324 



THE FUNGI WHICH CAUSE PLANT DISEASE 



verrucosely sculptured, borne singly, or sometimes in chains, 

 (5) teliospores, smooth or variously sculptured but not echinulate, 

 borne singly or in chains. In every species the mycelium even- 

 tually gives rise to teliospores, which produce in germination 

 four basidia, either remaining within the spore-cell or borne in 

 the air on a short promycelium, each basidium supporting a single, 

 stalked or sessile basidiospore. 



The order of some two thousand species, constituting the 

 "rust" fungi, many of them living on cultivated plants of high 

 value, is of great economic significance. Its members are strict, 

 obUgate, parasites which in no stage of the life except in the 

 promycelial stage can develop other than on the living host. 

 The complexities of the life histories of the species, with their five 

 distinct spore forms, inhabiting at different seasonal periods two 



or even three different host 

 plants, renders the order both 

 difficult and exceedingly in- 

 teresting. 



The hfe history of the most 

 complete of these fungi may 

 be stated as follows: 



I. .£cia (secidia) and O. 

 pycnia (often called spermo- 

 gonia or pycnidia). The my- 

 celium arising from a basidio- 

 spore invades the host plant, 

 and vegetates until vigor suf- 

 ficient to spore formation is 

 attained, meantime often pro- 

 ducing local spotting, hyper- 

 trophy, or other injury to the 

 host. The mycelium then de- 

 velops a stroma which pro- 

 duces spore beds (son) and ruptures the epidermis. These sori 

 are usually deeply sunken in the host and cup-shaped and take 

 the common name "cluster cups," Fig. 239, technically secia or 

 aecidia. The sporophores arise from a hyphal plexus at the base 

 of the cup and the spores are borne catenulate in acropetal suc- 



& 



Fig. 239. — ^jScium and pycnium. 

 Tavel. 



After 



