252 



DEVELOPMENT OF A RUST 



bodies which rupture the epidermis and finally open out into cups 

 filled with chains of yellowish spores. An examination of 

 Fig. 162 shows that these spores are formed in rows at the end of 

 hyphae 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 aeciospores. Often smaller spore-bear- 



FIG. 162. Cluster cups as seen in section of leaf of spring beauty, Clay- 

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

 Below a small cup, pycnium, is discharging pycniospores that are possibly 

 functionless male gametes. 



ing cups, known as pycnia (sing, pycnium), 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 parasite. They have been looked 

 upon as male gametes that originally effected fertilization in a 

 female organ from which developed the spore-bearing cluster cup, 

 the process being similar to that noted in the Red Algae. On 

 the other hand they are regarded by some authorities as spores 

 that have lost their power to germinate and so breed the fungus 

 asexually. The process of spore formation in the cluster cup, 

 as will be seen from the outline given below, is so different from 

 that of the algae and is known in so few forms that any interpre- 

 tation of the phenomena should be deferred for the present. 



