104 Minnesota Plant Diseases. 



a fusion process takes place and this has been interpreted as a 

 breeding act by some botanists. 



The algal fungi (Phycomycetes). It has already been stated 

 that these fungi are for the most part aquatic in habit but that 

 some forms, as the insect molds and black molds, have aban- 

 doned the water and taken to dryer situations. It is a noticea- 

 ble fact, however, that even insect molds and black molds re- 

 quire very moist conditions. It is in the algal fungi, therefore, 

 that swimming spores are commonly produced and especially, 

 though not exclusively, in the aquatic forms. White rust of 

 mustard produces swimming spores at certain stages, but this 

 only occurs when an abundance of water is present, so that the 

 fungus may be considered aquatic during a part of its life and 

 terrestrial during the remainder. Spores are produced by a 

 breeding act and often these are special spores for resting pur- 

 poses; they are therefore provided with very thick walls. In 

 some families, as in the black molds, large numbers of spores 

 are produced in cases which to the naked eye look like tiny 

 black spheres about the size of a pin-point. These spores are 

 not provided with swimming lashes but depend upon the wind 

 for aerial distribution. The algal fungi are structurally pecul- 

 iar in that the threads have no crosswalls except when spores or 

 spore-cases are about to be formed. The following groups are 

 the most important among the algal fungi : Chytridines or 

 Lowly Algal Fungi, Water Molds, Fish Molds, Sewer and 

 Drainpipe Molds, Damping-off Fungi, Downy Mildews, White 

 Rusts, Black Molds and Black Mold Parasites. 



Lowly algal fungi (Chytridincce). In this group are found, 

 in general, very simple fungi. All of them are minute and it 

 requires strong powers of the microscope for the observation 

 of most of them. The simplest are tiny single-celled spheres 

 and resemble much the Flower Pot Algae, except that they 

 have no leaf-green. In fact, it is very probable that they have 

 descended from these algae. Some have become elongated 

 into simple threads and still others are even considerably 

 branched. Spore cases and breeding spores are produced and 

 each forms, by internal division, swimming spores. These 

 swimming spores are the chief agents of distribution and are 

 provided with one or two lashes which, by whipping about in 



