28 Sporidiola or Promycelial Spores. 



with at least another stage, that when found alone ot isolated they are 

 regarded as incomplete, and it is taken for granted that the associated form 

 has yet to be found. In the case of teleutospores, however, there are 

 numerous species which produce them alone, and in such cases, all the other 

 spore- forms aje considered to have been suppressed or never formed. And 

 of these surviving teleutospores, in some species they are able to germinate 

 at once, and thus produce successive generations in the course of a year, 

 while in others they can only do so after a winter's rest, and are thus pro- 

 duced only once a year. 



In the heteroecious rusts there is no evidence to show that the sporidiola 

 can infect the plant bearing the teleutospores, hence it would appear that 

 P. gramhris in Australia can only be propagated, as far as spores are con- 

 cerned, by means of the uredospores. It has not been definitely proved 

 how the sporidiola are distributed, but no doubt the wind is an important 

 factor, and probably also animals assist in the distribution. In connexion 

 with the teleutospores of Gymno sporangium, Plowright 2 says: " It is prob- 

 able that the promycelial spores are implanted upon the ovary by insects 

 which had previously visited the Podisoma under the delusion that it was a 

 flower, and carried the minute spores with them to the hawthorn." The 

 wind, however, is likely to be the common agency. Since the leaves and 

 stalks bearing the germinaiting teleutospores often lie upon the ground, it 

 might seem at first sight as if they were not favorably situated for the wind 

 to act upon them, but as they are usually produced in large numbers, suffi- 

 cient of the sporidiola are likely to be suspended in the air to infect fresh 

 plants when the conditions are suitable. 



How long the sporidiola retain (their germinating power, and how far they 

 can stand drying up, are questions not yet satisfactorily determined. 



