70 A TEXT-BOOK OF BOTANY. 



last over winter on the old wheat plant, and in the spring begin 

 again the life-cycle of the rust. The plant which results from 

 the germination of a teleutospore gives rise to sporidia, which 

 are carried to the barberry leaves where secidiospores are pro- 

 duced. The latter are then carried to growing wheat, forming 

 first uredospores and later teleutospores. It should be remembered 

 that these are all asexual spores. In regions where there are no 

 barberry plants to act as host the aecidiospore stage is omitted. 



The Fungi Imperfecti. — The miscellaneous fungi included in 

 this group are of importance because of the great damage which 

 they cause to agricultural crops. The potato scab is an especially 

 destructive pest in New England and in Canada. The scab not only 

 develops on the growing tubers in the soil, but can be spread from 

 a few affected potatoes to a whole bin of clean ones if they come 

 in contact with them. Prevention of this disease usually consists 

 in disinfecting the tubers which are used for seed so as not to 

 carry the minute organisms into the soil. 



A disease affecting the leaves of the potato and thereby destroy- 

 ing the crop is due to a fungus whose spores, settling on the leaves, 

 germinate and penetrate to the interior through the stomata, 

 finally weakening or killing the plant. 



Some of the other important forms produce a pink mold on 

 apples, scabs on peaches and other fruits, mold on onions and 

 other garden crops. The blight of ginseng and the blight of 

 cotton, the dry rot of various vegetables and the blotches on many 

 of our common fruits can be traced to the development of these 

 fungi. The study of these forms is a very difficult one, and re- 

 searches are constantly being carried on at the government experi- 

 ment stations, as well as by individual workers. 



For a description of these forms, as well as many other 

 harmful fungi, consult '' Fungous Diseases of Plants," by Duggar. 



Detection of Fungus in Host. — Unless special means are 

 employed, it is ofttimes rather difficult to trace the mycelial of the 

 fungus in among the cells of the host plant. Vaughan (Annals 

 of the Missouri Botanical Garden, 1914, p. 241 ) has used the stain 

 known as " Pianeze Illb " in differentiation of the fungus from 

 the plant substratum. The host tissue stains green and the my- 

 celium a deep pink. This stain, devised by Dr. Pianeze for the 



