222 



LIFE HIST< )RY OF A RUST 



pear in the life history of a rust. One phase of the life of this 

 parasite appears upon the leaves of the barberry. During May 

 and June the mycelium growing in the leaves forms roundish 

 bodies which rupture the epidermis and finally open out into 

 cups filled with chains of yellowish spores. An examination of 

 Fig. 159 shows that these spores are found at the end of hyphac 



Fig. 159. Cluster cups as seen in section of leaf of spring beauty, Clay- 

 tonia. At right one of the cups is ruptured, exposing the aecicliospores. 

 Below a small cup, pycnidium, is discharging pycnidiospores that are pos- 

 sibly functionless male gametes. 



as in the powdery mildews and surrounded by a layer of rather 

 thick-walled hyphae. This stage of the rust is known as the 

 cluster cup or aecial stage and the spores are called aecidiospores. 

 Often smaller spore-bearing cups, known as pycnidia (sing, pyc- 

 nidium), are associated with this, phase of the fungus. These 

 small spores, while capable of germinating, do not appear to 

 enter into the life history of the fungus by producing a new para- 

 site. They have been looked upon as male gametes that origi- 

 nally effected fertilization in a female organ from which de- 

 veloped the spore-hearing cluster cup, the process being similar 

 to that noted in the Red Algae where fertilization resulted in 

 the formation of a spore containing cystocarp. The sexual proc- 



