "imper- fect" is in a sense a misnomer. In a vast number of forms, the connection is indicated to a certain degree by the connections of an- alogous forms. For instance, the accessory spore forms of the powdery mildew is of a definite type known as an Oidium, and when one meets with such isolated spore forms, if they occur in the usual habitats of mildews, one may refer them to the powdery mil- dew group. Indeed it may be that all so-called imperfect fungi are actually traceable in their connections with sac-spore forms, but many have, as yet, frustrated all attempts to prove such connections. We may sum up these forms in this re- spect into three groups: first, those isolated forms whose con- nection with sac-spore forms is known; second, those isolated forms whose sac-spore connection is not known, but suspected from analogy with known forms; third, those isolated forms whose sac-spore connections are not even suspected or have become actually Fic. 69.—Truffle. 1. Fruiting body cut open; surface furrow which corresponds to the opening of a cup fungus is seen below and the convoluted sur- faces of the cup interior above. 2. A portion of the interior showing the sacs, each with four spores (highly magnified). 3. A single sac show- ing four spiny spores. Very highly magnified
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