290 DISEASES OF CULTIVATED PLANTS 



traces prove to have been at one time present, the family 

 appears, relatively to other groups, to be of ancient origin. 

 Many biologic forms are on record. 



The great majority of parasitic fungi pass through the 

 entire course of their development on one and the same 

 host-plant. Such are termed autoedous species. As opposed 

 to this condition, many species, belonging to the Uredinaceae 

 or rusts, grow during different periods in the cycle of their 

 development on different host-plants. In many instances 

 these different hosts belong to widely separated families in 

 some instances one host-plant may be a dicotyledon, and the 

 other a monocotyledonous plant, as in the case of the 

 gooseberry cluster-cup fungus, where the aecidium or cluster- 

 cup phase of the fungus grows on the leaves or fruit of the 

 gooseberry, whereas the puccinia or winter stage grows on 

 the leaves of a sedge. Fungi growing at different periods of 

 their development on two different hosts are said to be 

 heteroecious, or metoedous^ as preferred by some. Heteroecism 

 is characteristic of the large genus Pucdnia, many of the 

 species having four markedly different types of fruit pro- 

 duced during different periods of the cycle of their life- 

 history. In the case of Puccinia graminis (D. C.), the 

 earliest spring stage of the fungus develops on young, living 

 leaves of the common barberry. The first indication of the 

 presence of the parasite is the appearance of one or more 

 yellowish blotches, which show at the same points on the 

 upper and under surface of the leaf. Minute flask-shaped 

 bodies, called spermogonia, are the first to make their appear- 

 ance, sunk in the substance of the blotches on the under 

 surface of the leaf. These flask-shaped bodies, opening by a 

 pore at the surface of the leaf, contain myriads of very 

 minute bodies called spermatia. Spermogonia appear to be 

 functionless structures at the present day, and it is surmised 

 that at some earlier period during the evolution of the 

 Uredinaceae, they were male or fertilising organs, the 

 spermatia becoming blended with the trichogne of the 

 female organ, as occurs at the present day in some groups 

 of cryptogams. 



Soon following, or contemporaneous with the spermogania, 

 a second type of fruit known as the aecidium stage, popularly 

 known as 'cluster-cups,' appears on the yellow blotch, but 

 on the upper surface of the leaf. The aecidia, when mature, 

 resemble miniature cups with notched and recurved edges 



