THE FUNGI WHICH CAUSE PLANT DISEASE 325 



cession. The whole structure is usually red or yellow. The outer 

 layer of the cup usually consists of a paUsade of sterile sporo- 

 phores bearing sterile cells and constitutes the peridium. The 

 seciospores are usually nearly globular, or angular by compression, 

 reddish and rough and sometimes bear germ pores. They are ca- 

 pable of germination at once and on germination give rise to germ 

 tubes which may infect susceptible hosts, leading to a mycelium. 

 This in turn again produces sori which in some species may be 

 aecia, in others teUa, but in most species, uredinia. 



Associated with the aecia, occasionally with other spore forms, 

 but never borne alone, are minute pycnia with sporophores 

 arising from their walls and bases. These bear unicellular pyenio- 

 spores. Sterile hairs usually protrude from the ostioles. The 

 whole structure in gross appearance is much like the pycnidium 

 of Phoma or Phyllosticta but it is reddish or orange in color. 

 These pycnia were formerly often spoken of as "spermogonia" 

 and the spores as "spermatia," due to the thought that they stood 

 for degenerated male organs; a view supported by the fact that 

 the spores were not observed to germinate. Germination ^^ has 

 now been observed and there is no longer reason to regard them 

 as sexual organs. 



II. Uredinia (uredo-sori). The aeciospores may infect the 

 same species of plant that produced the aeciospores (autce- 

 cious) or plants of an entirely different species (heteroecious). 

 The mycelium produced by the seciospore develops within the 

 host; usually remains local, and causes spotting. When it has 

 attained sufficient vigor and age, usually after about two weeks, 

 it produces a sub-epidermal hyphal plexus from which arises a bed 

 of sporophores which bear unicellular, hyaline to brown, nearly 

 globose, thin-walled, usually echinulate or rough spores, each with 

 from 2 to 10 germ-pores variously placed. These are the ure- 

 diniospores borne in uredinia (uredo-sori). They may germinate 

 at once producing a germ tube which develops to a mycelium. 



These spores falling on susceptible tissues, by infection, usually 

 stomatal, continue the production of uredinia and spread the 

 disease. The urediniospores are usually short-lived and function 

 to spread summer infection. They continue to form throughout 

 the growing season. 



