142 Minnesota Plant Diseases. 



off and the spores are thrown out in a tiny drop of liquid. Very 

 often comparatively large areas of sacs in a cup throw out their 

 spores simultaneously and then one sees small dust-like clouds 

 arising from the cup. If a little slip of glass be placed over 

 the cup at such times the spores will be found in groups of 

 eight in little drops of liquid on the glass. If cups be placed 

 in a moist chamber and allowed to remain undisturbed for sev- 

 eral hours they will often, upon the removal of the moist cham- 

 ber lid, begin to send up the dust cloud of spores. The change 

 of moisture conditions seems to initiate the expulsion of these 

 clouds. A few cup fungi have another device by which the en- 

 tire unopened sacs are thrown out. Accessory spore-forms are 

 known in a great many cup fungi though not nearly so numerous 

 nor in such great variety as those of the black fungi. The great 

 majority of cup fungi live on the ground or on dead wood and 

 are saprophytes, but not a few are parasitic and some cause 

 serious diseases of their host plants. As agents in the disinte- 

 gration of plant debris they are important economically, though 

 not nearly so conspicuous in this effect as are the black fungi 

 and the gill- and pore-fungus allies. The two following groups 

 contain most of the common forms of cup fungi. 



Tar-spot fungi and their allies (Phacidiinccs and Hysteninccc). 

 This group of fungi may be considered as a transitional group 

 between those black fungi whose sac-capsules have large oval or 

 slit-like mouths and the true cup fungi. They produce densely- 

 woven mycelial masses which form crusts with the substrata 

 and upon these burnt-wood-like masses arise the little cups 

 which are similar in texture. The cups are at first closed 

 and simulate the spore-sac-capsules of the black fungi, but 

 the sac-bearing area is soon exposed. The sterile threads 

 between the sacs are usually longer than the sacs and the ends 

 come together above the sacs forming a covering. Accessory 

 spore bodies are not uncommon. These fungi occur on leaves 

 and branches of trees and have the habit of leaf- and dead-stick- 

 inhabiting black fungi. They are, moreover, usually sapro- 

 phytic though tree tar-spots are parasites of economic impor- 

 tance. The tar spots of willow and maple leaves are very abun- 

 dant in Minnesota. The mycelial mass which forms on the 

 leaves in summer and in fall looks like a drop of tar and does 



