176 



BOTANY 



They are endoparasites ; the mycelium, which is often colored 

 orange by the presence of an oily pigment, grows vigorously within 

 the host, upon which sometimes there are produced distorted growths 

 or galls. 



While the Smuts produce but one type of spores, many of the 

 Rusts are characterized by the production of several quite different 

 forms. This polymorphy is complicated in some species by heteroe- 

 cism; i.e. the different stages may be borne upon entirely different 

 hosts, often quite unrelated. This has resulted in much confusion 

 in naming the Rusts, as different stages of the same plant have been 

 named under the impression that they belonged to quite unrelated 

 Fungi. 



Five forms of spores are known, the ^Ecidiospores, Uredospores, 

 Teleutospores, Sporidia, and Spermatia. The last named are very 

 minute cells cut off from slender basidia contained in special flask- 

 shaped receptacles (Pycnidia, Spermogonia), which usually accom- 

 pany the ^Ecidia. It has been supposed that the spermatia may be 



male reproductive 

 cells, but there 

 is no direct evi- 

 dence as to their 

 real nature, no 

 oogonium, or cor- 

 responding struc- 

 ture having been 

 demonstrated in 

 any of them. 



The duration of 

 the mycelium in 

 the Rusts is vari- 

 ous. Where the 

 host is an annual, 

 the life of the 

 parasite may be 

 limited to a few 

 weeks, but where 

 the host is peren- 

 nial, the mycelium 

 often persists from 

 year to year, grow- 

 ing with the de- 

 veloping tissues of the host-plant, upon which the same mycelium 

 produces annual crops of spores. 



The number of Rusts is very large, probably not far from two 

 thousand species, which may be arranged in two categories, the 



FIG. l^l. Uromyces caladii. A, section of*the leaf of 

 Arissema triphyllum, with young aecidirim (x 150). B, 

 section of spermogonium. C, section of ripe secidium 

 (X about 40) ; p, peridium. 



