HUME FUNGI OF COLORADO, WYOMING AND NEBRASKA. 247 



materially the abundance of these host plants on the plains. The only 

 way to account for the abundance of these at lower altitudes is that the 

 mycelium is perennial in the tissues of the hosts. 



In places where there are dews or where moisture is more abundant, 

 as in the flood-plains of the Platte River, parasitic fungi, like Piiccinia 

 Xafj/hii, occur in close proximity to the water. Entire plants are cov- 

 ered by it. Moisture, especially in the form of dew, is a very import- 

 ant factor in the distribution of fungi. As an illustration, it is not 

 uncommon to find rusted wheat plants in close proximity to the irriga- 

 tion ditches while in other parts the disease is not so common. It is 

 well known that in and around these irrigation ditches heavy dews 

 cover the grass in the morning. 



We might note the fact here that grain rusts are not abundant at 

 any point in Colorado, at least not nearly so conmion as we find them 

 in the Mississippi Valley, and this is undoubtedly due to the fact that 

 the plants are supplied with water by irrigation. 



Farther up in the foot-hills, especially in the narrow cafions, fungi 

 are more numerous. Frequently thousands of plants are affected, as, 

 for instance, we find the ALcidiiim alenii on Shephcrdia Canadensis. 

 Near the timber-line and in the swamps and bogs, fungi are more 

 numerous. As an illustration, the Phragmidium Fragariasfri is found 

 on nearly every plant of Potentilla gracilis in places. 



Of the fungi on woody plants mention may be made of the Pcrider- 

 mun cerebrum on the Lodge-pole Pine. Although this fungus is widely 

 distributed it is nowhere abundant. It usually occurs on the yqunger 

 branches where it forms large swellings. The limb, in the course of a 

 year or two, dies because the fungus cuts off the supply of nutrient 

 material. 



Among other interesting fungi of trees mention may be made of the 

 Witches' Broom (^Exoascus nanus) on Betula occidentalis. Hundreds 

 of branches on a single tree were affected, giving it quite a strange 

 appearance. This species has never been reported for this continent 

 so far as I know and was first described by Johanson from specimens 

 collected in Finland. Other Exoasci were not uncommon both at and 

 around Pike's Peak and a large number of leaves of the Rocky Moun- 

 tain Oak {Quercus Gunnisoni) in North Chyenne Canon were affected. 

 Several species of the genus Prunus were quite commonly affected 

 along the Manitou trail. 



